[Federal Register Volume 85, Number 225 (Friday, November 20, 2020)]
[Rules and Regulations]
[Pages 74560-74593]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-25844]



[[Page 74559]]

Vol. 85

Friday,

No. 225

November 20, 2020

Part III





Department of Transportation





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Federal Aviation Administration





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14 CFR Part 39





Airworthiness Directives; The Boeing Company Airplanes; Final Rule

Federal Register / Vol. 85 , No. 225 / Friday, November 20, 2020 / 
Rules and Regulations

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2020-0686; Product Identifier 2019-NM-035-AD; Amendment 
39-21332; AD 2020-24-02]
RIN 2120-AA64


Airworthiness Directives; The Boeing Company Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: The FAA is superseding Airworthiness Directive (AD) 2018-23-
51, which applied to all The Boeing Company Model 737-8 and 737-9 (737 
MAX) airplanes. AD 2018-23-51 required revising certificate limitations 
and operating procedures of the Airplane Flight Manual (AFM) to provide 
the flightcrew with runaway horizontal stabilizer trim procedures to 
follow under certain conditions. This AD requires installing new flight 
control computer (FCC) software, revising the existing AFM to 
incorporate new and revised flightcrew procedures, installing new MAX 
display system (MDS) software, changing the horizontal stabilizer trim 
wire routing installations, completing an angle of attack (AOA) sensor 
system test, and performing an operational readiness flight. This AD 
also applies to a narrower set of airplanes than the superseded AD, and 
only allows operation (dispatch) of an airplane with certain 
inoperative systems if specific, more restrictive, provisions are 
incorporated into the operator's existing FAA-approved minimum 
equipment list (MEL). This AD was prompted by the potential for a 
single erroneously high AOA sensor input received by the flight control 
system to result in repeated airplane nose-down trim of the horizontal 
stabilizer. The FAA is issuing this AD to address the unsafe condition 
on these products.

DATES: This AD is effective November 20, 2020.
    The Director of the Federal Register approved the incorporation by 
reference of a certain publications listed in this AD as of November 
20, 2020.

ADDRESSES: For service information identified in this final rule, 
contact Boeing Commercial Airplanes, Attention: Contractual & Data 
Services (C&DS), 2600 Westminster Blvd., MC 110-SK57, Seal Beach, CA 
90740-5600; telephone 562-797-1717; internet https://www.myboeingfleet.com. You may view this service information at the 
FAA, Airworthiness Products Section, Operational Safety Branch, 2200 
South 216th St., Des Moines, WA. For information on the availability of 
this material at the FAA, call 206-231-3195. It is also available on 
the internet at https://www.regulations.gov by searching for and 
locating Docket No. FAA-2020-0686.

Examining the AD Docket

    You may examine the AD docket on the internet at https://www.regulations.govby searching for and locating Docket No. FAA-2020-
0686; or in person at Docket Operations between 9 a.m. and 5 p.m., 
Monday through Friday, except Federal holidays. The AD docket contains 
this final rule, any comments received, and other information. The 
address for Docket Operations is U.S. Department of Transportation, 
Docket Operations, M-30, West Building Ground Floor, Room W12-140, 1200 
New Jersey Avenue SE, Washington, DC 20590.

FOR FURTHER INFORMATION CONTACT: Ian Won, Manager, Seattle ACO Branch, 
FAA, 2200 South 216th St., Des Moines, WA 98198; phone and fax: 206-
231-3500; email: [email protected].

SUPPLEMENTARY INFORMATION:

Discussion

Summary of NPRM

    The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 
CFR part 39 and supersede AD 2018-23-51, Amendment 39-19512 (83 FR 
62697, December 6, 2018; corrected December 11, 2018 (83 FR 63561)) (AD 
2018-23-51). AD 2018-23-51 applied to all Boeing Model 737-8 and 737-9 
(737 MAX) airplanes. The NPRM proposed to apply only to the 737 MAX 
airplanes identified in Boeing Special Attention Service Bulletin 737-
31-1860, dated June 12, 2020, which identifies line numbers for 
airplanes with an original airworthiness certificate or original export 
certificate of airworthiness issued on or before the effective date of 
the original Emergency Order of Prohibition. Airplanes that have not 
received an original airworthiness certificate or original export 
certificate of airworthiness on or before the date of the original 
Emergency Order of Prohibition will have been modified to incorporate 
the changes required by this AD prior to receiving an original, or 
original export, airworthiness certificate.
    The NPRM published in the Federal Register on August 6, 2020 (85 FR 
47698). The NPRM was prompted by the potential for a single erroneously 
high AOA sensor input received by the flight control system to result 
in repeated airplane nose-down trim of the horizontal stabilizer. To 
address this unsafe condition, the NPRM proposed to require installing 
new FCC software, revising the existing AFM to remove the AFM revisions 
required by AD 2018-23-51 and to incorporate new and revised AFM 
flightcrew procedures, installing new MDS software, changing the 
horizontal stabilizer trim wire routing installations, completing an 
AOA sensor system test, and performing an operational readiness flight. 
The NPRM also proposed to allow operation (dispatch) of an airplane 
with certain inoperative systems only if certain more restrictive 
provisions are incorporated into the operator's existing FAA-approved 
MEL.

Related Actions

    During September 2020, the FAA conducted an operational evaluation 
of the operating procedures (checklists) in the proposed AD, to assess 
their effectiveness. The FAA also evaluated pilot training proposed by 
Boeing pertaining to the 737 MAX. The FAA conducted the evaluation 
jointly with the Ag[ecirc]ncia Nacional de Avia[ccedil][atilde]o Civil 
(ANAC) Brazil, Transport Canada Civil Aviation (TCCA), and the European 
Union Aviation Safety Agency (EASA). This joint evaluation is referred 
to as the Joint Operational Evaluation Board (JOEB). The operational 
evaluation included airline pilots with varied levels of experience 
from the United States, Canada, Brazil, and the European Union. The FAA 
and the other civil aviation authorities (CAAs) concluded that air 
carrier pilots operating the 737 MAX need to complete special training 
on the 737 MAX, including ground and flight training in a full flight 
simulator (FFS). The FAA also identified additional special emphasis 
areas to be included in 737 MAX recurrent or continuing qualification 
pilot training.
    The FAA documented the results of the JOEB evaluation in the draft 
FAA Flight Standardization Board (FSB) Report, The Boeing Company 737, 
Revision 17 (draft 737 FSB Report). As described in an addendum to the 
draft 737 FSB Report, the JOEB evaluation identified three areas in the 
proposed Airspeed Unreliable checklist for potential refinement.\1\ On 
October 6, 2020, the FAA made the draft 737 FSB Report and the Addendum 
available to the public for comment (85 FR 63641,

[[Page 74561]]

October 8, 2020). The comment period closed November 2, 2020.
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    \1\ These areas are described in the 737 FSB Report Addendum, 
which is in the docket for this rulemaking.
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    The FAA issued the final FSB Report, The Boeing Company 737, 
Revision 17, dated November 16, 2020 (final 737 FSB Report), after 
considering the relevant comments received to the 737 FSB Report docket 
(Docket No. FAA-2020-0928). The FAA considered the conclusions of the 
JOEB, comments received during the NPRM comment period regarding the 
AFM procedures, and comments received during the draft 737 FSB Report 
comment period in determining the final AFM procedures contained in 
this final rule. For information on the refinements to AFM procedures 
identified in the proposed AD, please refer to the section of this 
preamble titled, ``Suggestions for Crew Procedure Changes.''
    Additionally, the FAA has also finalized the ``Preliminary Summary 
of the FAA's Review of the Boeing 737 MAX,'' dated August 3, 2020, 
which the FAA placed in the docket at the time of publication of the 
NPRM. This ``Summary of the FAA's Review of the Boeing 737 MAX,'' dated 
November 18, 2020, is also included in the docket for this rulemaking. 
The final Summary includes additional explanation regarding 737 MAX 
design changes, certification efforts, maintenance considerations, 
pilot training, and final disposition of the Technical Advisory Board 
(TAB) findings. The TAB is an independent team of experts that 
evaluated efforts by the FAA and efforts by Boeing associated with the 
redesign of the maneuvering characteristics augmentation system (MCAS). 
The conclusions from the TAB and resolution of the findings directly 
informed the FAA's decision-making on MCAS.\2\ The TAB included FAA 
certification specialists and chief scientific and technical advisors 
not involved in the original 737 MAX certification program. TAB members 
also included subject matter experts from the U.S. Air Force, the Volpe 
National Transportation Systems Center, and the National Aeronautics 
and Space Administration. All findings that the TAB members identified 
as required for return to service of the 737 MAX were resolved to their 
satisfaction.
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    \2\ The TAB Report has been included in this docket.
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Summary of Final Rule

    After careful consideration of the comments submitted \3\ and 
further review of the proposal, the FAA adopts this final rule. This 
final rule mandates corrective action that addresses an unsafe 
condition on the 737 MAX. This unsafe condition is the potential for a 
single erroneously high AOA sensor input received by the flight control 
system to result in repeated airplane nose-down trim of the horizontal 
stabilizer, which, in combination with multiple flight deck effects, 
could affect the flightcrew's ability to accomplish continued safe 
flight and landing.
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    \3\ In developing this final rule, the FAA considered comments 
submitted to the NPRM docket and also comments submitted to the 737 
FSB Report docket.
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    As proposed in the NPRM, the corrective actions mandated by this AD 
include a revision of the airplane's flight control laws (software).\4\ 
The new flight control laws now require inputs from both AOA sensors in 
order to activate MCAS. They also compare the inputs from the two 
sensors, and if those inputs differ significantly (greater than 5.5 
degrees for a specified period of time), will disable the Speed Trim 
System (STS), which includes MCAS, for the remainder of the flight and 
provide a corresponding indication of that deactivation on the flight 
deck. The new flight control laws now permit only one activation of 
MCAS per sensed high-AOA event, and limit the magnitude of any MCAS 
command to move the horizontal stabilizer such that the resulting 
position of the stabilizer will preserve the flightcrew's ability to 
control the airplane's pitch by using only the control column. This 
means the pilot will have sufficient control authority without the need 
to make electric or manual stabilizer trim inputs. The new flight 
control laws also include FCC integrity monitoring of each FCC's 
performance and cross-FCC monitoring, which detects and stops erroneous 
FCC-generated stabilizer trim commands (including MCAS).
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    \4\ In the NPRM, the FAA used several terms (including ``new,'' 
``updated,'' and ``revised'') when describing the FCC software 
(including MCAS and control laws) required by paragraph (g) of this 
AD. This software change is a complete replacement of the original 
FCC software, including a new part number. This final rule requires 
installation of the same FCC software as described in the NPRM and 
refers to it as the new FCC software, new MCAS, and new control 
laws. For example, where this final rule uses the term ``new MCAS,'' 
this term reflects the same meaning as ``revised MCAS'' or ``updated 
MCAS'' used in the NPRM.
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    This AD further mandates changes to the airplane's AFM to add and 
revise flightcrew procedures to facilitate the crew's ability to 
recognize and respond to undesired horizontal stabilizer movement and 
the effects of a potential AOA sensor failure.
    This AD also mandates an AOA DISAGREE alert, which indicates 
certain AOA sensor failures or a significant calibration issue. The 
alert is implemented by revision of MDS software; as a result, certain 
stickers (known as INOP markers) will be removed.
    Additionally, this AD mandates adequately separating certain 
airplane wiring, and conducting an AOA sensor system test and an 
operational readiness flight on each airplane before the airplane is 
reintroduced to service.
    Finally, this AD requires that operators that wish to dispatch 
airplanes with certain inoperative systems must first have incorporated 
specific provisions that are more restrictive into their existing FAA-
approved MEL.

Differences From the NPRM

    This final rule differs from the NPRM in minor respects. After 
review of input from the operational evaluations and public comments, 
the FAA adjusted two AFM procedures: The Airspeed Unreliable and the 
ALT Disagree non-normal checklists. This AD simplifies and corrects 
grammatical and typographical errors in the Airspeed Unreliable non-
normal checklist (figure 2 to paragraph (h)(3) of this AD), and revises 
the ALT Disagree non-normal checklist (figure 8 to paragraph (h)(9) of 
this AD) to correct a typographical error in the NPRM.
    The FAA has reviewed and approved new and updated service 
information that is mandated by this AD, including Boeing Alert 
Requirements Bulletin 737-22A1342 RB and Alert Service Bulletin 737-
22A1342, both dated November 17, 2020, for the new FAA-approved FCC 
software; Boeing Special Attention Service Bulletin 737-31-1860, 
Revision 1, dated July 2, 2020, for the MDS software change; and Boeing 
Special Attention Service Bulletin 737-27-1318, Revision 2, dated 
November 10, 2020, for the horizontal stabilizer wiring change. This AD 
also provides credit for accomplishment of certain prior actions as 
specified in paragraph (o) of this AD.

Public Comment

    The FAA provided the public with an opportunity to comment on the 
proposed AD and received approximately 230 submissions to Docket No. 
FAA-2020-0686. The FAA received comments from individual commenters as 
well as from organizations. The majority of the comments were from 
individuals.
    Organizations submitting comments included the Families of 
Ethiopian Airlines Flight 302; the civil aviation authorities of Turkey 
(Turkish DGCA) and the United Arab Emirates (UAE GCAA); the National 
Transportation Safety Board (NTSB); the National Air

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Traffic Controllers Association (NATCA); Flyers Rights; Aerospace 
Safety and Security, Inc.; the Aerospace Safety Research Institute, 
Inc.; Boeing; Airlines for America (A4A); the Ethiopian Airlines Group; 
the Joint European Max Operators Group (JEMOG); the British Airline 
Pilots Association (BALPA); the Allied Pilots Association; the 
Association of Flight Attendants-CWA (AFA-CWA); Air China; Ameco; 
Travelers United, Inc.; Southwest Airlines Pilot Association (SWAPA); 
and the Air Line Pilots Association, International (ALPA).
    The following summarizes the comments received on the NPRM, and 
provides the FAA's responses.

A. Support for the NPRM

    The FAA received supportive comments on the NPRM from Travelers 
United, Inc., and numerous other commenters. Commenters who expressed 
support for the NPRM noted the benefits of the proposed design changes 
based on lessons learned and applied by the FAA, the resolution of 
issues related to the airplane's MCAS, the relative ease of 
accomplishing the proposed changes, a general appreciation for the 
airplane design and handling, and the length and intensity of the 
review of the unsafe condition, corrective action, and the airplane, 
which the commenters said resulted in a safe design. The NTSB expressed 
general support for the NPRM as it relates to MCAS, noting ``positive 
progress on meeting the intent of the overall recommendation regarding 
system safety assessments (SSAs) for the Boeing 737 MAX relating to 
uncommanded flight control inputs.''

B. Fundamental Design/Approach Concerns

    The Boeing 737 MAX uses MCAS to change the handling characteristics 
for the flightcrew in order to comply with certain regulations during 
high-AOA maneuvers. In the NPRM, the FAA proposed to require the 
installation of new FCC software with new MCAS control laws to replace 
the earlier FCC software installed on 737 MAX airplanes. Several 
commenters questioned the fundamental design of the airplane, 
especially the inclusion and availability of MCAS.

Comments Regarding Inclusion and Availability of MCAS

    Comment summary: Several commenters stated that MCAS should not be 
retained as a function on the airplane, and other commenters including 
the Families of Ethiopian Airlines Flight 302 had fundamental concerns 
with the basic design and availability of MCAS. More specifically, 
these comments focused on the availability of MCAS after failure, 
whether the airplane remained safe and compliant, and on the redundancy 
of the system and its inputs.
    FAA response: The FAA determined that the 737 MAX with the new MCAS 
implemented by the new FCC software, as proposed in the NPRM and 
required by paragraph (g) of this AD, meets FAA safety standards.
    The MCAS on the 737 MAX improves the pilot handling qualities 
(maneuvering characteristics) during non-normal flight conditions, 
specifically when the airplane is at high AOAs. During normal flight, 
the 737 MAX should never be at an AOA high enough to be within the 
range that MCAS would activate. FAA regulations require that airplanes 
be designed and tested over the entire range of potential angles of 
attack, including high AOAs. FAA regulations also require column force 
to increase as AOA increases (14 CFR 25.143(g), 25.251(e), and 25.255).
    In a 737 MAX, if a pilot is maneuvering the airplane with the flaps 
retracted and encounters a high AOA (outside of the normal flight 
envelope), MCAS will activate and command the stabilizer to move in the 
airplane nose-down direction, which changes the handling 
characteristics such that the pilot would need to pull with increasing 
force on the control column to maintain the current AOA or further 
increase the AOA. MCAS-commanded stabilizer movement results in 
increased column forces such that the airplane meets FAA handling 
characteristics requirements for airplane operation at high AOAs. 
Existing FAA regulations (14 CFR 25.21, 25.671, and 25.672) allow for 
use of stability augmentation systems (such as MCAS) in showing 
compliance with FAA handling characteristics requirements. The 737 MAX 
airplane with MCAS operative is therefore compliant.
    To be approved by the FAA, the proposed designs of transport 
category airplane flight control systems must comply with applicable 14 
CFR part 25 regulations. The assessment of compliance must consider the 
airplane in the as-designed, fully operational configuration (no 
failures) and also, in accordance with 14 CFR 25.671 and 25.1309, in 
potential failure conditions. When assessing those failure conditions, 
the applicant must take into account both the probability of the 
failures and their airplane-level consequences. The outcome must show 
that the airplane is capable of continued safe flight and landing after 
single failures and any failure combination not shown to be extremely 
improbable (14 CFR 25.1309). For example, a twin-engine transport 
airplane complies with all regulations while both engines are 
operating, but if there is a single engine failure, the airplane must 
be capable of continued safe flight and landing with only the one 
remaining engine operating.
    With MCAS inoperative, the Boeing 737 MAX is capable of continued 
safe flight and landing and is therefore compliant with 14 CFR 25.671 
and 25.1309. If at high AOAs, with MCAS inoperative, MCAS will not move 
the stabilizer, and the resultant incremental change in column force 
will not be experienced by the pilot. In this situation, the pilot 
maintains control and can decrease the airplane's AOA by moving the 
column forward. Through comprehensive analysis, simulation testing, and 
flight testing, the FAA determined that the airplane meets applicable 
14 CFR part 25 standards, with MCAS operative and with failures, 
including failures that render MCAS inoperative. With MCAS inoperative 
after a failure, the 737 MAX is capable of continued safe flight and 
landing, as required by 14 CFR 25.671 and 25.1309.
    If a system must be functional at all times to ensure continued 
safe flight and landing, the system must be available to function after 
a single failure. Conversely, if an inoperative system does not prevent 
continued safe flight and landing, then it is acceptable under FAA 
regulations for the system to not be available after a single failure; 
this is how MCAS is implemented on the 737 MAX.
    The foregoing discussion focuses on an inoperative MCAS. All 
failure modes must be considered and assessed by the manufacturer and 
the FAA for compliance with 14 CFR 25.671 and 25.1309. The new MCAS is 
designed such that most failures will result in the MCAS function 
becoming inoperative, with maintenance required before a subsequent 
flight to return MCAS to being fully operative and available. The 
manufacturer and the FAA have assessed potential failure modes of the 
system to ensure that no single failure will prevent continued safe 
flight and landing and that any combination of failures that could 
occur in service, except for those shown to be extremely improbable, 
would similarly not prevent continued safe flight and landing.
    Failures of MCAS are annunciated to the flightcrew. MCAS is 
implemented as part of the airplane's STS. During flight, STS failures 
(including MCAS failures) are annunciated by illumination of the master 
caution light, the SPEED TRIM FAIL light, and the system annunciator

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panel (FLT CONT). Per training, the flightcrew will follow applicable 
crew procedures for continued safe flight and landing.
    Based on analyses, simulation, and flight testing to establish 
consequences of failures and the capability for continued safe flight 
and landing, the FAA has determined that the new MCAS meets FAA safety 
standards, and that it is acceptable for STS (including MCAS) to remain 
inoperative for the remainder of a flight after the system fails. 
Therefore, the additional redundancy requested by commenters, to 
increase the availability of the system, is not required.

C. Specific Concerns About MCAS

1. Comments Regarding Redundancy of Two AOA Sensors
    Comment summary: The Families of Ethiopian Airlines Flight 302 
asked whether the two AOA sensor inputs to MCAS are truly redundant.
    FAA response: The two AOA sensors and the data they provide are 
independent, and are therefore redundant in that the failure of one AOA 
sensor does not impede the operation of the other AOA sensor. For MCAS 
inputs, the left and right air data/inertial reference units (ADIRUs) 
receive direct input from the AOA sensors installed on the left and 
right sides of the airplane, respectively. Each ADIRU transmits the 
current AOA sensor position to the left and right FCCs via databuses. 
The signal path to each FCC is independent of the other FCC (e.g., the 
left AOA data does not travel through the left FCC to reach the right 
FCC).
2. Comments Regarding Additional AOA Sensors or Data
    Comment summary: Numerous commenters including the Families of 
Ethiopian Airlines Flight 302 and BALPA contended that three or more 
AOA values are required for the system to be able to continue operating 
after a failure of a single AOA sensor. Commenters assert that if the 
two AOA values diverge, the system cannot detect which value is 
erroneous; but with three AOA inputs, if one value deviates from the 
other two, the deviant value could be excluded while the system 
continues to operate using data from the remaining two sensors. In 
support of their requests for additional AOA sensors or inclusion of a 
derived value (synthetic AOA), some commenters noted that AOA sensors 
are exposed to the elements or other external factors such as bird 
strikes.
    FAA response: As explained earlier in this preamble, the 737 MAX is 
capable of continued safe flight and landing with MCAS inoperative. 
Accordingly, continued safe flight and landing can be accomplished when 
MCAS is disabled following the failure of a single AOA input. The new 
MCAS, as proposed in the NPRM and mandated by this AD, utilizes two AOA 
inputs and compares the difference between them. If there is a 
significant difference (greater than 5.5 degrees for a specified period 
of time), then MCAS will be disabled (unavailable) for the remainder of 
that flight, annunciation will alert the flightcrew to the failure, and 
maintenance will be required before subsequent flight.
    Regarding exposure to the elements (that is, weather conditions but 
not a bird strike), AOA sensors are designed, tested, and qualified for 
their operational environment as part of certification (14 CFR 
25.1301). The new MCAS design accounts for safe operation after AOA 
sensor failures due to environmental causes including bird strikes that 
bend or break the vane of the AOA sensor, as discussed in subsequent 
responses.
3. Comments Regarding Keeping MCAS Partitioned
    Comment summary: Commenters suggested that MCAS be partitioned such 
that each FCC would receive input from only a single AOA sensor, with 
the pilots responsible for switching control from one FCC to the other.
    FAA response: The change suggested by the commenters would not 
improve the safety of the airplane, because it would remove the AOA 
sensor comparison feature of the new design and allow a single AOA 
sensor failure to activate MCAS as in the original MCAS. Regarding the 
request to make the pilots responsible for switching control from one 
FCC to the other, the FAA evaluated the design presented by the 
applicant. It is likely, however, that the commenters' proposal would 
increase pilot workload and may also introduce unreasonable reaction 
time requirements for pilot actions. Contrary to the commenters' 
proposed single-input configuration, which could allow for MCAS 
activation following a single failure, the new MCAS design mandated by 
this AD addresses the unsafe condition by not allowing for that exact 
event.
4. Comments Regarding MCAS Response After Failure(s)
    Comment summary: Several commenters, including BALPA and the 
Turkish DGCA, requested that the FAA require that MCAS not activate if 
there is a disagreement between AOA sensor inputs or a dual AOA sensor 
failure, and that MCAS should not remain available following certain 
AOA sensor failures.
    FAA response: The FAA confirms that most AOA sensor failures will 
result in the MCAS function becoming inoperative, and if MCAS is 
activated, it will activate only once for each high-AOA event, which 
does not preclude continued safe flight and landing. AOA sensor 
failures can be divided into two broad categories: (1) Detected 
failures of the electrical circuit that measures the angular position 
of the AOA sensor such that the AOA data is labeled as invalid and not 
used by user systems (including MCAS); and (2) undetected failures that 
do not damage the electrical circuit such that AOA data is transmitted 
from the ADIRU to the FCC as valid. Both 737 MAX accidents involved the 
second category of AOA sensor failures; the AOA sensor electrical 
circuit was unaffected and therefore perceived by the ADIRU to be 
valid, and the transmitted value was used by the MCAS function in the 
FCC.
    With the new MCAS, the second type of AOA sensor failure will 
result in disparate inputs to the FCCs. When disparate inputs are 
received by the FCCs, the FCCs will disable the MCAS function, 
preventing it from activating for the remainder of that flight. When 
MCAS is disabled in this way, the master minimum equipment list (MMEL) 
does not allow for dispatch of the airplane again until the system is 
repaired.
    If a single AOA sensor is damaged due to a bird strike, the bent or 
broken AOA sensor vane will affect the AOA measurement. If the AOA 
sensor vane breaks off, the AOA sensor will provide a high AOA value 
due to a counterweight falling within the sensor. With a significant 
difference between valid AOA sensor inputs, the FCCs will disable MCAS. 
Later, if the other AOA sensor is damaged (resulting in a high AOA 
value), MCAS will already have been disabled and there will be no MCAS 
activation. The sequential failure of two AOA sensors during the same 
flight is unlikely; even more unlikely would be a case where two 
sensors are damaged simultaneously and symmetrically such that there is 
not a difference sensed between the two AOA sensors as they both 
transition to similar high AOA values. Even if such a simultaneous and 
symmetrical failure were to occur, MCAS would activate only once. The 
FAA confirmed through testing and analysis during certification that a 
single activation of MCAS will not prevent continued safe flight and

[[Page 74564]]

landing. The pilots can control the change in pitch using only the 
control column, or trim inputs, or any combination of the two.
    The other concern raised by these commenters was that if during a 
flight there is a detected AOA sensor circuit failure (the first 
category described previously), MCAS will continue to be available to 
operate with only a single AOA sensor input for the remainder of that 
flight. During the remainder of the flight when the first circuit 
failure occurred, a subsequent independent failure of the other AOA 
sensor, that is not detected (second category, e.g., a bird strike) and 
results in an erroneous valid AOA input, would be extremely improbable. 
Nevertheless, if this failure combination were to occur (first category 
followed by the second category), the outcome would not prevent 
continued safe flight and landing; MCAS would activate only one time, 
with the pilots able to control the airplane using either the control 
column, the electric trim switches, or both. This scenario was analyzed 
and tested by FAA engineers and pilots and found to be compliant with 
the FAA's safety standards.
5. Comments Regarding MCAS Operation at Low Altitude
    Comment summary: A commenter stated that MCAS should not operate in 
certain phases of flight, such as takeoff, climb, and landing, because 
there should not be a potential for a failure to cause the airplane to 
lose altitude during those phases of flight. Another commenter 
suggested MCAS should not operate at low altitudes due to the potential 
for a wake turbulence encounter or a bird or animal strike.
    FAA response: MCAS is functional only during flight with the flaps 
fully retracted. When the airplane is at low altitudes near the airport 
for takeoff, and later during approach and landing, flaps are extended, 
typically below 1,000 feet; therefore, MCAS is not operational for the 
take-off and landing phases of flight. For other phases of flight 
including climb, AOA disagreement due to an incident such as a bird 
strike will be detected by the FCCs, and the FCCs will disable MCAS for 
the remainder of that flight. Since the new MCAS function is consistent 
with the commenters' requests, no change to this AD is necessary.
6. Comments Regarding MCAS Availability for Multiple Activations
    Comment summary: Two commenters expressed concern that limiting 
MCAS to a single activation would render MCAS unavailable for more 
activations later in the flight, if needed, and that MCAS would not be 
available to perform its intended function.
    FAA response: The commenters' concerns do not accurately reflect 
the new MCAS functionality. The new MCAS is designed to activate one 
time for each high-AOA event (above the MCAS activation threshold). The 
new MCAS will activate when there is a high-AOA event (above activation 
threshold as previously described), and then will reset after the 
airplane returns to a low AOA that is sufficiently below the MCAS 
activation threshold, such that it will be available for a subsequent 
activation if there is a subsequent high-AOA event. As a result, after 
the new MCAS activates once, it will be available for more activations 
later in the same flight. Only if there has been a failure during the 
flight that disables MCAS, which is indicated by the SPEED TRIM FAIL 
light, will MCAS not be available during a high-AOA event with the 
flaps retracted.
7. Comments Regarding Disabling of Column Cutout Switches
    Comment summary: Two commenters suggested changing the design and 
function of the column cutout switches on the 737 MAX to be more 
similar to those on earlier Boeing Model 737 designs.
    FAA response: The column cutout switch function of earlier Boeing 
Model 737 models would not allow for MCAS activation.
    Column cutout switches on earlier Boeing Model 737 models allow the 
flightcrew the capability to interrupt (cut out) a stabilizer command 
in one direction by making a control column input in the other 
direction (e.g., an airplane nose-down stabilizer command will be 
interrupted by pulling the control column aft). The 737 MAX has the 
same column cutout feature, but it is temporarily disabled during the 
short duration of an MCAS activation.
    MCAS operates only during high-AOA events, which are typically 
caused by the flightcrew pulling aft on the control column. To allow 
MCAS to operate as intended, the FCC temporarily disables the column 
cutout switches when MCAS is activated (makes a command). Without this 
temporary disable feature, the MCAS command to move the stabilizer in 
the airplane nose-down direction would otherwise be interrupted by the 
column cutout switches.
    After the MCAS activation, the column cutout switches revert to a 
configuration where control column inputs will interrupt stabilizer 
commands in the opposite direction. When MCAS is not making a command, 
the column cutout switches operate like they do on earlier models of 
the Boeing Model 737. It is only during the short duration of an MCAS 
command that the column cutout switches on 737 MAX airplanes operate 
differently than those on other Boeing Model 737 airplanes.
    The new MCAS includes cross-FCC monitoring, which detects and stops 
erroneous FCC-generated stabilizer trim commands (including MCAS). This 
protects against an erroneous FCC-generated stabilizer trim command 
throughout the entire flight, including when the column cutout switches 
are temporarily disabled.
8. Comments Regarding Erroneous MCAS Enable Command
    Comment summary: A commenter expressed concern that the MCAS enable 
command, which disables column cutout, could be asserted during a 
horizontal stabilizer trim runaway due to hardware faults on the 
stabilizer interface.
    FAA response: The scenario set forth by the commenter would result 
from the simultaneous occurrence of an erroneous FCC-generated command 
that disables the column cutout feature and an erroneous command (from 
either the pilot or the FCC) to move the stabilizer. The potential for 
this combination of failures to occur simultaneously is mitigated by 
integrity monitoring of the MCAS enable command by the new FCC 
software, which monitors for proper FCC performance. Furthermore, 
periodic maintenance checks, implemented by new tasks in the Boeing 737 
Maintenance Planning Document (MPD), verify the function of the cutout 
switches (located on the aisle stand) and the MCAS enable command. 
Finally, the cross-FCC monitor also reduces the likelihood of any FCC-
generated stabilizer trim runaway command.
9. Comments Regarding MCAS Vulnerability to Single Failures
    Comment summary: A commenter stated that the system should not be 
vulnerable to a single failure, and expressed concern that the new MCAS 
remains vulnerable to a single failure. Another commenter asked whether 
there is a scenario where any single failure, or probable combination 
of failures, requires the flightcrew to stop moving the stabilizer by 
grabbing the manual stabilizer trim wheel in the flight deck; this 
commenter also asked whether that is in the crew procedure.
    FAA response: The FAA determined that the new MCAS is compliant 
with 14 CFR 25.671 and 25.1309, such that no single failure, or 
combination of

[[Page 74565]]

failures not shown to be extremely improbable, will prevent continued 
safe flight and landing. Nevertheless, the AFM revisions required by 
this AD include a runaway stabilizer procedure with guidance for 
arresting any potential runaway stabilizer event. The final step of 
that procedure is to ``grasp and hold stabilizer trim wheel.'' That 
procedure is yet another layer of protection.
10. Comments Regarding MCAS Vulnerability to Sinusoidal AOA Input
    Comment summary: Several commenters expressed concern about 
perceived vulnerabilities of the new MCAS implemented by the new FCC 
software. A commenter expressed concern that MCAS is vulnerable to 
sinusoidal AOA sensor input. Another commenter expressed concern that 
the middle value select (MVS) function implemented to mitigate 
erroneous sinusoidal AOA sensor input as part of the new MCAS can 
diverge or cause a limit cycle oscillation. Another commenter expressed 
a concern with the MVS algorithm, specifically that if there is a fixed 
offset between the two AOA sensor values that is less than the 5.5-
degree threshold that will cause deactivation of MCAS, the MCAS 
function would be utilizing AOA sensor inputs that are offset by up to 
5.5 degrees.
    FAA response: The new FCC software compares the two AOA sensor 
inputs relative to each other and will disable STS (including MCAS) for 
the remainder of the flight if the difference between the two exceeds a 
threshold of 5.5 degrees. The new MCAS also uses an MVS algorithm to 
address the potential for a sinusoidal AOA input from a single AOA 
sensor. To demonstrate compliance with 14 CFR part 25 standards, the 
new MCAS was analyzed and tested with various failure scenarios, 
including a sinusoidal AOA sensor input. The results established that 
MVS is effective, that it will not result in divergence or limit cycle 
oscillation, and that the design is compliant and safe. The FAA also 
tested the new MCAS with the scenario of AOA sensors offset by up to 
5.5 degrees during certification and found the design to be compliant 
and safe.
11. Comments Regarding MCAS Vulnerability to Pilot Induced Oscillation
    Comment summary: A commenter expressed concern about the MCAS 
response to a pilot induced oscillation (PIO).
    FAA response: PIO, which is also known as airplane/pilot coupling 
(APC), is a phenomenon where the frequency of pilot inputs couples 
(matches) with an inherent airplane frequency. The susceptibility of 
the 737 MAX to PIO/APC was assessed throughout all of the FAA flight 
testing during certification of the 737 MAX. The FAA found the 737 MAX 
is not prone to PIO/APC. This remains true with and without MCAS being 
available. This also remains true during a valid or erroneous MCAS 
activation.
12. Comments Regarding Adequacy of MCAS
    Comment summary: A commenter was concerned that the new MCAS is 
inadequate with regard to the rate at which it can respond during a 
high-AOA event. The commenter noted that the rate at which the airplane 
AOA increases may be too great for MCAS to be effective.
    FAA response: MCAS has been analyzed and tested by the FAA and the 
manufacturer in various scenarios and flight conditions, which includes 
MCAS's rate of response, as part of the certification process, and was 
found to meet its intended function, and to be compliant with all 
applicable 14 CFR part 25 regulations.

D. Specific Concerns About Alerting

1. Comments Regarding Annunciating MCAS Activation and MCAS Failures
    Comment summary: Numerous commenters, including BALPA, the Families 
of Ethiopian Airlines Flight 302, and Ethiopian Airlines Group, 
commented regarding annunciations and alerting associated with MCAS. 
Some commenters wanted the system changed to add features to make the 
pilot aware when MCAS is making a valid command to the stabilizer 
system. They were concerned that without annunciation, pilots would 
have difficulty discerning normal from non-normal MCAS activation. They 
suggested illuminating a new light, displaying a message on the primary 
flight display (PFD), displaying a new flight mode annunciator, 
displaying the magnitude of the incremental MCAS command to the 
stabilizer, and generating a voice annunciation. Other commenters 
suggested that MCAS failures or deactivations be annunciated by the 
addition of a warning to alert the crew, a red MCAS FAIL warning, or a 
loud alert at the same time MCAS is disabled.
    FAA response: The new MCAS already alerts the pilot of an MCAS 
failure. The addition of more annunciation of valid MCAS activation is 
not necessary to address the unsafe condition.
    When the STS (including the speed trim function and the MCAS 
function) makes a command to move the stabilizer, the flightcrew is 
aware of the command because the manual trim wheels, located in the 
aisle stand between the two pilots in the flight deck, will rotate as 
the stabilizer moves. The STS has been a basic design feature of the 
Boeing Model 737 series for many years and is familiar to flightcrews. 
It is not necessary for a system to annunciate to the pilot that it is 
active. The pilot can both see and hear the manual trim wheels rotate 
when the stabilizer is moved. Normal MCAS activation occurs only during 
non-normal flight conditions when the airplane is at a high AOA, and 
high AOA maneuvering could potentially already be a high workload 
scenario for the flightcrew. Indications to the pilot that the airplane 
is at a high AOA include the appearance of the amber band on the 
airspeed tape, the appearance of amber pitch limit indicator (PLI), 
flashing amber airspeed digits on the airspeed tape, the appearance of 
the red and black barber pole on the airspeed tape on the PFD, 
increasing column force, and stick shaker.
    Additional annunciation of normal MCAS function during this time 
could distract the pilots from recovering from this non-normal high-AOA 
flight condition.
    Regarding the commenters' request for annunciation of FCC failures 
related to MCAS, the system alerts the flightcrew by illuminating the 
Master Caution, system annunciator panel (FLT CONT), and SPEED TRIM 
light. After landing, the SPEED TRIM FAIL and/or STAB OUT OF TRIM light 
will be illuminated. Therefore, the existing system already alerts the 
flightcrew to MCAS failures.
    The new FCC software monitors inputs and outputs for failures, 
including erroneous MCAS commands, and will disable MCAS for detected 
failures. During normal operation, the FCC commands horizontal 
stabilizer movement only for three cases: (1) When the autopilot is 
engaged and the stabilizer is moved to offload column movement, (2) as 
part of the speed trim function during manual flight, associated with 
changes in airspeed, and (3) as part of the MCAS function during manual 
flight at high AOA outside normal flight conditions. Pilots will learn 
about automated stabilizer trim operation in the special 737 MAX 
training. Pilots have the ability to override any FCC-generated 
stabilizer trim command, because pilot stabilizer trim commands via the 
thumb switches

[[Page 74566]]

on the control wheel always have priority over FCC-generated commands.
    Finally, if the flightcrew deactivates MCAS by moving the 
stabilizer trim cutout switches (located on the aisle stand) to the 
cutout position using the Runaway Stabilizer NNC (non-normal 
checklist), there is no associated annunciation. When the FCC generates 
an STS command (speed trim or MCAS) after the trim cutout switches are 
moved to the cutout position, the system will detect the lack of trim 
motor response to the STS command and illuminate the master caution 
light, the SPEED TRIM FAIL light, and the system annunciator panel (FLT 
CONT). If the autopilot is engaged, when the FCC generates an autopilot 
command after the trim cutout switches are moved to the cutout 
position, the system will detect the lack of trim motor response to the 
autopilot command and illuminate the STAB OUT OF TRIM light. Therefore, 
the requested additional annunciation is not necessary.
2. Comments Regarding Display of AOA DISAGREE Alert
    Comment summary: Several commenters, including the UAE GCAA, 
requested that the AOA DISAGREE alert be displayed in the pilot's 
primary field of view and/or on the Head Up Display (HUD).
    FAA response: Paragraph (j) of this AD requires installation of new 
MDS software including functionality to display the AOA DISAGREE alert 
on each pilot's PFD if the left and right AOA values differ by more 
than 10 degrees for more than 10 seconds. The PFDs are in the primary 
field of view in front of each pilot, and are therefore consistent with 
the commenters' request. Regarding the message also showing on the HUD, 
the FAA notes that HUDs are optional equipment. For airplanes with HUDs 
installed, updated HUD software will display AOA DISAGREE on the HUD if 
it is being displayed on the PFD. The HUD software is not required by 
this AD. No change to this AD is necessary based on this comment.
3. Comments Regarding Omission of AOA DISAGREE Alert From 737 MAX
    Comment summary: Several commenters asked why the AOA DISAGREE 
alert was not included in the original 737 MAX design.
    FAA response: The AOA DISAGREE alert is a standard design feature 
on the 737 NG fleet (600/700/800/900/900ER) and was intended to be 
standard for the 737 MAX, but it was instead erroneously linked by the 
manufacturer to an optional AOA indicator (which some refer to as a 
gauge). The optional AOA indicator is a round dial that provides 
graphic and numeric AOA position information on both PFDs. Because of 
this error, only airplanes with the (optional) AOA indicator had a 
functioning AOA DISAGREE alert. This was incorrectly implemented by the 
manufacturer during the display software development, and was not 
identified until after the 737 MAX entered into service.
4. Comments Regarding Display of AOA Indicators
    Comment summary: Several commenters, including BALPA, suggested 
that the optional AOA indicators (gauges) be made basic to the 
airplane, or offered as a no-cost option, so they are available to 
check accuracy and enhance pilot situational awareness. Another 
commenter asked why there is no standby (third) AOA indicator.
    FAA response: The AOA position indicators are not required for 
compliance with design standards with regard to pilot situational 
awareness. The cues to the pilots as the airplane approaches stall are 
inherent in other airspeed and attitude information displayed on the 
PFDs, which provide situational awareness and are described earlier in 
this preamble. In response to the question about a third AOA indicator, 
the FAA notes that there is no requirement to have any AOA indicator 
for compliance with 14 CFR part 25 standards.\5\ The FAA has not 
changed this AD based on this comment.
---------------------------------------------------------------------------

    \5\ This preamble addresses elsewhere a comment suggesting the 
addition of a third independent AOA input, which would be required 
to provide data to a third independent AOA indicator.
---------------------------------------------------------------------------

5. Comments Regarding Additional Aural Alerts
    Comment summary: A commenter stated that the AOA DISAGREE alert, as 
well as IAS DISAGREE and ALT DISAGREE alerts, need a corresponding 
aural alert for immediate two-sense awareness of the condition by the 
flightcrew.
    FAA response: The AOA DISAGREE, IAS DISAGREE, and ALT DISAGREE 
alerts show on both PFDs in the pilots' primary field of view. This 
design has been assessed, tested, and found compliant with 14 CFR part 
25. The FAA has not changed this AD based on this comment.

E. Specific Concerns About Crew Interface

1. Comments Regarding Flightcrew Maintaining Control of Airplane
    Comment summary: Numerous commenters stated that the pilot must be 
able to maintain control of the airplane. A commenter expressed concern 
that MCAS remains vulnerable to a combination of MCAS commands and 
pilot inputs that would generate the repetitive MCAS activations that 
occurred during the accident flights. The commenters requested that the 
FAA ensure that the pilots have the physical strength required to make 
column inputs to counter system failures. These commenters stated that 
the system design should be changed to include an independent means to 
turn MCAS off via a dedicated MCAS shutoff switch, which would be 
different from and independent of the aisle stand cutout switches. The 
commenters suggested including a guard that would illuminate the MCAS 
shut-off switch when MCAS is inoperative and provide a corresponding 
aural warning.
    FAA response: None of the identified additional system changes are 
necessary to achieve the objective that the flightcrew must be able to 
maintain control of the airplane. The new MCAS design and associated 
pilot procedures and training focus on the pilot's ability to control 
and remain in control of the airplane.
    The new MCAS has several features to ensure that the pilot 
maintains control. With the new MCAS design, pilot inputs to the trim 
switches do not reset MCAS. Therefore, the new MCAS is not vulnerable 
to the same repetitive cycles of MCAS activation that occurred during 
the accident flights.
    The new MCAS design will (1) detect failures and not command MCAS 
if those failures occur; (2) result in only a single activation of MCAS 
for certain dual failures; and (3) in the event the airplane 
experiences multiple high AOA events, it will limit the stabilizer 
movement so the pilot can always maintain control of the airplane using 
only the control column.
    The FAA also notes that the Runaway Stabilizer NNC (as revised and 
required by paragraph (h) of this AD) is a means for a pilot to stop 
MCAS commands and any electric command to the stabilizer trim motor. 
That procedure is another safety feature in the unlikely event the 
airplane experiences erroneous stabilizer trim movement.
    Regarding the comments suggesting a dedicated switch to disable 
MCAS to include a guard, light, or aural warning, the FAA notes that 
when MCAS is

[[Page 74567]]

disabled due to detected faults, the Master Caution and system 
annunciator panel (FLT CONT), as well as the SPEED TRIM light on the P5 
overhead panel, will be illuminated. The new MCAS is compliant with 14 
CFR part 25 certification standards and addresses the unsafe condition, 
so it is not necessary to change the design to add a dedicated switch 
to disable MCAS or add an additional light or aural alert.
2. Comments Regarding Function of Aisle Stand Cutout Switches
    Comment summary: Numerous commenters suggested changing the design 
of the aisle stand stabilizer trim cutout switches to resemble the 
design on pre-MAX versions of Model 737 airplanes. On those earlier 
Model 737 airplanes, two guarded switches on the aft end of the center 
aisle stand, aft of the throttle levers, are used to stop electric 
commands to the stabilizer trim motor. The pilots are directed to use 
the switches by two NNCs: Runaway Stabilizer and Stabilizer Trim 
Inoperative. In both procedures, the pilot is directed to ``place both 
STAB TRIM cutout switches to CUTOUT.'' On the earlier models of the 
Boeing Model 737, the switches have distinct functions (labeled 
``main'' and ``auto'') where one (auto) would cut out all FCC-generated 
stabilizer commands (autopilot and speed trim) and the other (main) 
would cut out pilot-generated commands (from the pilot thumb switches). 
On the 737 MAX, however, the switches are wired in series, and both 
perform the same function (primary and backup): To cut out all electric 
commands to the stabilizer (both FCC-generated commands and pilot 
commands). The commenters asserted that the configuration of the 
earlier (pre-MAX) Boeing Model 737 airplanes would allow the pilot to 
disable MCAS commands while retaining the ability to make electric trim 
inputs using the thumb switches. The commenters expressed concern that 
pilots would be required to use manual trim for the remainder of that 
flight.
    FAA response: No change to the design or this AD is necessary to 
address the commenters' concerns. The new MCAS has redundancy (receives 
inputs from two AOA sensors and is implemented by two FCC computers) 
and will automatically disable MCAS for the remainder of the flight if 
certain failures are detected. For detected failures where MCAS stops 
making commands, the pilot does not use the aisle stand cutout 
switches, and retains the ability to use thumb switches to control the 
stabilizer. The only time the thumb switches would be unavailable is if 
the pilot moves the aisle stand cutout switches to the cutout position; 
in that event, the pilot has the option to use manual trim to move the 
stabilizer. As discussed in the next paragraph, manual trim forces have 
been assessed and deemed acceptable.
3. Comments Regarding Manual Trim Forces
    Comment summary: Many commenters, including the Allied Pilots 
Association, ALPA, BALPA, Ethiopian Airlines Group, and the UAE GCAA, 
expressed concerns regarding the 737 MAX manual trim system and the 
forces required to control and trim the aircraft following a failure of 
the STS (including MCAS). Some questioned the mechanical advantage 
provided by the manual trim system and whether it had been evaluated in 
flight testing. A commenter stated that it takes 15 turns of the pitch 
trim wheel to get just one degree of horizontal stabilizer movement, 
and some pilots may lack the strength to make those turns if the 
required force is too high. The commenter suggested pilots should be 
required to take a yearly strength test to determine whether they are 
capable of pulling a yoke or turning the pitch trim wheel in simulated 
emergency conditions.
    FAA response: Following the Ethiopian Airlines accident, the 737 
MAX manual trim system design and force requirements were an area of 
intense focus by the Ethiopian Aircraft Accident Investigation Bureau, 
the FAA, Boeing, and other CAAs, which continued throughout the FAA's 
evaluation and testing of the new FCC software and new MCAS during 
certification. The data from the Ethiopian Airlines accident indicates 
that the high trim wheel forces experienced during that accident were 
the result of significant horizontal stabilizer mis-trim combined with 
excessive airspeed. The new FCC software limits the maximum mis-trim 
that could occur for any foreseeable failure of the STS, thus ensuring 
the pilot can maintain control of pitch using the column only, without 
requiring exceptional pilot skill, strength, or alertness. 
Additionally, the FAA evaluated the manual trim system for the unlikely 
event that manual trim will be necessary. This included detailed 
analysis of manual trim wheel forces as a function of both dynamic 
pressure and out-of-trim state, testing to measure and assess the 
strength capability of an anthropometric cross-section of male and 
female subjects, and FAA flight testing to quantitatively validate 
manual trim wheel forces and qualitatively evaluate the ability to 
control the airplane for continued safe flight and landing. These 
flight test conditions and the associated analysis included maximum 
out-of-trim conditions well beyond those possible for any failure 
conditions in the new MCAS design and included the most critical 
aircraft configurations and airspeeds to the operational airspeed limit 
of the flight envelope (referred to as Vmo/Mmo). The FAA determined 
that manual trim wheel forces meet FAA safety standards and do not 
require exceptional pilot skill or strength nor any special or unique 
handling techniques as suggested by some of the commenters. 
Improvements to the Runaway Stabilizer non-normal procedure proposed in 
the NPRM and mandated by this final rule include steps to help ensure 
column forces remain manageable and reduce manual wheel trim forces in 
the unlikely case where manual trim may be needed. Additionally, this 
AFM procedure and pilot training emphasize the first priority in an 
emergency is to maintain control of the airplane, and also include 
specific information about the manual trim system including techniques 
for effectively using manual trim. Therefore, the FAA has made no 
changes in finalizing this AD related to the manual trim system or 
related AFM non-normal procedures.
4. Comments Regarding Availability of Automation After MCAS Failure
    Comment summary: A commenter stated that the autopilot and 
autothrottle should be available following an MCAS failure. The 
commenter expressed concern that MCAS will be triggered routinely due 
to turbulence and gusts during cruise, and its shutdown would render 
the autopilot inoperative. The commenter noted that when autopilot is 
not available, airplanes are prohibited from flight at higher altitudes 
where airplanes fly with reduced vertical separation minima (RVSM).
    FAA response: In most cases, autopilot and autothrottle are 
available following an MCAS failure. Flight testing of the new MCAS has 
demonstrated that it will not be triggered due to turbulence and gusts. 
The new MCAS design is such that following certain MCAS failure 
scenarios, the system will allow for engagement of the autopilot and 
autothrottle. Flightcrew training and procedures identify when the 
flightcrew may attempt to engage the autopilot and/or autothrottle. If 
the Runaway Stabilizer NNC is used, the use of autopilot is prohibited 
by the procedure.

[[Page 74568]]

5. Comments Regarding Selection of Air Data Source
    Comment summary: A commenter wanted the air data system to be 
revised to allow for selection of offside data if onside data is 
erroneous (i.e., the captain can select to display first officer's 
data, or vice versa), and ideally to automate it to prevent the display 
of erroneous data.
    FAA response: This comment regarding the air data system is not 
related to the unsafe condition addressed by this AD. The Boeing 737 
air data system is federated such that independent air data (altitude, 
airspeed, and AOA) from the captain's side is used to provide 
information on the captain's PFD, while independent air data from the 
first officer's side is used to provide information on the first 
officer's PFD. The unsafe condition addressed by this AD concerns a 
single high erroneous AOA generating repetitive MCAS behavior, which, 
in combination with multiple flight deck effects, could affect the 
flightcrew's ability to accomplish continued safe flight and landing. 
The requirements of this AD address the MCAS issue.
6. Comments Regarding Suppression of Overspeed Warning
    Comment summary: A commenter stated that the warning system needs 
to be revised so that the overspeed aural warning can be suppressed 
manually by the flightcrew.
    FAA response: This comment is not related to the unsafe condition 
addressed by this AD. Like the airspeed and stick shaker, the overspeed 
aural warning is federated in a left/right configuration aligning with 
the captain's and first officer's sides of the airplane. The system 
meets the certification standards applicable to this airplane and was 
certificated without a provision for suppressing the aural warning.
7. Comments Regarding Crew Procedure To Extend Flaps
    Comment summary: Two commenters suggested adding a crew procedure 
to extend the flaps in the event of an MCAS failure. They noted that 
MCAS is available only when the flaps are retracted, which indicates 
that the airplane does not need MCAS when the flaps are extended.
    FAA response: It is not necessary to add a new flightcrew procedure 
for extending the flaps in order to counter an MCAS failure. With the 
new MCAS design, time-critical crew procedures are not required to 
mitigate MCAS failures. Furthermore, extending the flaps at high 
airspeeds could damage the flaps and cause controllability problems. 
The FAA has not changed this AD regarding this issue.

F. Suggestions for Crew Procedure Changes

1. Comments Regarding AFM Crew Procedure Adequacy
    Comment summary: Several commenters, including BALPA, NATCA, ALPA, 
Boeing, the Allied Pilots Association, the JEMOG, Ethiopian Airlines 
Group, A4A, and SWAPA, requested that the FAA modify the emergency and 
non-normal procedures contained in the proposed AD. These comments 
covered several of the proposed checklists, with an emphasis on the 
Airspeed Unreliable and Runaway Stabilizer checklists. The comments 
included requests to make small changes involving typographical errors, 
to add information to checklists, to simplify checklists, to shorten or 
reduce the number of memory items, and to develop checklists for 
certain specific failure cases. Three commenters, including BALPA and 
Ethiopian Airlines Group, recommended providing a combined Airspeed 
Unreliable and Runaway Stabilizer checklist for certain specific 
failure conditions.
    Finally, ALPA commented that, while it supported in principle the 
potential changes to the Unreliable Airspeed checklist described in the 
addendum to the draft 737 FSB Report, it cannot provide support or 
opposition to any such changes without reviewing the checklist as 
modified. ALPA proposed that the FAA release the final Airspeed 
Unreliable Checklist for public review and comment after modification 
with the potential refinements described in the addendum.
    FAA response: The FAA has made several changes to the checklists, 
taking into consideration not only comments provided in the context of 
the NPRM, but also in response to the outcomes from the FAA FSB 
evaluation. The inputs from the FAA FSB were the result of 
collaboration with other CAAs during the JOEB. The JOEB conducted an 
extensive evaluation of the proposed procedures and training conducted 
by a wide variety of crews, including line pilots with levels of 
experience ranging from high to low and regulatory pilots from four 
separate CAAs during the NPRM comment period.
    The AFM procedures specified in the proposed AD were the result of 
procedural development conducted by FAA test pilots, human factors, and 
operations personnel (along with other engineering and operational 
experts from other CAAs and from Boeing), which considered a myriad of 
similar aspects as the procedures were developed and evaluated. 
Additionally, the procedures were evaluated during FAA certification, 
including human factors evaluations to determine compliance to 14 CFR 
25.1302, and system safety assessments to determine compliance to 14 
CFR 25.1309. The FAA convened a team of test pilots, operational 
pilots, and human factors experts during the development of the AFM 
procedures specified in the proposed AD. The FAA convened a similar 
team to consider each procedural comment made during the NPRM comment 
period and to determine if changes were warranted to improve safety.
    A4A and SWAPA expressed concern that there are too many recall 
items in the Runaway Stabilizer non-normal procedure, and included a 
suggestion for how to reduce the number of steps. The suggestion 
included combining some recall items to achieve fewer numbered steps, 
but with multiple embedded actions in each recall item, such that the 
suggested changes would result in the same number of required 
flightcrew actions. The FAA agrees that it is desirable to minimize 
recall items when appropriate. The recall steps in the non-normal 
procedures required by paragraph (h) of this AD reflect flightcrew 
actions required to address a runaway stabilizer condition. Based on 
the FAA's evaluation and in coordination with human factors 
specialists, the FAA determined that the commenters' proposed changes 
would complicate the recall steps and would increase the likelihood 
that a critical flightcrew action is forgotten or missed. The FAA 
considered all of the commenters' requests in the context of crew 
workload, clarity of instruction, consistency with training objectives, 
and consistency with other procedures contained in the AFM. The FAA 
declines the request to combine checklists because checklists must be 
applicable to all potential failure conditions, not just the specific 
failure conditions noted by the commenters. Additionally, the failure 
conditions where a combined checklist might be useful were evaluated by 
multiple flightcrews, resulting in a conclusion by the FAA that, 
primarily due to the new MCAS required by this AD, the order and 
content in which these two checklists were accomplished is not critical 
to continued safe flight and landing.
    The FAA made minor changes to the procedures that were proposed in 
the NPRM. The changes simplify and

[[Page 74569]]

correct grammatical and typographical errors in, the Airspeed 
Unreliable non-normal checklist (figure 2 to paragraph (h)(3) of this 
AD) as follows:
     Removed the words ``using performance tables from an 
approved source,'' which contradicted the next sentence.
     Corrected a typographical error to specify actions if the 
``captain's and first officer's altitude indications are both 
unreliable'' instead of the proposed ``captain's or first officer's 
altitude indications are both unreliable.''
     Revised a note to correct a typographical error; the 
corrected text refers to ``DA/MDA,'' while the previous text referred 
to ``DH/MDA,'' and revised the last sentence for clarity.
     Revised a sentence to specify that the pitch bar may 
``automatically'' be removed, thus clarifying that removal does not 
require pilot action.
     Revised a sentence to specify ``An AFDS pitch mode'' 
instead of ``Selection of an AFDS pitch mode.''
     Added a note to specify ``only use flight director 
guidance on the reliable PFD.''
    The FAA also revised the ALT Disagree non-normal checklist (figure 
8 to paragraph (h)(9) of this AD) to correct a typographical error in 
the proposed AD. The corrected text refers to ``DA/MDA,'' while the 
proposed text referred to ``DH/MDA.''
    To the extent that ALPA suggests the addendum contained 
insufficient information to provide a meaningful comment, the FAA notes 
that the addendum identified the areas of potential checklist 
refinement and the reasons why refinement may be necessary. The JOEB's 
operational evaluation of the proposed checklists generated potential 
refinements that did not result in any substantive change to the 
checklists proposed in the NPRM. Rather, the results of the evaluation 
indicated that minor revisions to the unreliable airspeed checklist, 
which are reflected in this AD, may be appropriate. As such, there was 
no need for the FAA to publish the ``final checklist'' with the 737 FSB 
Report. However, because the FAA was aware that additional information 
obtained during the operational evaluation could have an impact on the 
final checklists, it provided notice of the findings in an addendum to 
the 737 FSB Report and sought comment from the public. The FAA finds 
that the addendum provided sufficient information for commenters to 
assess the potential revisions and offer alternatives to the proposed 
checklist to address the concerns suggested by the operational 
evaluation.
2. Comments Regarding Crew Procedure To Disable Stick Shaker
    Comment summary: Several commenters, including the Allied Pilots 
Association, ALPA, BALPA, Ethiopian Airlines Group, and the UAE GCAA, 
expressed concerns regarding the attention-getting nature of the stick 
shaker and requested a change to the procedures to include a means to 
suppress an erroneous stick shaker, including procedures to pull the 
associated stick shaker circuit breaker. In contrast, a commenter 
expressed a concern with the possible safety risks of including a 
procedure to pull the stick shaker circuit breaker in order to silence 
the warning.
    FAA response: The FAA infers that the commenters are suggesting 
there is an unacceptably high flightcrew workload when stick shaker is 
activated erroneously. The 737 stall warning/stick shaker is, by 
design, attention getting and can be a distraction during an 
erroneously high-AOA event. However, after careful evaluation, the FAA 
has not changed the AFM non-normal procedure to include pulling the 
stick shaker circuit breakers in this final rule, for the following 
reasons.
    The FAA evaluated all failure conditions of the new FCC software as 
part of certification of the proposed system changes. The new FCC 
software removes the potential for repeated, uncommanded MCAS inputs in 
the presence of an erroneous high AOA sensor input. This new design 
therefore removes the most significant contributor to unacceptably high 
flightcrew workload. With the new FCC software on the 737 MAX, the FAA 
tested and assessed all remaining flight deck effects, including 
erroneous stick shaker, during all foreseeable failure conditions, 
including high-AOA sensor failures during the most critical phases of 
flight (such as during takeoff or go-around). With the remaining flight 
deck effects and associated crew workload, these failures and effects 
were found compliant and safe.
    The FAA considered the commenters' concerns that an erroneous stick 
shaker may pose a distraction for the crew, and evaluated that scenario 
with procedures that include steps to silence an erroneous stick shaker 
stall warning via a circuit breaker pull. The FAA finds that an 
erroneous stick shaker, while it may pose a distraction to the 
flightcrew, does not affect controllability of the airplane. The stick 
shaker circuit breaker locations also do not meet FAA requirements for 
convenient operation for emergency controls for the complete range of 
pilots from their normal seated position in the flight deck, leading to 
possible distraction from their primary duties to safely control and 
monitor the aircraft. Furthermore, inclusion of these additional steps 
would add cognitive and physical workload to an already substantial 
Airspeed Unreliable non-normal procedure, and errors in locating and 
pulling the correct circuit breaker may lead to other airplane hazards. 
Balancing the concerns associated with adding a procedure to pull 
circuit breakers against the distraction of an erroneous stick shaker, 
the FAA has concluded that the design is compliant and safe, and 
therefore no change to the proposed non-normal procedures related to 
silencing the 737 MAX stall warning is required for this AD.
3. Comments Regarding Changes Associated With Crew Procedures
    Comment summary: The FAA received comments from A4A, JEMOG, Air 
China, Ameco, and several other commenters regarding the new AFM non-
normal procedures that were primarily administrative in nature rather 
than specific recommended changes. A commenter recommended referring to 
the AFM non-normal procedures as ``updates'' versus ``new'' as stated 
in the NPRM. Another commenter stated that the proposed new non-normal 
procedures were different and more complicated than previous Boeing 
Model 737 non-normal procedures. Another commenter disagreed with the 
FAA's proposed allowance to insert the figures containing the non-
normal procedures directly into the AFM. A4A expressed concern with the 
memory items in the proposed AFM non-normal procedures and use of Quick 
Reference Cards (QRCs) by some operators. Finally, a commenter 
requested that the FAA assess the proposed procedures in light of one 
pilot instead of a crew of two.
    FAA response: While it is true that some of these non-normal 
procedures can be viewed as updates to existing procedures, such as 
those in the operator's Quick Reference Handbook, this AD addresses AFM 
non-normal procedures that are part of the required type design change 
to the 737 MAX. The FAA is mandating removal of old, and replacement 
with new, AFM non-normal procedures. These AFM changes will result in 
corresponding changes to flightcrew training and operations materials 
including applicable Quick Reference Handbook Non-Normal Checklists 
such that they reflect these new AFM procedures.
    Regarding the comment about the added complexity in the new AFM 
non-normal procedures compared to

[[Page 74570]]

previous Boeing Model 737 procedures, as previously noted the AFM 
procedures specified in the proposed AD were thoroughly vetted by the 
FAA and others, as previously described in the ``Related Actions'' 
section. The AFM procedures are required by this AD as part of the 737 
MAX design changes; their complexity has been reduced during the FAA's 
certification activity, and they have been validated by the FSB during 
the JOEB evaluation.
    To facilitate immediate incorporation of new AFM non-normal 
procedures, the FAA allows for copies of the figures to be inserted 
directly into the existing AFM if needed. That provision is specified 
in paragraph (h) of this AD. The FAA agrees that revised AFMs should be 
provided to operators, and the FAA expects those revisions will be 
available from Boeing following issuance of this final rule.
    The FAA did not assess use of QRCs, which are operator specific. 
Should an operator wish to use QRCs that deviate from the AFM 
procedures specified in paragraph (h) of this AD, the operator must 
coordinate with its principal inspector or responsible Flight Standards 
Office and submit a request for an alternative method of compliance 
(AMOC) to the requirements of this AD.
    Finally, while most tasks in the flight deck could be accomplished 
by a single pilot, the FAA notes that the 737 MAX is certified with two 
pilots as the minimum crew, in accordance with 14 CFR 25.1523.
    No change to this AD is necessary based on these comments.
4. Comments Regarding Disabling Elevator Feel Shift
    Comment summary: A commenter requested that the flight control 
system disable differential feel in the event it is triggered falsely 
by an erroneous high AOA condition.
    FAA response: The FAA infers the commenter is referring to the 
Elevator Feel Shift (EFS), which is associated with identification of a 
stall on 737 NG and 737 MAX airplanes based on AOA sensor data. 
Although both MCAS and EFS use AOA data, only MCAS can move the 
horizontal stabilizer. The EFS changes control column feel force, but 
does not use the horizontal stabilizer trim system to initiate the 
changed feel force. This comment is unrelated to MCAS and the unsafe 
condition addressed by this AD. The FAA considered this system during 
the analysis, flight testing, and human factors assessments performed 
prior to approval of the new MCAS implemented by the FCC software 
required by paragraph (g) of this AD. No change to this AD is necessary 
based on this comment.
5. Comments Regarding Timeliness of Flightcrew Procedures
    Comment summary: Boeing recommended that the FAA revise a sentence 
in the sixth paragraph of the Proposed Design Changes section of the 
NPRM to clarify the use of ``timeliness'' as it relates to the 
flightcrew performing a non-normal procedure. Boeing stated that there 
is an element of timeliness expected in flightcrew responses to all 
non-normal events.
    FAA response: The FAA intentionally referred to the ``timeliness'' 
of the flightcrew performing a non-normal procedure in the proposed AD. 
The 737 MAX flight control design at the time of the Lion Air and 
Ethiopian accidents relied on pilot use of secondary flight controls 
(i.e., the electric trim switches) in a particular way (large 
continuous commands versus several short duration commands) or use of 
the Runaway Stabilizer non-normal crew procedure (using aisle stand 
cutout switches or grasping the manual trim control wheel), in a 
relatively short amount of time, for certain failure conditions 
(erroneous MCAS command) to retain aircraft control and ensure 
continued safe flight and landing. Control of the airplane during this 
failure scenario depended on these timely crew actions. With the new 
MCAS implemented by the FCC software required by this AD, basic control 
of the airplane is ensured for all potential failure conditions through 
the use of only the primary flight controls (i.e., control column), 
without the need for particular and timely pilot reactions on non-
primary controls. Therefore, the FAA has determined that no change to 
this AD is warranted.

G. Suggestions Regarding Monitors/Maintenance/Operations

1. Comments Regarding AOA Sensor Checks and Monitoring
    Comment summary: Several commenters offered input regarding 
suggested additional checks and monitoring of the AOA sensors, 
including doing a visual inspection before flight, continuously 
monitoring the AOA sensor electrical circuits, comparing AOA sensor 
values before flight, and continuously monitoring them throughout the 
flight. The commenters asked whether the monitors can detect damage 
(e.g., damage that occurs while at the gate) to an AOA sensor while on 
the ground. The commenters noted that the NPRM did not mention ground 
operations actions regarding vulnerable AOA vanes. The commenters 
requested expansion of the one-time AOA sensor system test (required by 
paragraph (l) of this AD) to a regularly scheduled repetitive action 
(not just one time before the airplane is returned to service).
    FAA response: The vane-style AOA sensor used on the 737 MAX is a 
common instrument installed on many transport airplanes. The existing 
preflight walk-around inspection of the airplane includes a visual 
check of the condition of the AOA sensors. These AOA sensors include 
electrical circuits that measure the angle of the sensor. The position-
sensing electrical circuits are continuously monitored and can detect 
if an electrical circuit is compromised. The AOA sensors also include 
electrical heaters in the body of the sensor and within the vane that 
aligns with local airflow and rotates within the sensor as AOA changes. 
The electrical current to the AOA heaters is monitored to detect a 
heater failure. The left and right AOA sensor values are not compared 
before flight because AOA sensors can be moved by winds. The left and 
right AOA sensor values are compared during flight and before the data 
is used by MCAS. If the difference between them is more than 5.5 
degrees, MCAS will be disabled. If an AOA sensor is damaged while at 
the gate, the typical damage would be a bent or broken vane. This 
damage could be detected during the preflight inspection. If the heater 
circuit is damaged, the heater failure will be annunciated. If a vane 
is bent only a small amount, there may be small differences between the 
captain's and first officer's altitude and airspeed indications. 
Paragraph (l) of this AD requires a one-time check of the AOA sensors 
to verify that the AOA sensors are calibrated correctly and the AOA 
heaters are working properly. Scheduled checks of the AOA sensors are 
not necessary due to the preflight inspections, the continuous circuit 
monitors, and the pilots' use of altitude and airspeed data affected by 
the AOA sensors.
2. Comments Regarding AOA Sensor Calibration and Testing
    Comment summary: A commenter requested improved calibration and 
testing of critical AOA sensors.
    FAA response: The Collins Aerospace Component Maintenance Manual 
(CMM) that is used for calibrating the 737 MAX AOA sensors as they are 
assembled has been updated with a new final check to verify that the 
AOA sensor has been calibrated correctly. This new check uses a simple 
independent electrical test that will

[[Page 74571]]

detect whether the more sophisticated calibration equipment was 
configured and used correctly. The AOA sensor is tested on the airplane 
using the AOA sensor system test in the AMM. This test is specified in 
Boeing Special Attention Service Bulletin 737-00-1028, dated July 20, 
2020, which is required by paragraph (l) of this AD. The test is 
required to ensure that all 737 MAX AOA sensors are properly calibrated 
and the heaters are operational prior to return to service. Therefore 
no change to this AD is necessary based on this comment.
3. Comments Regarding Discerning AOA Sensor Failures
    Comment summary: The Turkish DGCA, Ethiopian Airlines Group, and 
other commenters proposed to integrate information from the various AOA 
sensor electrical circuits and other data available on the airplane to 
establish when there is an AOA sensor failure and when data from the 
AOA sensor should not be used. Data from the Ethiopian Airlines Flight 
302 accident shows a detected AOA heater failure coincident with the 
sensed AOA transitioning rapidly to a large AOA value.\6\ The 
commenters also noted that with the failure of the AOA sensor heater, 
the AOA sensor is more vulnerable to icing and consequently could 
provide unreliable AOA output values. Proposed scenarios that would 
cause AOA sensor data to be disregarded include the following: Heater 
failure, heater failure combined with a rapid change in the AOA sensor 
position to a position consistent with vane departure, AOA disagree at 
90 knots during takeoff, unreasonable AOA for flight conditions, and an 
AOA that disagrees with the estimated (synthetic) AOA.
---------------------------------------------------------------------------

    \6\ Figure 56, ``AOA Values During the Beginning of the 
Flight,'' of Report No. AI 01/19, ``Interim Investigation Report on 
Accident to the B737-8 (MAX) Registered ET-AVJ operated by Ethiopian 
Airlines on 10 March 2019,'' dated March 9, 2020, of the Federal 
Democratic Republic of Ethiopia Ministry of Transport Aircraft 
Accident Investigation Bureau.
---------------------------------------------------------------------------

    FAA response: FAA regulations do not require the integrated failure 
detection capability requested by the commenters, and the 737 MAX air 
data system does not include this capability. The FAA has determined 
that no change to this AD is necessary because heater failures are 
annunciated, and the Unreliable Airspeed NNC provides guidance for 
pilots to establish whether there is reliable available data.
4. Comments Regarding Use of Erroneous AOA Sensor Data
    Comment summary: A commenter noted that it would be preferable to 
suppress the effects of a faulty AOA sensor by declaring it failed and 
disregarding it.
    FAA response: The unsafe condition identified in this AD is 
addressed by the required actions, including installation of the new 
FCC software (with the new MCAS) which compares AOA sensor data 
supplied to it. The actions required by this AD do not change the 
existing 737 MAX air data system, which includes monitoring and 
determination of AOA sensor failures, which was certificated without 
the capability suggested by the commenter.
5. Comments Regarding Use of STAB OUT OF TRIM Light
    Comment summary: Several commenters, including ALPA and the UAE 
GCAA, had questions and concerns regarding the STAB OUT OF TRIM light 
function and use. The commenters noted the new use of the light to 
annunciate FCC failures, and had questions about where the light is 
located, when the light would be illuminated, whether pilots would see 
it, and whether depressing the RECALL button would be required. Other 
commenters were concerned that a light with a dual meaning could lead 
to what they referred to as a ``Helios'' type of event, and therefore 
there should be a new separate light.
    FAA response: On the 737 MAX, there is one STAB OUT OF TRIM light 
located on the captain's forward instrument panel above the inboard 
display. Per figure 6 to paragraph (h)(7) of this AD, on the ground the 
light will illuminate if there is a partial failure of an FCC. In 
flight, the light will illuminate if the autopilot does not set the 
stabilizer trim correctly. Dispatch is prohibited when the STAB OUT OF 
TRIM light is illuminated while on the ground. With electrical power 
on, for certain failures of an FCC, the light will be illuminated 
continuously, such that no recall action is required of the pilot to 
have the light annunciate a fault. The light is in a location that is 
visible by both pilots.
    The FAA infers that the commenter's reference to Helios is 
regarding the Helios Airways Flight 522 accident on August 14, 2005,\7\ 
related to confusion with a single flight deck warning used for a dual 
purpose. On that 737-300 airplane, a single warning served to 
annunciate two different, unrelated issues: Takeoff configuration 
warning and cabin altitude warning, with two associated distinct 
flightcrew procedures. The function of the STAB OUT OF TRIM light 
implemented by this AD (it is in the FCC software) is associated with 
only one flightcrew procedure (the Stabilizer Out of Trim NNC required 
by this AD). Per that procedure, if the light is illuminated on the 
ground the flightcrew is directed to not takeoff. Therefore, a new 
separate light is not required. No change to this AD is necessary based 
on these comments.
---------------------------------------------------------------------------

    \7\ Hellenic Republic Ministry of Transport & Communications Air 
Accident Investigation & Aviation Safety Board (AAIASB) Helios 
Airways Flight HCY522 Aircraft Accident Report, dated November 2006 
(https://data.ntsb.gov/Docket/?NTSBNumber=DCA05RA092).
---------------------------------------------------------------------------

6. Comments Regarding Periodic Testing of MCAS
    Comment summary: A commenter suggested that MCAS have either an 
automatic or a manual self-test that could be tied to the stall warning 
system test.
    FAA response: Based on the suggestion to tie a self-test to the 
stall warning system test, the FAA infers that the commenter is 
suggesting that this test be conducted every day. Frequent testing of 
MCAS is not required to comply with FAA reliability requirements (14 
CFR 25.1309). Even though MCAS is intended only for use during non-
normal flight conditions, the elements of the air data and flight 
controls system associated with MCAS are used during every flight and 
are continuously monitored. These include AOA sensors and associated 
wiring, ADIRUs, databuses, FCCs, and FCC-generated stabilizer trim 
commands, such as STS commands or autopilot commands. An existing CMR 
(22-CMR-01 in the Boeing MPD) does an operational check of speed trim 
and stabilizer trim discrete associated with the FCC computers. 
Certification of the new MCAS required implementing a new CMR (22-CMR-
02), which requires periodic testing to verify proper functioning of 
the stabilizer trim enable ground path and autopilot arm cutout switch. 
In summary, while MCAS is not explicitly tested each flight, any 
problem with AOA, ADIRU, FCC, software, etc., will be evidenced 
immediately by existing monitors and alerts to be resolved by 
maintenance prior to subsequent dispatch, and therefore does not need 
to be tested. The FAA has not changed this AD based on this comment.
7. Comments Regarding Maintenance of MCAS
    Comment summary: A commenter noted that there is little mention of

[[Page 74572]]

maintenance in the NPRM. Another commenter asked whether dispatch is 
prohibited after MCAS failure. Another commenter inquired about 
procedures for recording, diagnosing, and repairing the system before 
another flight.
    FAA response: Design changes mandated via an AD often have new or 
revised maintenance documents associated with them.
    All of these 737 MAX maintenance-related documents have been 
revised:

 Boeing 737 Fault Isolation Manual (FIM)
 Boeing 737 Aircraft Maintenance Manual (AMM)
 Boeing 737 Maintenance Planning Document (MPD)
 FAA Maintenance Review Board Report
 FAA Master Minimum Equipment List (MMEL) (referenced in 
paragraph (i) of this AD)
 Collins Aerospace Component Maintenance Manual (CMM) for AOA 
Sensor

    This AD requires accomplishment of certain Boeing service bulletins 
that reference sections of the AMM. Paragraph (i) of this AD requires 
actions related to the MMEL. The FAA has released a maintenance Safety 
Alert for Operators (SAFO), SAFO 20015, Boeing 737-8 and 737-9 
Airplanes: Return to Service,\8\ that identifies related documents.
---------------------------------------------------------------------------

    \8\ SAFO 20015 is available at  https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/.
---------------------------------------------------------------------------

    U.S. airlines must have an approved maintenance program as a 
condition of their approval to operate in the U.S. In response to the 
comment pertaining to operation after MCAS failure, the MMEL does not 
allow dispatch of the airplane with failure of the STS, which includes 
MCAS. Maintenance will utilize the FIM and AMM to assess the system, 
isolate the fault, resolve the issue, and then return the airplane to 
service.
    For shop repair of AOA sensors, the Collins Aerospace CMM was 
updated to add a final check using different equipment to ensure the 
sensor was not mis-calibrated.
    For scheduled periodic maintenance, two new tasks are included in 
the FAA's Maintenance Review Board Report and in the Boeing MPD. The 
first is Item 22-011-00 in the Boeing MPD, which is an operational 
check of the MCAS discrete to verify the integrity of MCAS. The other 
new task is Item 22-030-00 in the Boeing MPD, which is also a CMR (22-
CMR-02) that operationally checks the stabilizer trim enable ground 
path and autopilot arm cutout switch.
    Boeing notified 737 MAX operators that these documents were revised 
and published via customary communication methods. U.S. part 121 and 
part 135 operators must use current CMRs per their OPS SPECS D072 
Aircraft Maintenance--Continuous Airworthiness Maintenance Program 
(CAMP) Authorization. Continued eligibility for a CAMP authorization 
depends on the operator incorporating MPD revisions (which include 
CMRs) into their maintenance programs.
8. Comments Regarding Oversight of Maintenance Program
    Comment summary: A commenter asked who and what documents and/or 
procedure ensures that the maintenance program is enforced.
    FAA response: For airplanes registered in the United States, 
operators must have an approved maintenance program and must adhere to 
it. The FAA oversees U.S. operators. Foreign operators are regulated 
and overseen by the civil aviation authority of their country.
9. Comments Regarding Redundancy in the Master Minimum Equipment List
    Comment summary: A commenter noted that figure 10 to paragraph (i) 
of the proposed AD contained redundant information. The commenter 
stated that within figure 10 to paragraph (i) of the proposed AD, both 
step (2) and step (8) specify that the autopilot disengage aural 
warning system must be operating normally for dispatch. The commenter 
added that item 22-10-02 (which is discussed in note 2 to paragraph (i) 
of the proposed AD; now note 3 to paragraph (i) of this AD) was deleted 
in revision 2 of the MMEL.
    FAA response: The FAA agrees that the items mentioned are 
redundant. However, this redundancy does not affect compliance with the 
AD. In addition, this redundancy will be addressed in the next revision 
of the MMEL. No change to this AD is necessary based on this comment.
10. Comments Regarding Inclusion of AOA Sensors in MMEL
    Comment summary: A commenter asked if the AOA sensors and MCAS are 
in the MEL. The commenter stated that if the AOA and MCAS are 
essential, then they must be included in the MEL so that pilots cannot 
take off if the AOA sensor or the connection between the AOA and MCAS 
is degraded or failed.
    FAA response: The FAA infers that the commenter is asking that the 
AOA sensors and MCAS be excluded from the MMEL, meaning that the 
equipment must be operative for dispatch. On April 10, 2020, the FAA 
published the FAA-approved Boeing 737 MAX B-737-8/-9 MMEL, Revision 2, 
after public notice and opportunity for comment. The 737 MAX MMEL does 
not allow dispatch with the STS (which includes MCAS) inoperative, and 
it does not allow dispatch with the position sensing circuit in an AOA 
sensor inoperative. The monitoring that would prevent this dispatch 
would also detect a failure in the communication between the AOA 
sensors and the MCAS function in the FCCs. The MMEL, which includes AOA 
sensor heaters, allows for limited dispatch with inoperative AOA 
heaters, provided the airplane is not operated in known or forecast 
icing conditions. No change to this AD is necessary based on this 
comment.
11. Comments Regarding Inclusion of AOA Sensor Heaters in MMEL
    Comment summary: The UAE GCAA noted that currently ``AOA heating 
system, flight control system, and AP/YD'' are MMEL ``go'' items in 
most cases, except for long-range operations and in-icing conditions. 
The UAE GCAA noted that it is sometimes difficult for flightcrews to 
avoid icing in some flight conditions. The UAE GCAA asked that the FAA 
and Boeing make these items ``no go'' in the MMEL.
    FAA response: As previously noted, the FAA approved revisions to 
the MMEL that removed provisions for dispatch related to MCAS failures. 
The MMEL continues to include provisions for limited dispatch for other 
unrelated degradation of the flight control system, the autopilot, and 
yaw damper. Regarding the AOA heating system, no changes are required 
for MMEL item 30-31-02. The MMEL currently states that the AOA sensor 
heaters may be inoperative, provided the aircraft is not operated in 
known or forecast icing conditions. However, if icing conditions are 
encountered, the potential effects due to unheated vanes, including to 
air data and to MCAS, do not rise to a hazardous level.
12. Comments Regarding Typographical Error in Note 2 to Paragraph (i) 
of the Proposed AD
    Comment summary: A4A stated that note 2 to paragraph (i) of the 
proposed AD incorrectly refers to MMEL item 22-11-06-2B instead of MMEL 
item 22-11-06-02B.
    FAA response: The FAA concurs and has revised this note, now note 3 
to paragraph (i) of this AD, to refer to MMEL item 22-11-06-02B.

[[Page 74573]]

13. Comments Regarding Removal of Note in Item (4) Within Figure 10 to 
Paragraph (i) of the Proposed AD
    Comment summary: A4A stated that the FAA should correct conflicts 
between the NPRM and policies regarding MEL items pertaining to several 
aspects of the flight control system (FCS). A4A noted that figure 10 to 
paragraph (i) of the proposed AD contains a note under item (4) stating 
that both FCCs must be operative to dispatch. A4A explained that there 
are several FCC functions that will continue to have MMEL deferral 
relief, as specified in figure 10 to paragraph (i) of the proposed AD 
and Revision 2 of the MMEL. A4A added that the item (4) statement in 
figure 10 to paragraph (i) of the proposed AD (which states that speed 
trim function must be operative for dispatch), combined with the 
deletion of the Speed Trim deferral allowance from Revision 2 of the 
MMEL, provides a clear indication that Speed Trim must operate normally 
for dispatch. For these reasons, A4A recommended that the note be 
removed.
    FAA response: The FAA has removed the note identified in the A4A 
comment. The intent of the note was to emphasize that FCC deactivation 
is no longer permitted; this deactivation was associated with Speed 
Trim Function relief in previous MMEL revisions. This deactivation came 
as part of a required maintenance procedure supported by Boeing in the 
Dispatch Deviation Guide (DDG). The FAA acknowledges that the note is 
unnecessary, and the revised MMEL itself addresses the condition 
specified in the note. For these reasons, the FAA has revised this AD 
to remove the note that was under item (4) in figure 10 to paragraph 
(i) of the proposed AD.

H. Suggestions for Crew Reporting and Crew Procedures

1. Comments Regarding Crew Reporting of Irregularities
    Comment summary: A commenter stated that a procedure should exist 
mandating that every 737 MAX operator inform Boeing, the FAA, and local 
authorities when any stall warning activation, airspeed disagree alert, 
altitude disagree alert, or AOA disagree alert occurs in normal 
operation (excluding test flights or readiness flights).
    FAA response: For U.S. operators, 14 CFR 121.563 requires the pilot 
in command to ensure all mechanical irregularities occurring during 
flight time are entered into the maintenance log of the airplane at the 
end of that flight time. 14 CFR 121.533, 121.535, and 121.537 also 
place responsibility for operational control with the operator and 
require operators to exercise operational control through approved or 
accepted procedures that lead to the safe dispatch and operation of a 
flight. Operators may also provide additional reporting and/or data 
collection such as irregularity reports, Aviation Safety Action Program 
reports, flight operational quality assurance data, or ad-hoc data 
collection from flight data recorders or from aircraft communicating 
and reporting system (ACARS) as part of their operational control 
system. 14 CFR 121.703 requires reporting of emergency actions during 
flight, such as stick shaker activations. The FAA has not changed this 
final rule regarding this issue.
2. Comments Regarding Consistency of 737 MAX and 737 NG AFM Procedures
    Comment summary: The BALPA questioned whether applicable procedure 
changes from the 737 MAX AFM would be applied to the Boeing 737 NG AFM 
to avoid confusion if pilots serve in both the Boeing 737 MAX and the 
Boeing 737 NG.
    FAA response: The FAA expects Boeing will update the eight non-
normal procedures included in this final rule in the Boeing 737 NG AFM. 
The FAA is considering mandating these 737 NG AFM changes by a separate 
AD rulemaking action. Additionally, the new special emphasis areas \9\ 
described in section 9.2 of the 737 FSB Report, also apply to the 
Boeing 737 NG. Therefore, pilots serving in mixed fleet operations of 
the Boeing 737 MAX and the Boeing 737 NG will have consistent 
procedures and training in both airplanes. The FAA has not changed this 
final rule regarding this issue.
---------------------------------------------------------------------------

    \9\ 737 FSB Report, paragraph 6.11, defines a ``special emphasis 
area'' as ``A training requirement unique to the aircraft, based on 
a system, procedure, or maneuver, which requires additional 
highlighting during training. It may also require additional 
training time, specialized FSTD, or training equipment.''
---------------------------------------------------------------------------

3. Comments Regarding Flight Crew Operations Manual Content
    Comment summary: The Turkish DGCA commented that a comprehensive 
description of the flight director bias out of view needed to be 
included ``in FCOM'' (the FAA infers the commenter is referring to a 
Flight Crew Operations Manual) to ensure pilots will understand that 
manual flight is necessary. Another commenter stated that the ``MAX 
system'' (which the FAA infers means MCAS) must be included in the 
pilot's manual.
    FAA response: The information requested by the commenters is in the 
AFM. In addition, the FAA has confirmed that Boeing will include the 
information requested by the commenter in the FCOM (which is not 
mandated by this AD) after publication of this AD.

I. Comments Related to Pilot Training and the Use of Simulators for 
Pilot Training

    The FAA received several comments to the NPRM docket related to 
pilot training and certification and the qualification and use of 
simulators for pilot training. The FAA appreciates this input and, 
where appropriate, considered the information in other related actions 
(e.g., finalizing the 737 FSB Report). Although the comments are beyond 
the scope of this rule, the FAA provides the following responses.
1. Comments Regarding Simulator Training
    Comment summary: Several commenters, including Flyers Rights, ALPA, 
and the Turkish DGCA, stated that the FAA must require simulator 
training for pilots operating the Boeing 737 MAX including training on 
specific areas.\10\ Two commenters also recommended that the FAA 
address perceived deficiencies in 737 MAX simulators related to 
accurate representations of the force required by pilots to turn the 
pitch trim wheel manually.
---------------------------------------------------------------------------

    \10\ Commenters suggested the following areas be included in 
simulator training: Stall recovery, flight displays, what to do if 
the AOA disagree light illuminates, maneuvers with the AOA sensor 
failed, training that mimics the forces needed by pilots, 
intricacies of the manual trim wheel and how to implement two-pilot 
intervention, autopilot disconnect and flight director bias out of 
view, dependencies between MCAS and the other aircraft systems, and 
differences in behavior when MCAS is operational versus when MCAS 
has failed. Another commenter also noted that computer-based 
training (CBT) should include the AOA disagree warning system and 
the instrument panel gauges.
---------------------------------------------------------------------------

    FAA response: As noted, this AD does not mandate pilot training. 
However, consistent with the results of the JOEB operational evaluation 
and in accordance with 14 CFR 121.405(e), the FAA is requiring air 
carriers to revise all Boeing 737 MAX training curricula to include the 
special training as described in the 737 FSB Report. This special 
training includes training on all of the areas identified by the 
commenters, including the use of manual stabilizer trim in an FFS. The 
FAA has taken steps to verify that, in accordance with 14 CFR 60.11(d), 
flight simulation training device (FSTD) sponsors have evaluated the 
manual stabilizer trim system for proper control forces and travel on 
each

[[Page 74574]]

FAA-qualified Boeing 737 MAX FFS. If the forces do not meet the 
specified requirements of 14 CFR part 60, Appendix A, the FSTD sponsor 
must not allow use of the FFS to conduct training on the manual 
stabilizer trim wheel.
    The FAA recommends that commenters review the 737 FSB Report and 
SAFO 20014, Boeing 737-8 and 737-9 Airplanes: Pilot Training and Flight 
Simulation Training Devices (FSTDs) Updates for more information on air 
carrier pilot training requirements for the MAX.\11\
---------------------------------------------------------------------------

    \11\ The 737 FSB Report is available at https://fsims.faa.gov/PICResults.aspx?mode=Publication&doctype=FSBReports; and SAFO 20014 
is available at https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/
---------------------------------------------------------------------------

2. Comments Regarding New Pilot Type Rating
    Comment summary: Some commenters suggested that the FAA establish a 
new type rating for the Boeing 737 MAX because, according to the 
commenters, the 737 MAX behaves differently than the Boeing 737 Next 
Generation (NG), and differences training is not adequate to address 
the changes in the 737 MAX from the previous series. Commenters 
suggested that a new type rating would ensure that 737 MAX pilots are 
properly trained especially in abnormal and emergency situations. The 
UAE GCAA raised concerns regarding a mixed fleet consisting of both the 
Boeing 737 MAX and the Boeing 737 NG, suggesting that the FAA needed to 
examine the impact of mixed fleet operations on crew training.
    FAA response: The FAA establishes type ratings through an 
operational evaluation of an aircraft conducted by a Flight 
Standardization Board. The same process determines the differences 
training required for a variation of the aircraft type (e.g., a new 
series). For each new series of Boeing Model 737 airplanes, the FAA 
conducted the described evaluation and determined that the same pilot 
type rating applies to all Boeing Model 737 airplanes. The FAA finds 
that this evaluation process has properly determined that the Boeing 
737 type rating is appropriate for the 737 MAX. However, in accordance 
with 14 CFR 121.400(c)(5), differences training is required for air 
carrier pilots to serve on a new series of the Boeing 737. As outlined 
in the 737 FSB Report, the differences training from the Boeing 737 NG 
to the 737 MAX includes ground and flight training on abnormal and 
emergency situations.
    Regarding concerns about mixed fleets, the FAA notes that the new 
special emphasis areas described in section 9.2 of the 737 FSB Report 
also apply to the Boeing 737 NG. Therefore, pilots serving in mixed 
fleet operations of the Boeing 737 MAX and the Boeing 737 NG will have 
consistent training in both airplanes. The FAA refers commenters to the 
737 FSB Report for further information specific to this issue.
3. Comments Regarding Manual Flying Proficiency
    Comment summary: Several commenters asserted that pilots have an 
over-reliance on automation and need training on manual flying skills 
to ensure proficiency.
    FAA response: Although these comments are not within the scope of 
the proposed rule, the FAA notes that air carrier pilots are required 
to demonstrate and maintain proficiency of manual flying skills.\12\ 
The FAA's commitment to ensuring manual flying proficiency is evident 
in its publication of several advisory circulars (ACs) and SAFOs 
related to this topic.\13\
---------------------------------------------------------------------------

    \12\ See 14 CFR 121.423, 121.424, 121.427, 121.441, and part 121 
Appendices E and F.
    \13\ See AC 120-109A, Stall Prevention and Recovery Training; AC 
120-111, Upset Prevention and Recovery Training; AC 120-114, Pilot 
Training and Checking (14 CFR part 121, subparts N and O, including 
Appendices E and F); SAFO 13002 Manual Flight Operations; and SAFO 
17007 Manual Flight Operations Proficiency.
---------------------------------------------------------------------------

    The FAA continues to emphasize proficiency in manual flying skills 
for air carrier pilots by requiring 737 MAX special pilot training that 
focuses on manual trim operations, manual flight during MCAS 
demonstration at high angles of attack, and manual flight with an 
unreliable airspeed condition. The 737 MAX special training is 
described in Appendix 7 of the 737 FSB Report.
    In September 2019, the FAA presented a working paper at the 
International Civil Aviation Organization (ICAO) Assembly seeking the 
establishment of a new panel that would address pilot training and 
automation dependency. This panel would be an important step in 
understanding the scope of automation dependency globally and bring the 
international community together to work towards accepted solutions 
that could reduce the variability in how the issue is addressed by 
individual CAAs.
    With broad support for establishing a panel at the Assembly, the 
ICAO Air Navigation Commission approved the establishment of a new 
Personnel Training and Licensing Panel (PTLP) in June 2020. The U.S. 
has been named a member of this panel and the panel's work is 
anticipated to begin in early 2021. The FAA will continue to advocate 
for taking steps to address automation dependency, manual flight 
operations proficiency, and improving pilot management of automated 
systems globally. No change to this AD is necessary based on these 
comments.
4. Comments Regarding Inclusion of Low-Time Pilots in Operational 
Evaluation
    Comment summary: The UAE GCAA stated the operational evaluation 
should include low-time pilots with a commercial pilot license.
    FAA response: As previously described in the ``Related Actions'' 
section, the FAA completed the operational evaluation jointly with 
EASA, ANAC, and TCCA in September 2020. The operational evaluation of 
the 737 MAX with the new MCAS included pilots from multiple countries 
with varying levels of experience, including a low-time pilot with a 
commercial pilot license.

J. Requests for Clarification

    Several commenters sought additional information about operation 
and behavior of certain systems on the 737 MAX.
1. Comments Regarding Various AOA Thresholds
    Comment summary: Several commenters asked questions regarding the 
different thresholds used by the new FCC and MDS software when 
comparing AOA values. They asserted that use of different thresholds 
and different computers should be eliminated. They were concerned that 
different thresholds for the two monitors could cause confusion. They 
noted that if the difference in AOA values is between the two 
thresholds, MCAS would be disabled but the AOA DISAGREE annunciation 
would not take place.
    FAA response: The FAA provides the following clarification. At 
lower speeds (flaps extended), the acceptable difference between the 
left and right AOA values is larger. MCAS operates with flaps fully 
retracted (higher airspeeds), where the acceptable difference is 
smaller.
    Airplanes experience significantly different sideslip conditions 
during low-speed flight compared to high-speed flight, resulting in 
larger differences between left and right sensed AOA values at low 
airspeed when compared to high airspeed. It is therefore appropriate 
for MCAS, which operates only at high airspeeds (with the flaps 
retracted), to have a smaller acceptable

[[Page 74575]]

difference (tighter tolerance) than the AOA DISAGREE alert, which 
functions throughout the flight envelope (low and high airspeeds). With 
this tighter tolerance, MCAS will be disabled with the smaller 
difference between AOA sensor inputs; thus, preventing erroneous MCAS 
commands. No change to this AD is necessary based on these comments.
2. Comments Regarding MCAS Activation Prior to Stick Shaker
    Comment summary: Several commenters stated that the thresholds for 
MCAS activation and for stick shaker activation should ensure that 
stick shaker occurs after MCAS activation.
    FAA response: The AOA threshold associated with MCAS activation is 
less than the AOA threshold associated with stick shaker. Therefore, 
MCAS will activate prior to stick shaker.
3. Comments Regarding Function of Column Cutout Switches
    Comment summary: Several commenters stated that the NPRM did not 
explain the hardware and software modifications that provide new 
functionality for control column cutout. They stated that there are 
three conditions of control column cutout: Main electric stabilizer 
trim column cutout, FCC trim column cutout, and FCC trim software 
column cutout. They asked that the FAA explain the significant 
modification on the control column cutout as part of this AD.
    FAA response: The functionality of the column cutout switches is 
described in section 6 of the ``Preliminary Summary of the FAA's Review 
of the 737 MAX,'' dated August 3, 2020, which was included in the 
docket for this AD at the time of publication of the NPRM. At the base 
of the control column are column cutout switches. They inhibit 
stabilizer trim commands if the control column moves more than a few 
degrees in a direction opposite to the trim command. For example, if 
the stabilizer trim command is in the airplane nose-down direction and 
the pilot pulls the column aft to raise the nose of the airplane, then 
the column cutout switches will inhibit the command to the stabilizer. 
There are column cutout switches for commands initiated by the pilot 
using the thumb switches on the control wheels, and for commands 
initiated by the FCC for autopilot and speed trim commands. The new FCC 
software installed as required by paragraph (g) of this AD includes a 
redundant software equivalent of the physical switches that interrupt 
FCC commands. An FCC will not make a stabilizer command if the column 
position is more than a few degrees in the opposite direction of the 
pending stabilizer command. The exception occurs when there is an MCAS 
airplane nose-down command during high-AOA flight, when the pilot is 
typically pulling aft on the control column. During the short duration 
of an MCAS activation, the physical and software column cutouts will be 
temporarily bypassed to allow the MCAS command.
4. Comments Regarding Term Used in NPRM for Wiring Change
    Comment summary: A commenter suggested changing the description of 
wiring associated with the horizontal stabilizer trim system. The NPRM 
described one of the wires as ``arm'' wiring, and the commenter 
suggested that the wiring be referred to as ``power'' wiring.
    FAA response: The wiring nomenclature in the NPRM is consistent 
with that of the service information required by paragraph (k) of this 
AD. No change has been made to this AD based on this comment.
5. Comments Regarding Autopilot Engagement During Stick Shaker
    Comment summary: A commenter asked whether the autopilot can be 
engaged with the stick shaker active. The commenter noted that flight 
data recorder data from the ET302 flight shows that the autopilot was 
engaged while the stick shaker was active.
    FAA response: Flightcrew training informs pilots how to recover 
from a stall, which does not include engagement of the autopilot. In 
some cases, the autopilot can be engaged or remain engaged while a 
single stick shaker is active. For example, an AOA sensor failure 
(e.g., ET302 flight) can cause persistent erroneous stick shaker that 
would also affect airspeed and altitude displayed to one of the pilots. 
The Airspeed Unreliable procedure required by paragraph (h) of this AD 
directs flightcrews to disengage the autopilot, then later allows for 
autopilot engagement, but only after a reliable airspeed indication has 
been determined. No change has been made to this AD based on this 
comment.
6. Comments Regarding Retention of INOP Markers
    Comment summary: Several commenters questioned why the FAA proposed 
to mandate removing ``INOP'' markers as part of paragraph (j) of the 
proposed AD. They suggested that the INOP markers be retained as a 
backup or to draw the attention of the flightcrew.
    FAA response: The INOP markers are simply stickers that are 
covering one of the selectable positions of a dial on the electronic 
flight instrument system (EFIS) panel. After installation of the 
software required by paragraph (j) of this AD, a display setting that 
had been inoperative will be operative. Removal of the INOP marker will 
allow the flightcrew to select and use the now operative display 
setting. No change to this AD has been made based on these comments.
7. Comments Regarding Boeing Model 737 STS Failures
    Comment summary: Several commenters noted that the STS has been on 
Boeing Model 737 airplanes since the Boeing Model 737 Classic 
airplanes, implemented with a single FCC in control of the function. 
They stated that the STS has always been subject to the failure 
conditions that drove MCAS to require a dual FCC solution. They 
asserted that the STS has not failed to date, but seems vulnerable to a 
future failure. They asked whether there is a plan to address STS on 
prior models, or if the unhindered aft column cutout saves those 
airplanes from further hazards.
    FAA response: These comments do not pertain directly to the unsafe 
condition of the Boeing 737 MAX that this AD addresses, and therefore 
no change to this AD is required based on these comments. Relevant to 
these comments, however, the new FCC software installed on the 737 MAX 
includes a cross-FCC monitor that will detect and stop any erroneous 
FCC-generated stabilizer commands, including STS/MCAS commands. Earlier 
Boeing 737 models (pre-MAX) include full-time column cutout switches, 
which effectively protect against an erroneous stabilizer trim command. 
The pilot stops, or cuts out, the trim command by moving the control 
column to oppose the uncommanded trim input. Because of this design 
difference between the 737 MAX and earlier versions of the Boeing Model 
737, the FAA is not aware of any need to change earlier Boeing 737 
models in this respect.

K. Changed Product Rule/Regulations Allowance

    This section addresses comments regarding how the FAA certificates 
new and derivative aircraft, the overall configuration of the 737 MAX, 
whether it is appropriate to include systems like MCAS on airplanes, 
and specific comments suggesting changes to crew alerting and 
indication on the 737 MAX.

[[Page 74576]]

1. Comments Regarding Certification of Derivative Airplane Models
    Comment summary: Several commenters, including the Families of 
Ethiopian Airlines Flight 302 and NATCA, did not consider it 
appropriate that FAA regulations allowed for 737 MAX airplanes to be 
certificated as derivative airplanes of the older, existing Boeing 737 
Type Certificate. They highlighted that all Model 737 airplanes are 
included on the same type certificate. They stated that FAA regulations 
related to this practice should be amended to disallow this. A 
commenter suggested that type certificates should expire. Some 
commenters contended that FAA regulations allow for existing type 
certificates of older designs to be modernized excessively to avoid 
complying with new more restrictive requirements. They stated that 
every variation needs to be thoroughly reviewed as if it were new. They 
also stated that when certifying a derivative aircraft, standard 
improvements should be required, such as to include brake temperature 
gauges, to make upgrades to the airspeed system, and to introduce 
triple redundancy for critical systems. Lastly, they stated that the 
737 MAX airplane needs to be recertified with a new type certificate. 
Specific to the 737 MAX, they cited the new, larger engines installed 
on the old airframe, the age of stabilizer trim system, and the flight 
deck caution and warning system.
    FAA response: The comments recommend broader reforms to 14 CFR 
21.19 and 21.101 and associated guidance that address the criteria and 
process used by the FAA, and the other major civil aviation 
authorities, when assessing proposed changes to existing products. 
These comments do not pertain specifically to correcting the unsafe 
condition addressed in this AD. The corrective action mandated by this 
AD addresses the identified unsafe condition.
2. Comments Regarding Configuration of 737 MAX
    Comment summary: Several commenters, including the Families of 
Ethiopian Airlines Flight 302, Flyers Rights, and Aerospace Safety and 
Security, Inc., expressed fundamental concerns with the configuration 
of the 737 MAX. They stated that the design should be changed, and 
should not have been certificated originally. They cited the new, 
larger engines installed on the older airplane in a new location that 
is forward and higher, and potential associated impacts to 
aerodynamics, weight and balance, and pitch-up tendency. Redesign 
suggestions include the following: Reverting to using the old engines, 
replacing the engines with smaller engines, redesigning the nacelles so 
they do not generate lift, and increasing the height of the airplane by 
extending the landing gear.
    FAA response: The FAA does not prescribe particular designs, but 
rather assesses the regulatory compliance and safety of designs 
proposed by an applicant. In this case, the FAA certificated the 
configuration of the MAX with its current configuration of wing, 
engine, landing gear, nacelles, etc., with MCAS as part of the design. 
Since the initial certification of the MAX, an unsafe condition was 
identified and is addressed by the actions mandated by this AD. The FAA 
has determined that the resultant configuration, which includes the new 
MCAS, is compliant with the 14 CFR part 25 regulatory requirements and 
is safe.
3. Comments Regarding Inclusion of MCAS
    Comment summary: Several commenters, including the Families of 
Ethiopian Airlines Flight 302, stated that MCAS should not be retained 
on the airplane. Some asserted that FAA regulations do not (or, if they 
do, they should not) allow for inclusion of a stability augmentation 
system like MCAS on an airplane. They stated the airplane should be 
redesigned via an aerodynamic configuration change, as discussed 
previously, such that it is stable without MCAS, instead of relying on 
automation like MCAS to make it stable. They stated that if MCAS is 
installed, it would be unacceptable for the airplane to become unstable 
with MCAS inoperative. They questioned how much divergent pitch 
instability is permitted in commercial aircraft. They stated MCAS 
should be replaced with an elevator system solution to resolve a column 
force issue.
    FAA response: The FAA does not have a factual basis to mandate 
removing MCAS from the airplane and finds that the unsafe condition is 
appropriately addressed by the requirements of this AD. In addition, 
FAA regulations 14 CFR 25.21, 25.671, and 25.672 provide for inclusion 
of stability augmentation systems in showing compliance to those 
standards. Stability augmentation systems are common features included 
in the design of modern transport category airplanes. Subpart B of 14 
CFR part 25 requires transport airplanes to have stable pitch 
characteristics. The 737 MAX airplane is stable both with and without 
MCAS operating. This has been demonstrated on the MAX during FAA flight 
testing. Regarding the suggestion to revise the elevator system, the 
FAA does not prescribe design, but rather assesses proposed designs, 
and the FAA finds the new MCAS meets FAA safety standards.
4. Comments Regarding Crew Alerting System
    Comment summary: The Families of Ethiopian Airlines Flight 302 
suggested simplifying the Crew Alert System on the 737 MAX so that 
flightcrews are not overwhelmed by multiple warning systems. They 
asserted that due to provisions of 14 CFR 21.101, the 737 MAX does not 
fully comply with 14 CFR 25.1322 concerning flightcrew alerts. They 
asserted that an FAA rule (14 CFR 21.101) allows for determining that 
it would be ``impractical'' to comply with later amendments of 
regulations because the anticipated safety benefits do not justify the 
costs necessary to comply with later amendments. They asserted that the 
Boeing 737 MAX does not fully comply with 14 CFR 25.1322(b)(3), which 
requires advisory alerts ``for conditions that require flightcrew 
awareness and may require subsequent flightcrew response''; 14 CFR 
25.1322(c)(2), which mandates that warning and caution alerts ``must 
provide timely attention-getting cues through at least two different 
senses by a combination of aural, visual, or tactile indications''; and 
14 CFR 25.1322(d), which states that ``the alert function must be 
designed to minimize the effects of false and nuisance alerts.''
    Separately, NATCA recommended that all changes to the 737 MAX 
comply with the flightcrew alerting requirements in 14 CFR 25.1302 
amendment 25-137 and 25.1322 amendment 25-131. Specifically, NATCA 
contended that the exception to 14 CFR 25.1322(b)(2), (b)(3), (c)(2), 
(d)(1), and (d)(2) granted by the FAA for the 737 MAX should not be 
granted for the cockpit changes that would be implemented by the 
proposed AD.
    Finally, another commenter suggested conducting a holistic 
evaluation of flight deck human factors and crew alerting, at least 
ensuring all alerts comply with regulations, and reevaluate the 
exception to the crew alerting regulation, and to ideally require 
installation of an engine indication and crew alerting system (EICAS) 
on the 737 MAX.
    FAA response: The 737 MAX complies with 14 CFR 25.1322, as 
specified in that airplane's certification basis. The 737 MAX crew 
alerting system is not substantially changed

[[Page 74577]]

from the 737 NG crew alerting system, which has been shown through 
service history to be reliable and safe. The FAA has determined the 
existing certification basis for the 737 MAX airplane is appropriate 
for the design changes necessary to correct the identified unsafe 
condition.
    The FAA lacks a factual basis to require any changes (simplifying 
the crew alerting system or converting to EICAS) other than those 
proposed in the NPRM and mandated by this AD. The unsafe condition 
associated with this AD is related to MCAS and how it contributed to 
pilot workload. The changes mandated by this AD effectively address the 
unsafe condition.
    This AD includes two changes related to the crew alerting system. 
First, the MDS software change required by paragraph (j) of this AD 
implements the AOA DISAGREE alert that was certificated, but 
erroneously not implemented, during the initial certification of the 
737 MAX. The other change is implemented by the new FCC software 
required by paragraph (g) of this AD, which changes the conditions for 
which the existing SPEED TRIM FAIL and STAB OUT OF TRIM lights are 
illuminated. No change to this AD is necessary based on these comments.
5. Comments Regarding Autothrottle Indication
    Comment summary: NATCA asked the FAA to require design changes to 
the autothrottle indication to meet current certification regulations, 
which are 14 CFR 25.1329(k) at amendment 25-119 and 25.1322.
    NATCA stated that the Autothrottle Disconnect alert on the 737 MAX 
is a red flashing light with no aural component, which does not meet 
the standard alert definitions in 14 CFR 25.1322 and 25.1329(k).
    FAA response: This request is unrelated to the unsafe condition 
addressed by this AD. There are no changes to the autothrottle 
associated with this AD.

L. Certification Process

1. Comments Regarding Compliance and Certification Rigor of MCAS
    Comment summary: Some commenters had several questions regarding 
the certification associated with the new MCAS, including the basis for 
assessing the change, whether the change complies with applicable 
regulatory requirements, and the rigor associated with the 
certification effort. The commenters questioned the aviation standards 
that the FAA used to certify MCAS, including whether the certification 
basis is the latest (as commenters believe it should be), whether MCAS 
complies, and whether MCAS would comply if it were installed as part of 
a new airplane. The comments were associated with hazard 
classifications of the software and of certain failures of MCAS, Speed 
Trim, and the pitch trim systems. The commenters asserted that a 
single-channel system cannot be upgraded to a dual-channel system via a 
software change only, and that a hardware change must also be required. 
Another commenter asked whether certification testing was done with 
MCAS failed. Other commenters suggested specific flight test scenarios.
    FAA response: The initial 737 MAX certification and the recent 
certification of changes to the 737 MAX used the 737 MAX certification 
basis as documented in the Type Certificate Data Sheet. In some areas, 
the regulations in the certification basis are at earlier amendment 
levels, as allowed by 14 CFR 21.101. The new MCAS complies with those 
design standards, and addresses the unsafe condition identified in this 
AD. While certifying the new MCAS, the FAA determined the hazard levels 
associated with potential failure scenarios after thorough review, 
including failure scenarios assessed by FAA pilots.
    The new MCAS software was certified as Level A using Radio 
Technical Commission for Aeronautics, Inc. (RTCA) DO-178 ``Software 
Considerations in Airborne Systems and Equipment Certification'' as a 
means of compliance, per Advisory Circular 20-115. Regarding the 
assertion that the new MCAS software is insufficient and that a 
hardware change is needed, the existing hardware on the 737 MAX 
airplane includes two AOA sensors and two FCCs; therefore, with only a 
software change to the existing dual-FCC and dual-AOA hardware 
configuration, MCAS became a dual-channel system. In addition to the 
dual architecture, the new FCC software that implements MCAS includes 
integrity monitoring and cross-FCC monitoring. The flight test program 
included flights with MCAS failures, and the FAA determined the set of 
test scenarios to be sufficient for demonstrating compliance with 
applicable 14 CFR part 25 regulations.
2. Comments Regarding Embedding Pilots in Certification Process
    Comment summary: Several commenters, including BALPA, suggested 
that pilots should be embedded in the certification process and that 
average airline pilots should be considered. BALPA stated that the MAX 
accidents were due to modifying aircraft with a commonality of design 
that precluded the need for a level of certification rigor that the 
modification deserved. BALPA cited the Kegworth accident with B737 
Engine Instrument System (EIS) change that did not necessitate a new 
type rating for EIS-equipped models. BALPA asserted that had line 
pilots been involved in certification of that EIS and assessing its 
efficiency in imparting information to the pilots, then a different 
outcome may have occurred.
    FAA's response: The FAA confirms that operational pilots were an 
integral part of the certification of the 737 MAX. Several types of 
pilots were embedded in the certification process. The FAA has flight 
test pilots from its Aircraft Certification Service and aviation safety 
inspector pilots from the Flight Standards Service participate in 
various parts of the certification process. Additionally, the 
certification process involves a cooperative effort from not just the 
FAA, but also the aircraft manufacturers, who closely consult with 
their customers. The 737 MAX procedures and training were evaluated by 
the FAA, EASA, ANAC, and TCCA, including evaluations by pilots from 
foreign CAAs and airline pilots from many different countries 
representing a wide range of experience. Associated with the actions 
required by this AD, 737 MAX flightcrew procedures and training have 
been updated and evaluated by the FSB to ensure flightcrews are 
provided information about MCAS and that flightcrews will be trained on 
the new system before operating the 737 MAX.
3. Comments Regarding Assessment of Flightcrew Response Times
    Comment summary: The FAA received two comments, including one from 
the Families of Ethiopian Airlines Flight 302, expressing concern 
regarding what they described as unrealistic expectations for pilot 
response times after failures. The commenters noted that the flightcrew 
is a key part of the aircraft control system, and pilot reaction and 
response used for certification must be operationally representative 
and scientifically validated. A commenter stated that Boeing failed to 
examine sufficiently the hazard of repeated MCAS activation due to 
erroneously high AOA and failed to consider properly the real-world 
pilot reaction to flight deck effects during these potential failures.
    FAA response: The FAA agrees that pilot reaction and response used 
for certification should be operationally

[[Page 74578]]

representative and validated. The FAA utilized the findings and 
recommendations from the accident reports and auditing entities to 
drive a closer evaluation of airmanship and pilot response. This 
resulted in extensive FAA design reviews and validations conducted in 
engineering simulators and in-flight tests. With the original MCAS 
design, pilots had full control authority over MCAS, but had to use the 
electric stabilizer trim switches, and could disable the system using 
the stabilizer trim cutout switches. The new MCAS design eliminates the 
need for time-critical pilot actions beyond normal pitch attitude 
control using the column alone for any foreseeable failures. The FAA 
evaluated possible failures, including AOA failures, during all phases 
of flight under the most critical (i.e., takeoff and go-around) phases 
of flight and conditions. All associated flight deck effects were 
replicated, and the workload and effect of each in combination was 
considered and validated. These evaluations were conducted using a wide 
range of FAA test pilots, FAA operations pilots, training pilots, and 
domestic and international pilots of varying experience. The 
evaluations were monitored by human factors specialists to validate 
pilot reactions to possible failures of the new design.
    The changes to the 737 MAX required by this AD address the unsafe 
condition. Therefore, the FAA has not changed this final rule based on 
these comments.
4. Comments Regarding Integrated Review Including MCAS
    Comment summary: Flyers Rights commented that MCAS should be 
evaluated from an integrated whole-aircraft system perspective, and 
evaluated with the appropriate catastrophic failure designation.
    FAA response: The FAA evaluated MCAS from an integrated whole-
aircraft system perspective. During certification of the new MCAS, 
Boeing developed and the FAA approved an integrated SSA that assessed 
systems that interface with MCAS. The FAA also approved an analysis of 
single and multiple failures, which considered comprehensive impacts of 
single and multiple failures. The FAA concluded that for certification 
of the new MCAS, Boeing applied the appropriate hazard category 
designations.

M. Proposed AD Revisions and Data Requests

1. Comments Regarding Clarification of the Unsafe Condition
    Comment summary: A commenter suggested the FAA clarify that the 
agency's intent is to address the following unsafe condition: 
``Failures that results in repeated nose-down trim commands of the 
horizontal stabilizer, that if not addressed, could cause the 
flightcrew to have difficulty controlling the airplane, and lead to 
excessive nose-down attitude, significant altitude loss, and possible 
impact with terrain.''
    FAA response: The FAA's description of the unsafe condition in this 
AD is accurate. The commenter's proposed description of the unsafe 
condition is specific to the narrow accident scenarios. However, the 
unsafe conditions and corrective actions addressed by this AD encompass 
not only those scenarios described by the commenter, but also other 
related scenarios, to ensure they do not occur in service.
2. Comments Requesting Additional Information
    Comment summary: The FAA received a variety of requests for 
additional information from numerous commenters, including the Families 
of Ethiopian Airlines Flight 302 and the Turkish DGCA. These requests 
ranged from general to specific. The most broadly-worded included 
requests for ``all'' data used by the agency to make its findings and 
to propose this rule, and for ``technical details of the proposed 
fixes.'' Slightly more tailored requests asked for all data that showed 
the airplane's stall characteristics were safe. Very specific requests 
also were made, such as for the MCAS SSA including its fault trees and 
failure modes and effects analyses (FMEAs), a full description of 
system input signals and functions, and details of the in-depth reviews 
that a commenter stated took place to establish the acceptability of 
implementing MCAS through tailplane movement. Another commenter asked 
for internal objections by FAA employees to the NPRM.
    FAA response: In reviewing whether a particular issue is an unsafe 
condition that requires corrective action, the FAA relies upon data 
provided by the manufacturer, including the manufacturer's contractors 
and suppliers, which they have designated as proprietary.
    The records submitted by the manufacturer to show compliance with 
FAA regulations consist of highly technical data and proprietary 
compliance methods that the manufacturer developed specific to the 737 
MAX design changes. The Trade Secrets Act (TSA) prohibits the FAA and 
its employees from disclosing companies' proprietary information. 18 
U.S.C. 1905. The information is likewise protected from disclosure 
under Freedom of Information Act (FOIA) Exemption 4, and would not be 
available to members of the public through a FOIA request for public 
access. 5 U.S.C. 552(b)(4).
    The FAA supports the public's rights to be reasonably informed of 
the basis for agency rulemaking. This does not, however, require 
putting interested members of the public in a position to reconstruct 
for themselves the underlying technical analyses that are based on 
proprietary data; rather, the FAA has provided, as the law specifies, 
``either the terms or substance of the proposed rule or a description 
of the subjects and issues involved.'' 5 U.S.C. 553. If the FAA were to 
disclose or force the disclosure of manufacturers' proprietary data, 
there is risk of a chilling effect that would make U.S. aviation less 
safe. Manufacturers could become hesitant to provide the FAA with 
fulsome design and manufacturing information that best supports the FAA 
in addressing potential unsafe conditions, instead seeking to provide 
only a bare minimum of information required by 14 CFR 21.3 and 121.703. 
FAA analysts would have difficulty obtaining needed technical data, or 
such details could be slow in forthcoming during what are sometimes 
very urgent analyses.
    This particular NPRM was accompanied by the service bulletins for 
all of the design changes except for one, and a nearly 100-page summary 
of technical information in the ``Preliminary Summary of the FAA's 
Review of the Boeing 737 MAX,'' dated August 3, 2020. This information 
fairly apprised the public of the issues under consideration in this 
rulemaking and enabled informed responses, as evidenced by the more 
than two hundred submitted comments, many of which were highly 
technical.
    For example, the FAA received thirty comments regarding the 
adequacy of two AOA sensors on the 737 MAX, with many suggesting that 
three sensors are necessary to address the unsafe condition. Some of 
these commenters provided detailed engineering rationale, which was 
possible based on generally available knowledge of how AOA sensors 
work; their reliability; and general principles on system design, 
system architecture, and system safety analysis techniques. The 
information

[[Page 74579]]

that the FAA supplied thus enabled the public to provide thoughtful 
comments on the agency's proposal. As another example, regarding the 
new FCC software, the NPRM provided a detailed explanation of how the 
new MCAS functions (as implemented by the new FCC software), and how 
the FAA proposed that those functions would address the unsafe 
condition. Also, in the ``Preliminary Summary of the FAA's Review of 
the Boeing 737 MAX,'' dated August 3, 2020, the FAA explained the 
safety standards that the agency applied to the software, and how the 
agency validated that the new software would function as intended. 
Without the need for underlying detail such as the actual MCAS software 
code, which could not be interpreted unless it is installed in the 
airplane or simulator, the information that the FAA supplied enabled 
meaningful comments on the software's functions and how those functions 
address the unsafe condition.
    Regarding the request for internal objections by FAA employees to 
the NPRM, this final rule represents the considered position of the FAA 
based on the totality of the agency's work.
3. Comments Regarding Inclusion of Wiring Change in Proposed AD
    Comment summary: Several commenters noted that the proposed AD 
would mandate wiring separation; however, it was not clear to the 
commenters how separating wiring prevents the repeated nose-down trim 
commands that this AD is intended to correct. The Boeing service 
information indicates that a short circuit between the ``Arm,'' one of 
the Control signal lines, and a 28 VDC source will cause a stabilizer 
trim runaway. A commenter noted that a continuous trim runaway command 
is a different scenario from repeated nose-down trim commands, and 
stated that continuous trim runaway should be addressed via an AFM 
procedure. While the commenter agreed that future production aircraft 
should incorporate this corrective action, the commenter did not find 
that an AD mandating corrective action was warranted.
    FAA Response: As noted in the NPRM, Boeing re-assessed the 
stabilizer trim control system and identified areas of non-compliance 
with applicable regulations. The Boeing system safety analysis for the 
stabilizer trim control system assessed compliance of the revised 
system (with wires separated). Boeing and the FAA determined that wire 
separation is needed on the Boeing Model 737 MAX to bring the airplanes 
into compliance with the FAA's wire separation safety standards (14 CFR 
25.1707).
    Regarding the commenter's statement about continuous trim runaway, 
the Runaway Stabilizer NNC required by figure 3 to paragraph (h)(4) of 
this AD is the AFM procedure to be used ``[i]f uncommanded stabilizer 
movement occurs continuously or in a manner not appropriate for flight 
conditions.''
4. Comments Regarding Operational Readiness Flight
    Comment summary: Several commenters, including Air China, Ameco, 
and the UAE GCAA, had questions about the operational readiness flight 
required by paragraph (m)(1) of this AD. They did not think the 
``Operational Readiness Flight'' (ORF) is sufficiently defined in 
Boeing Special Attention Service Bulletin 737-00-1028, July 20, 2020. 
They suggested that Boeing publish a separate flight test document for 
the 737 MAX ORF rather than the profile in the service bulletin. They 
asked whether an AMOC is required if there is a deviation from the ORF 
requirements in this AD. They asked whether a subsequent ORF is 
required if a fault is identified during the ORF required by this AD.
    FAA response: The requirements of the ORF are intentionally brief 
and concise and are specified in the service bulletin. The requirements 
are to achieve flaps-up flight at or above 20,000 feet above mean sea 
level (MSL). If a flight achieves these two criteria, the ORF is 
completed. There are no specific test conditions or required maneuvers. 
The requirement is written to allow operators the flexibility to 
utilize their own typical procedures and flight profiles, provided they 
include flight with the flaps up, at or above 20,000 feet above MSL. 
The service bulletin includes a suggested flight profile, which an 
operator may choose to use. The FAA does not anticipate the need for 
AMOCs related to paragraph (m)(1) of this AD due to the brevity of the 
requirement.
    If a fault is identified during the ORF, a subsequent ORF is not 
required by this AD; however, the operator should resolve the 
discrepancy using standard procedures, which may require a test flight. 
Paragraph (m)(2) of this AD requires resolving any mechanical 
irregularities that occurred during the ORF following the operator's 
FAA-approved maintenance or inspection program, as applicable.
5. Comments Regarding Necessity for Flight Permit
    Comment summary: A4A noted that all Required for Compliance (RC) 
steps must be completed ``before further flight'' (including the ORF in 
paragraph (m) of the proposed AD) to fully address the NPRM referenced 
unsafe condition. A4A asked the FAA to clarify the airworthiness of the 
aircraft prior to completing the ORF.
    FAA Response: The FAA did not intend the reference to ``before 
further flight'' in paragraph (m)(1) of this AD to include the ORF. 
Therefore, the FAA has revised paragraph (m)(1) of this AD to require 
the ORF to be completed ``before any other flight.'' The FAA finds that 
completion of the actions specified in paragraphs (g) through (l) of 
this AD is adequate to accomplish the ORF safely. Ferry flights are 
permitted prior to or after the ORF as stated in paragraph (n) of this 
AD.
6. Comments Regarding Warranty Coverage of Wiring Change Costs
    Comment summary: A commenter asserted that the cost of the 
horizontal stabilizer wiring change would be borne by the operators, 
and suggested that the wiring change should be done at Boeing's 
expense.
    FAA response: Boeing Service Bulletin 737-27-1318, identified in 
the NPRM as the appropriate source of service information for the 
horizontal stabilizer wiring change, states that warranty remedies are 
available for airplanes in warranty as of March 6, 2020. Although the 
NPRM provided all costs, it also noted, ``[a]ccording to the 
manufacturer, some or all of the costs of this proposed AD may be 
covered under warranty, thereby reducing the cost impact on affected 
operators.'' No change to this AD is necessary based on this comment.
7. Comments Regarding Change to AOA Sensor System Test Costs
    Comment summary: Based on new data, Boeing clarified and updated 
the amount of time it will take to perform the AOA sensor system test: 
10 work-hours instead of 40 work-hours. Boeing noted that Boeing 
Special Attention Service Bulletin 737-00-1028, dated July 20, 2020 
(the source of service information identified in the NPRM for this 
test), overstated the time required. Boeing subsequently re-evaluated 
the time it takes to do the test and determined the 10-work-hour 
estimate better reflects the actual time required to do the AOA sensor 
system test. Boeing reported this update in Information Notice IN-737-
00-1028-00-01.
    FAA response: The FAA concurs with this requested change to the 
work-hour estimate for the reasons provided by the commenter, and has 
updated the ``Costs

[[Page 74580]]

of Compliance'' section in this final rule accordingly.

N. Requests for Clarification of Preamble Statements

    Various commenters requested clarification of preamble statements.
1. Comments Regarding Preamble Changes From Boeing
    Comment Summary: Request to clarify purpose of AOA sensors: 
Regarding the Proposed Design Changes section, Boeing requested that 
the FAA change ``[t]he updated FCC software would also compare the 
inputs from the two sensors to detect a failed AOA sensor'' to ``[t]he 
updated FCC software would also compare the inputs from the two sensors 
to detect a disagreement between the AOA sensors.'' Boeing stated that 
this comment is intended to add clarity and enhance the completeness of 
the information included in the NPRM. The software compares two AOA 
inputs to determine if they agree, within an appropriate range, and if 
the STS should be in an operative state.
    Comment Summary: Request to clarify conditions for multiple MCAS 
activations: Regarding the Proposed Design Changes section, Boeing 
requested that the FAA change ``[a] subsequent activation of MCAS would 
be possible only after the airplane returns to a low AOA state, below 
the threshold that would cause MCAS activation'' to ``[a] subsequent 
activation of MCAS would be possible only after the airplane returns to 
a low AOA state, below the threshold that would cause MCAS activation, 
and then increases above the activation threshold.'' Boeing stated that 
this comment is intended to improve clarity and completeness, and that 
the proposed language more fully describes the conditions under which 
multiple MCAS activations could occur. The airplane must return to a 
low AOA state, below the threshold that would cause MCAS activation, 
and then increase above the activation threshold.
    Comment Summary: Request to clarify purpose of AOA DISAGREE alert: 
Regarding the Proposed Design Changes section, Boeing requested that 
the FAA change ``[w]hile the lack of an AOA DISAGREE alert is not an 
unsafe condition itself, the FAA is proposing to mandate this software 
update to restore compliance with 14 CFR 25.1301 and because the 
flightcrew procedures mandated by this AD now rely on this alert to 
guide flightcrew action'' to ``[w]hile the lack of an AOA DISAGREE 
alert is not an unsafe condition itself, the FAA is proposing to 
mandate this software update to restore compliance with 14 CFR 25.1301 
and because the flightcrew procedures mandated by this AD now reference 
the presence of this alert.'' Boeing stated that this comment is 
included to add clarity and avoid confusion. The AOA DISAGREE alert is 
not relied upon to guide flightcrew action; it is one of several flight 
deck indications that may alert the flightcrew of an unreliable 
airspeed event. Due to those integrated flight deck effects, the 
flightcrew should execute the un-annunciated Airspeed Unreliable 
procedure.
    Comment Summary: Request for consistent terminology of non-normal 
procedures: Regarding the Proposed Design Changes section, Boeing 
requested that the FAA change ``[t]o facilitate the flightcrew's 
ability to recognize and respond to undesired horizontal stabilizer 
movement and the effects of a potential AOA sensor failure, the FAA 
proposes to mandate revising and adding certain operating procedures 
(checklists) of the AFM used by the flightcrew for the 737 MAX'' to 
``[t]o facilitate the flightcrew's ability to recognize and respond to 
undesired horizontal stabilizer movement and the effects of a potential 
AOA sensor failure, the FAA proposes to mandate revising and adding 
certain non-normal procedures (checklists) of the AFM used by the 
flightcrew for the 737 MAX.'' Boeing stated that this comment is 
intended to clarify and enhance consistency in the way the NPRM refers 
to procedures found in the AFM. The referenced procedures are 
technically referred to as ``non-normal procedures'' and the NPRM uses 
the ``non-normal procedure'' terminology in the subsequent sentences. 
This change simply makes the terminology consistent.
    Comment Summary: Request to clarify certain Quick Reference 
Handbook (QRH) provisions: Regarding footnote 15, in the Background 
section, Boeing requested that the FAA change ``[a]ll of the checklists 
that the FAA proposes to revise or add to the AFM are already part of 
Boeing's QRH, for the 737 MAX (except for the IAS Disagree checklist, 
which is new to both the AFM and the QRH)'' to ``[a]ll of the 
checklists that the FAA proposes to revise or add to the AFM are 
already part of Boeing's Quick Reference Handbook, or QRH, for the 737 
MAX.'' Boeing stated that this comment provides clarification. The IAS 
DISAGREE non-normal checklist is not new to the QRH.
    Comment Summary: Request to clarify revised Runaway Stabilizer 
checklist: Regarding the Proposed Design Changes section, Boeing 
requested that the FAA change ``[f]inally, the checklist would be 
revised to add a reference item to manually trim the horizontal 
stabilizer for pitch control, and note that a two-pilot effort may be 
used to correct an out-of-trim condition'' to ``[f]inally, the 
checklist would be revised to add a reference item to not reengage the 
autopilot or autothrottle, note that a two-pilot effort may be used to 
correct an out-of-trim condition, and note that reducing airspeeds will 
reduce the effort needed to manually trim the horizontal stabilizer for 
pitch control.'' Boeing stated that this comment is included to add 
clarity and avoid confusion. The existing checklist directs the 
flightcrew to manually trim the horizontal stabilizer. The revised 
checklist directs the flightcrew to not re-engage the autopilot or 
autothrottle and provides enhanced guidance that reducing airspeeds 
reduces the effort needed to manually trim.
    Comment Summary: Request to clarify conditions for AOA Disagree 
procedure: Regarding the Proposed Design Changes section, Boeing 
requested that the FAA change ``[t]herefore, this proposed checklist 
would be used when there is an indication, such as an AOA DISAGREE 
alert, that the airplane's left and right AOA vanes disagree'' to 
``[t]herefore, this proposed checklist would be used when there is an 
AOA DISAGREE alert, which indicates that the airplane's left and right 
AOA vanes disagree.'' Boeing stated that this comment is included to 
add clarity and avoid confusion. The current wording may be interpreted 
to suggest that there are multiple reasons to use the AOA Disagree non-
normal procedure. However, the only reason the flightcrew would perform 
the AOA Disagree procedure is if the AOA DISAGREE alert is annunciated.
    Comment Summary: Request to clarify conditions for certain 
checklist steps: Regarding the Proposed Design Changes section, Boeing 
requested that the FAA change ``[t]he checklist would also provide 
additional steps for the flightcrew to subsequently complete for the 
descent, approach, and landing phases of flight'' to ``[i]f IAS 
DISAGREE is not shown, the checklist would also provide additional 
steps for the flightcrew to subsequently complete the descent, 
approach, and landing phases of flight.'' Boeing stated that this 
comment is intended to improve clarity. The steps indicated are only 
executed by the crew if IAS DISAGREE is not present.
    FAA response: The FAA agrees with the foregoing assertions and 
Boeing's rationale for its proposed changes. However, because the 
proposed changes

[[Page 74581]]

would not affect any requirement of this AD, no change to this AD is 
necessary based on this comment.
2. Comments Regarding Credit for MEL Provisions
    Comment summary: Air China and Ameco requested that the FAA revise 
paragraph (i) of the proposed AD to state that the incorporation of FAA 
737 MAX MMEL Revision 2, dated April 10, 2020, into the operator's 
existing MEL would show compliance with the requirements of paragraph 
(i) of the proposed AD. The commenter also recommended revising 
paragraph (o) of the proposed AD to provide credit for the actions 
specified in paragraph (i) of the proposed AD, if Revision 2 of the 
MMEL was incorporated into the operator's existing MEL before the 
effective date of the AD.
    FAA response: Since operators are not required to have an MEL, the 
FAA cannot revise paragraph (i) of this AD to directly require 
operators to incorporate Revision 2 of the MMEL. Paragraph (i) requires 
that an operator update their MEL if they want to use it. The FAA 
agrees with the intent of the request for credit for incorporating 
Revision 2 of the MMEL before the effective date of this AD. Paragraph 
(f) of this AD requires that operators ``comply with this AD . . . 
unless already done.'' Therefore, in light of that provision, no change 
to this AD is necessary regarding these requests.
3. Comments Regarding Service Information: Boeing Special Attention 
Service Bulletin 737-27-1318
    Comment summary: Air China, Ameco, Boeing, A4A, and the Ethiopian 
Airlines Group requested that paragraph (k) of the proposed AD refer to 
revised service information for the horizontal stabilizer trim wire 
bundle routing change. (The NPRM referred to Boeing Special Attention 
Service Bulletin 737-27-1318, Revision 1, dated June 24, 2020, as the 
appropriate source of service information for this action, and provided 
credit for Boeing Special Attention Service Bulletin 737-27-1318, dated 
June 10, 2020.)
    The commenters requested credit for the prior accomplishment of 
previous revisions of this service information, if certain Installation 
Deviation Records (IDRs) identified in Boeing MOM-MOM-20-0608-01B(R3), 
dated November 3, 2020, have been incorporated. Boeing stated that the 
FAA and Boeing reviewed the IDRs that were issued to operators and 
maintenance repair organizations that completed the actions specified 
in Revision 1 of the service information, and determined that certain 
IDRs addressed installation issues identified in Revision 1 of the 
service information that needed to be addressed to ensure proper 
incorporation of the changes.
    A4A requested that the FAA also allow later FAA-approved revisions 
of this service information.
    FAA response: Boeing Special Attention Service Bulletin 737-27-
1318, Revision 2, dated November 10, 2020, was issued primarily to 
identify the IDRs that were issued to ensure proper incorporation of 
changes that were made in accordance with Revision 1 of the service 
information. As previously explained in the ``Differences from the 
NPRM'' section, the FAA is requiring Revision 2 for the actions 
required by paragraph (k) of this AD. The FAA further agrees to provide 
credit for the original and Revision 1 of this service information, 
provided the referenced 14 IDRs have been incorporated. The FAA also 
finds that incorporation of certain FAA-approved Boeing IDRs is 
acceptable in lieu of the corresponding RC step identified in the 
service information. The FAA has revised paragraphs (k) and (o) 
accordingly in this AD. The IDRs identified in Revision 2 of the 
service bulletin include an additional IDR that was not identified in 
Boeing Multi-Operator Message MOM-MOM-20-0608-01B(R3), dated November 
3, 2020; this AD therefore does not refer to the MOM since it is 
incomplete.
    Regarding the request to allow use of later-approved service 
information, an AD may not refer to any document that does not yet 
exist. To allow operators to use later revisions of the referenced 
document (issued after publication of the AD), either the FAA must 
revise the AD to refer to specific later revisions, or operators or the 
manufacturer must request approval to use later revisions as an AMOC 
for the AD. The FAA has therefore not changed this AD regarding this 
issue.
4. Comments Regarding Service Information: Boeing Special Attention 
Service Bulletin 737-31-1860
    Comment summary: Boeing requested that the FAA refer to Boeing 
Special Attention Service Bulletin 737-31-1860, Revision 1, dated July 
2, 2020, for installing/verifying MDS software and removing INOP 
markers, as specified in paragraph (j) of the proposed AD. (The 
proposed AD referred to Boeing Special Attention Service Bulletin 737-
31-1860, dated June 12, 2020, as the appropriate source of service 
information for these actions, and also the source of the applicability 
information in paragraph (c) of the proposed AD.) Boeing stated that 
allowing use of either version would enhance the completeness of the 
service information by providing up-to-date information in Revision 1, 
as well as credit for the original issue.
    FAA response: The FAA finds that the requested action would enhance 
the completeness of the service information, and leaves the effectivity 
and required actions unchanged. Therefore the FAA has revised 
paragraphs (c), (j), and (o) of this AD accordingly.
5. Comments Regarding Service Information: Boeing Alert Requirements 
Bulletin 737-22A1342 RB
    Comment summary: Paragraph (g) of the proposed AD would require 
installing new FCC OPS software. Although no specific compliance method 
was provided, the proposed AD referred to AMM 22-11-33 as a source of 
guidance for the service information. Ethiopian Airlines Group reported 
that it was notified by Boeing of the release of relevant service 
information for this software installation: Service Bulletin 737-
22A1342. Ethiopian requested that the FAA consider this service 
information as a method of compliance for the proposed FCC OPS 
software.
    FAA response: The FAA has reviewed Boeing Alert Requirements 
Bulletin 737-22A1342 RB, dated November 17, 2020, and determined that 
it is an appropriate source of service information for the FCC OPS 
software installation. The FAA has revised paragraph (g) of this AD to 
add this service information as a method of compliance.
6. Comments Regarding Effects Contributing to Flightcrew Workload
    Comment summary: The NPRM preamble stated that following the Lion 
Air Flight 610 accident, data from the flight data recorder indicated 
that a single erroneously high-AOA sensor input to the flight control 
system while the flaps are retracted can cause repeated airplane nose-
down trim of the horizontal stabilizer and multiple flight deck 
effects, including stall warning activation, airspeed disagree alert, 
and altitude disagree alert, and ``may affect the flightcrew's ability 
to accomplish continued safe flight and landing.'' Boeing commented 
that these effects instead should be characterized as ``contributing 
factors to crew workload.'' Boeing said that its comment was intended 
to provide a more specific description of the way in which stall 
warning activation, an airspeed disagree alert, and an altitude 
disagree alert may affect the flightcrew. Boeing reported that it has 
shown, and the FAA has found, that the effects of stall warning

[[Page 74582]]

activation and airspeed/altitude disagree alerts specifically affect 
flightcrew workload, an important factor that can affect continued safe 
flight and landing. Boeing added that flightcrew workload has been 
considered and accounted for in the development of the software update 
and non-normal procedures described in the NPRM.
    FAA response: The referenced flight deck effects can contribute to 
the flightcrew workload, but the FAA finds that the most adverse flight 
deck effect in the Lion Air 610 accident was a flight control problem 
that affected the flightcrew's ability to accomplish continued safe 
flight and landing. Because the proposed changes would not affect any 
requirement of this AD, no change to this AD is necessary based on this 
comment.

O. Additional Comments Unrelated to the Unsafe Condition

1. Comments Regarding Removal of 737 MAX Airplanes From Service
    Comment summary: Multiple commenters requested that the FAA prevent 
the 737 MAX from reentering service. Some asked that the FAA do so by 
removing the 737 MAX from the Boeing 737 Type Certificate; others 
requested that the FAA permanently prohibit the airplane's operation.
    The commenters expressed concern for the continued safety of Model 
737 MAX airplanes. Some of these commenters expressed concern about a 
design that they characterized as old, unsafe, or unstable, with 
inferior systems and an undue reliance on electronics and automated 
systems. Some commenters questioned the effect on pilot workload of 
complex procedures and multiple checklists. Other commenters contended 
that the MAX certification process was tainted by a lack of 
transparency, reliance on self-certification, a rush to complete 
certification, and certification decisions that prioritized profit, 
cost reduction, and expedience over safety.
    FAA response: The FAA finds that the requirements set forth in this 
AD appropriately address the unsafe condition and that upon completion 
of the mandated requirements, the 737 MAX airplane meets FAA safety 
standards. The FAA acknowledges all of the commenters' safety concerns, 
and those concerns align with the FAA's mission of ensuring safety in 
air commerce. However, the FAA bases its decisions on data, and because 
the corrective actions the FAA is mandating appropriately address the 
identified unsafe condition, the FAA lacks a factual basis to mandate 
that this airplane be permanently grounded.
2. Comments Regarding Assessment of Other Users of AOA Data
    Comment summary: Ethiopian Airlines Group noted that the proposed 
AD stated that MCAS logic that was dependent on a single AOA sensor 
input will be changed to using two AOA inputs. The commenter asked 
about other users of AOA data, either as a single input user or a dual 
input user, and whether the FAA can confirm the change to MCAS to use 
two AOA inputs does not affect other users requiring only one AOA 
input.
    FAA response: During the certification of the new MCAS, Boeing and 
the FAA scrutinized all users of AOA data and considered normal and 
failure conditions. There is no effect on other users of AOA data. 
Other users of AOA data are compliant and safe.
3. Comments Not Related to the Unsafe Condition Addressed by This AD
    The FAA received a variety of general comments and allegations 
related to the competence, ethics, motives, and resources of the 
agency, the manufacturer, and their component organizations such as the 
organization designation authorization (ODA) and the FAA Boeing 
Aviation Safety Oversight Office. These comments came from individuals 
and organizations that included the Families of Ethiopian Airlines 
Flight 302, Aerospace Safety and Security, Inc., Aerospace Safety 
Research Institute, Inc., AFA-CWA, Allied Pilots Association, BALPA, 
Ethiopian Airlines Group, and Flyers Rights. These comments are 
unrelated to the particular unsafe condition and corrective action, and 
therefore are not addressed here.
    The FAA also received a variety of comments related to other 
potential safety issues on the 737 MAX. The subjects of these comments 
include the airplane's susceptibility to high intensity radiated field, 
protection of the airplane's rudder cable, the reliability of the 
airplane's auto speedbrake system, engine bonding issues, electronic 
flight bags, slat track assemblies, the airplane's refueling system, 
the auxiliary power unit (APU) fuel tank float switch, the Landing 
Attitude Modifier, the airplane's fly-by-wire spoiler system, and the 
possibility of foreign object debris. These issues are unrelated to the 
particular unsafe condition that this AD addresses and therefore are 
not addressed here.
    The FAA also received a variety of comments related to proposed 
solutions other than those proposed in this rulemaking. These include 
limiting the 737 MAX's overwater operation; converting all 737 MAX 
airplanes to cargo airplanes; using the Boeing Model 757 instead; 
allowing passengers booked on this airplane to change flights; 
thoroughly redesigning the airplane's flight control surfaces; 
increasing engine power rather than decreasing pitch; limiting airplane 
nose up and installing an Alpha floor design used on Airbus airplanes; 
requiring certain data to be transmitted from the airplane mid-flight; 
requiring certain parameters to be recorded such as the status of 
manual electric trim switches; constraining the flight envelope using 
control laws or mechanical means; and changing the airplane's 
configuration. Some commenters also suggested that the FAA ask the U.S. 
Congress to increase the agency's budget and contract out its 
functions. These proposed solutions are unrelated to the corrective 
actions that were proposed in this rulemaking and therefore will not be 
addressed here.
    The FAA received a variety of comments and suggestions, including 
from the Families of Ethiopian Airlines Flight 302, related to other 
airplane models, and requests that the FAA review the safety of those 
other airplanes and future airplanes. The FAA is applying lessons 
learned on the 737 MAX to current and future FAA certifications and 
continued operational safety processes. However, these comments are 
unrelated to the unsafe condition addressed by this AD for the 737 MAX, 
and therefore will not be addressed here.
    The FAA received a variety of comments, including from the Families 
of Ethiopian Airlines Flight 302 and the Allied Pilots Association, 
related to the adequacy of the regulations that govern how the FAA 
processes applications, such as 14 CFR part 21 and 21.101 in 
particular, and the design standards in 14 CFR part 25 such as 25.1309 
and 25.1322, and how the FAA applies them, such as in AC 21.101 and AC 
25.1329. These comments included 13 requests from BALPA for regulatory 
and other oversight changes applicable to future aircraft models by the 
FAA and other authorities. The FAA's regulatory requirements are 
promulgated via notice-and-comment rulemaking as required by the 
Administrative Procedure Act (APA), and the public can petition for 
rulemaking at https://www.faa.gov/regulations_policies/rulemaking/petition/.
    The FAA received several comments, including from the Families of 
Ethiopian Airlines Flight 302, to improve its processes and oversight, 
such as those for approving proposed

[[Page 74583]]

designs, overseeing manufacturers (including conducting audits), 
overseeing the Boeing ODA and other designees including ensuring 
freedom from undue pressure, and overseeing all aspects of airline 
operations including maintenance practices and repair facilities. The 
FAA appreciates and considers all such input; however, it is outside 
the scope of this particular rulemaking.
    The FAA received requests, including from the Allied Pilots 
Association, regarding how the FAA should treat alternative methods of 
compliance, known as AMOCs. The FAA acknowledges the commenters' 
concern; however, it is premature for the FAA to limit or foreclose the 
methods by which an applicant can show compliance with this AD.
    The FAA also received requests that the agency create additional 
data for public review. These included a request for a comparative 
analysis of the difference in stability and control between the subject 
airplane and other airplane models. They also included a request for 
in-depth reviews to establish the acceptability of implementing MCAS 
through tailplane movement. The creation of such additional information 
is not necessary to find compliance with FAA regulations, or to find 
that the unsafe condition has been addressed.
    The FAA also received a request from the Families of Ethiopian 
Airlines Flight 302 to commission a new independent review board to 
prepare findings.
    The FAA commissioned an independent review board, called the 
Technical Advisory Board (TAB). The TAB is an independent team of 
experts that evaluated the design of the new MCAS. The TAB included FAA 
certification specialists and chief scientific and technical advisors 
not involved in the original 737 MAX certification program, and subject 
matter experts from the U.S. Air Force, the Volpe National 
Transportation Systems Center, and the National Aeronautics and Space 
Administration. The TAB findings are summarized in the ``Summary of the 
FAA's Review of the Boeing 737 MAX,'' which is posted in Docket No. 
FAA-2020-0686.
    The FAA also received comments that were out of scope for other 
reasons, such as doubting the technical ability of the public to 
comment on this proposal. Such comments are not being addressed.
    Commenters asked how the design changes to correct this unsafe 
condition would be distributed to and approved by the CAAs and 
implemented by operators worldwide. The FAA, as the airworthiness 
authority for the State of Design for these airplanes, is obligated by 
ICAO Annex 8 to provide Mandatory Continued Airworthiness Information 
to CAAs of other countries.\14\ The FAA will provide the AD to those 
authorities, and ICAO Annex 8 requires them to take appropriate action 
in response. Therefore, the FAA expects that foreign civil aviation 
authorities will adopt similar requirements to those mandated by this 
AD, and that foreign operators would then comply with those 
requirements.
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    \14\ https://www.icao.int/safety/airnavigation/Pages/nationality.aspx.
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Conclusion

    The FAA reviewed the relevant data, considered the comments 
received, and determined that air safety and the public interest 
require adopting this AD with the changes described previously, and 
minor editorial changes. The FAA has determined that these minor 
changes:
     Are consistent with the intent that was proposed in the 
NPRM for addressing the unsafe condition; and
     Do not add any additional burden upon the public than was 
already proposed in the NPRM.
    The FAA also determined that these changes will not increase the 
economic burden on any operator or increase the scope of this AD.

Related Service Information Under 1 CFR Part 51

    The FAA reviewed and approved the following service information.
     Boeing Alert Requirements Bulletin 737-22A1342 RB, dated 
November 17, 2020, describes procedures for installation of FCC OPS 
software on FCC A and FCC B, a software installation verification, and 
corrective actions.
     Boeing Special Attention Service Bulletin 737-31-1860, 
Revision 1, dated July 2, 2020, describes procedures for installation 
of MDS software, a software installation verification and corrective 
actions, and removal of certain INOP markers on the EFIS control 
panels.
     Boeing Special Attention Service Bulletin 737-27-1318, 
Revision 2, dated November 10, 2020, describes procedures for changing 
of the horizontal stabilizer trim wire routing installations.
     Boeing Special Attention Service Bulletin 737-00-1028, 
dated July 20, 2020, describes procedures for an AOA sensor system test 
and an operational readiness flight.
    This service information is reasonably available because the 
information is posted in the docket and because the interested parties 
otherwise have access to it through their normal course of business or 
by the means identified in the ADDRESSES section.

Effective Date

    Section 553(d) of the APA (5 U.S.C.) generally requires publication 
of a rule not less than 30 days before its effective date. However, 
section 553(d) authorizes agencies to make rules effective in less than 
thirty days, upon a finding of good cause. Due to the relationship 
between the Lion Air accident on October 29, 2018, and the Ethiopian 
Airlines accident on March 10, 2019, the FAA issued an Emergency Order 
of Prohibition on March 13, 2019, generally prohibiting the operation 
of 737 MAX airplanes subject to this AD. This AD now identifies the 
unsafe condition in the 737 MAX and mandates corrective actions to 
correct the unsafe condition so that general operations may resume. 
With the publication of this AD, the Emergency Order is no longer 
necessary. Accordingly, the FAA is rescinding the Emergency Order 
contemporaneously with publication of this final rule. These actions 
create the opportunity for operators to safely return the 737 MAX to 
service, following a fleet-wide grounding lasting over twenty months. 
Therefore, the FAA finds that good cause exists pursuant to 5 U.S.C. 
553(d) for making this amendment immediately effective to provide 
relief from the grounding restriction as operators take the required 
actions to address the unsafe condition.

Costs of Compliance

    The FAA estimates that this AD affects 72 airplanes of U.S. 
registry. The agency estimates the following costs to comply with this 
AD:

[[Page 74584]]



                                                 Estimated Costs
----------------------------------------------------------------------------------------------------------------
                                                                                                 Cost on U.S.
             Action                   Labor cost          Parts cost       Cost per product        operators
----------------------------------------------------------------------------------------------------------------
FCC OPS installation and          1 work-hour x $85   $0................  $85...............  $6,120.
 verification.                     per hour = $85.
AFM revisions...................  1 work-hour x $85   $0................  $85...............  $6,120.
                                   per hour = $85.
MDS installation and              1 work-hour x $85   $0................  $85...............  $6,120.
 verification, INOP marker         per hour = $85.
 removal.
Stabilizer wiring change........  Up to 79 work-      Up to $3,790......  Up to $10,505.....  Up to $756,360.
                                   hours x $85 per
                                   hour = Up to
                                   $6,715.
AOA sensor system test..........  10 work-hours x     $0................  $850..............  $61,200.
                                   $85 per hour =
                                   $850.
----------------------------------------------------------------------------------------------------------------

    The FAA has received no definitive data that would enable the 
agency to provide cost estimates for the operational readiness flight 
specified in this AD.
    Operators that have a MEL and choose to dispatch an airplane with 
an inoperative flight control system affected by this AD would be 
required to incorporate certain provisions into the operator's existing 
FAA-approved MEL. The FAA has determined that revising the operator's 
existing FAA-approved MEL takes an average of 90 work-hours per 
operator, although the agency recognizes that this number may vary from 
operator to operator. Since operators incorporate MEL changes for their 
affected fleet(s), the FAA has determined that a per-operator estimate 
is more accurate than a per-airplane estimate. Therefore, the FAA 
estimates the average total cost per operator to be $7,650 (90 work-
hours x $85 per work-hour).
    According to the manufacturer, some or all of the costs of this AD 
may be covered under warranty, thereby reducing the cost impact on 
affected operators.

Authority for This Rulemaking

    Title 49 of the United States Code specifies the FAA's authority to 
issue rules on aviation safety. Subtitle I, Section 106, describes the 
authority of the FAA Administrator. Subtitle VII, Aviation Programs, 
describes in more detail the scope of the Agency's authority.
    The FAA is issuing this rulemaking under the authority described in 
Subtitle VII, Part A, Subpart III, Section 44701, General requirements. 
Under that section, Congress charges the FAA with promoting safe flight 
of civil aircraft in air commerce by prescribing regulations for 
practices, methods, and procedures the Administrator finds necessary 
for safety in air commerce. This regulation is within the scope of that 
authority because it addresses an unsafe condition that is likely to 
exist or develop on products identified in this rulemaking action.

Regulatory Findings

    The FAA has determined that this AD will not have federalism 
implications under Executive Order 13132. This AD will not have a 
substantial direct effect on the States, on the relationship between 
the national government and the States, or on the distribution of power 
and responsibilities among the various levels of government.
    For the reasons discussed above, I certify that this AD:
    (1) Is not a ``significant regulatory action'' under Executive 
Order 12866,
    (2) Will not affect intrastate aviation in Alaska, and
    (3) Will not have a significant economic impact, positive or 
negative, on a substantial number of small entities under the criteria 
of the Regulatory Flexibility Act.

List of Subjects in 14 CFR Part 39

    Air transportation, Aircraft, Aviation safety, Incorporation by 
reference, Safety.

Adoption of the Amendment

    Accordingly, under the authority delegated to me by the 
Administrator, the FAA amends 14 CFR part 39 as follows:

PART 39--AIRWORTHINESS DIRECTIVES

0
1. The authority citation for part 39 continues to read as follows:

    Authority:  49 U.S.C. 106(g), 40113, 44701.


Sec.  39.13  [Amended]

0
2. The FAA amends Sec.  39.13 by:
0
a. Removing Airworthiness Directive (AD) 2018-23-51, Amendment 39-19512 
(83 FR 62697, December 6, 2018; corrected December 11, 2018 (83 FR 
63561)), and
0
b. Adding the following new AD:

2020-24-02 The Boeing Company: Amendment 39-21332; Docket No. FAA-
2020-0686; Product Identifier 2019-NM-035-AD.

(a) Effective Date

    This AD is effective November 20, 2020.

(b) Affected ADs

    This AD replaces AD 2018-23-51, Amendment 39-19512 (83 FR 62697, 
December 6, 2018; corrected December 11, 2018 (83 FR 63561)) (``AD 
2018-23-51'').

(c) Applicability

    This AD applies to The Boeing Company Model 737-8 and 737-9 
airplanes, certificated in any category, as identified in Boeing 
Special Attention Service Bulletin 737-31-1860, Revision 1, dated 
July 2, 2020.

(d) Subject

    Air Transport Association (ATA) of America Code 22, Auto flight; 
27, Flight controls; and 31, Indicating/recording systems.

(e) Unsafe Condition

    This AD was prompted by the potential for a single erroneously 
high angle of attack (AOA) sensor input received by the flight 
control system to result in repeated airplane nose-down trim of the 
horizontal stabilizer, which, in combination with multiple flight 
deck effects, could affect the flightcrew's ability to accomplish 
continued safe flight and landing.

(f) Compliance

    Comply with this AD within the compliance times specified, 
unless already done.

(g) Installation/Verification of Flight Control Computer (FCC) 
Operational Program Software (OPS)

    Before further flight, install FCC OPS software version P12.1.2, 
part number (P/N) 2274-COL-AC2-26, or later-approved software 
versions, on FCC A and FCC B, and do a software installation 
verification. During the installation verification, if the approved 
software part number is not shown as being installed on FCC A and 
FCC B, before further flight, do corrective actions until the 
approved software part number is installed on FCC A and FCC B. 
Later-approved software versions are only those Boeing software 
versions that are approved as a replacement for the applicable 
software, and are approved as part of the type design by the FAA 
after the effective date of this AD. Accomplishment of all 
applicable actions identified as ``RC'' (required for compliance) 
in, and in accordance with, the Accomplishment Instructions of 
Boeing Alert Requirements Bulletin 737-22A1342 RB, dated November 
17, 2020, is acceptable for compliance with the requirements of this 
paragraph.


[[Page 74585]]


    Note 1 to paragraph (g):  Guidance for doing the installation 
and installation verification of the FCC OPS software can be found 
in Boeing 737-7/8/8200/9/10 Aircraft Maintenance Manual (AMM), 
Section 22-11-33.


    Note 2 to paragraph (g):  Guidance for accomplishing the actions 
required by paragraph (g) can also be found in Boeing Alert Service 
Bulletin 737-22A1342, dated November 17, 2020, which is referred to 
in Boeing Alert Requirements Bulletin 737-22A1342 RB, dated November 
17, 2020.

(h) Airplane Flight Manual (AFM) Revisions

    Before further flight, revise the existing AFM to include the 
changes specified in paragraphs (h)(1) through (10) of this AD. 
Revising the existing AFM to include the changes specified in 
paragraphs (h)(2) through (10) of this AD may be done by inserting a 
copy of figure 1 to paragraph (h)(2) through figure 9 to paragraph 
(h)(10) into the existing AFM.
    (1) In the Certificate Limitations and Operating Procedures 
chapters, remove the information identified as ``Required by AD 
2018-23-51.''
    (2) In the Operating Procedures chapter, revise the General 
paragraph to include the information in figure 1 to paragraph (h)(2) 
of this AD.
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[GRAPHIC] [TIFF OMITTED] TR20NO20.031

    (3) In the Operating Procedures chapter, replace the existing 
Airspeed Unreliable paragraph with the information in figure 2 to 
paragraph (h)(3) of this AD.

[[Page 74586]]

[GRAPHIC] [TIFF OMITTED] TR20NO20.032


[[Page 74587]]


[GRAPHIC] [TIFF OMITTED] TR20NO20.033

    (4) In the Operating Procedures chapter, replace the existing 
Runaway Stabilizer paragraph with the information in figure 3 to 
paragraph (h)(4) of this AD.

[[Page 74588]]

[GRAPHIC] [TIFF OMITTED] TR20NO20.034

    (5) In the Operating Procedures chapter, replace the existing 
Stabilizer Trim Inoperative paragraph with the information in figure 
4 to paragraph (h)(5) of this AD.

[[Page 74589]]

[GRAPHIC] [TIFF OMITTED] TR20NO20.035

    (6) In the Operating Procedures chapter, add the information in 
figure 5 to paragraph (h)(6) of this AD.
[GRAPHIC] [TIFF OMITTED] TR20NO20.036

    (7) In the Operating Procedures chapter, add the information in 
figure 6 to paragraph (h)(7) of this AD.

[[Page 74590]]

[GRAPHIC] [TIFF OMITTED] TR20NO20.037

    (8) In the Operating Procedures chapter, add the information in 
figure 7 to paragraph (h)(8) of this AD.
[GRAPHIC] [TIFF OMITTED] TR20NO20.038

    (9) In the Operating Procedures chapter, add the information in 
figure 8 to paragraph (h)(9) of this AD.

[[Page 74591]]

[GRAPHIC] [TIFF OMITTED] TR20NO20.039

    (10) In the Operating Procedures chapter, add the information in 
figure 9 to paragraph (h)(10) of this AD.
[GRAPHIC] [TIFF OMITTED] TR20NO20.040

(i) Minimum Equipment List (MEL) Provisions for Inoperative Flight 
Control System Functions

    In the event that the airplane functions associated with the 
flight control system as modified by this AD are inoperative, an 
airplane may be operated (dispatched) only if the provisions 
specified in figure 10 to paragraph (i) of this AD are incorporated 
into the operator's existing FAA-approved MEL.

[[Page 74592]]

[GRAPHIC] [TIFF OMITTED] TR20NO20.041


    Note 3 to paragraph (i): The MEL provisions specified in figure 
10 to paragraph (i) of this AD correspond to Master Minimum 
Equipment List (MMEL) items 22-10-01B, 22-10-02, 22-10-03, 22-11-01, 
22-11-02, 22-11-05-02B, 22-11-06-02B, 22-11-08-01A, 22-11-08-01B, 
22-11-10A, 22-11-10B, and 27-41-01, in the existing FAA-approved 
Boeing 737 MAX B-737-8/-9 MMEL, Revision 2, dated April 10, 2020, 
which can be found on the Flight Standards Information Management 
System (FSIMS) website, https://fsims.faa.gov/PICResults.aspx?mode=Publication&doctype=MMELByModel.

(j) Installation/Verification of MAX Display System (MDS) Software, 
Removal of INOP Markers

    Before further flight, do all applicable actions identified as 
``RC'' in, and in accordance with, the Accomplishment Instructions 
of Boeing Special Attention Service Bulletin 737-31-1860, Revision 
1, dated July 2, 2020.

(k) Horizontal Stabilizer Trim Wire Bundle Routing Change

    Before further flight, do all applicable actions identified as 
``RC'' in, and in accordance with, the Accomplishment Instructions 
of Boeing Special Attention Service Bulletin 737-27-1318, Revision 
2, dated November 10, 2020.

(l) AOA Sensor System Test

    Before further flight, do all applicable actions identified as 
``RC'' for the ``Angle of Attack (AOA) Sensor System Test'' 
specified in, and in accordance with, the Accomplishment 
Instructions of Boeing Special Attention Service Bulletin 737-00-
1028, dated July 20, 2020.

(m) Operational Readiness Flight

    (1) After accomplishment of all applicable required actions in 
paragraphs (g) through (l) of this AD, do all applicable actions 
identified as ``RC'' for the ``Operational Readiness Flight'' 
specified in, and in accordance with, the Accomplishment 
Instructions of Boeing Special Attention Service Bulletin 737-00-
1028, dated July 20, 2020. The ``Operational Readiness Flight'' 
required by this paragraph must be accomplished before any other 
flight. A special flight permit is not required to accomplish the 
``Operational Readiness Flight'' required by this paragraph.
    (2) After the ``Operational Readiness Flight'' and before 
further flight, any mechanical irregularities that occurred during 
the ``Operational Readiness Flight'' must be resolved following the 
operator's FAA-approved maintenance or inspection program, as 
applicable.

(n) Special Flight Permits

    Special flight permits may be issued in accordance with 14 CFR 
21.197 and 21.199 to operate the airplane to a location where the 
actions of this AD can be performed.

(o) Credit for Previous Actions

    (1) This paragraph provides credit for the actions specified in 
paragraph (j) of this AD, if those actions were performed before the 
effective date of this AD using Boeing Special Attention Service 
Bulletin 737-31-1860, dated June 12, 2020.
    (2) This paragraph provides credit for the actions specified in 
paragraph (k) of this AD, if those actions were performed before the 
effective date of this AD using Boeing Special Attention Service 
Bulletin 737-27-1318, dated June 10, 2020, or Revision 1, dated June 
24, 2020, provided the 14 Installation Deviation Records (IDRs) 
identified in paragraph 1.D., ``Description,'' of Boeing Special 
Attention Service Bulletin 737-27-1318, Revision 2, dated November 
10, 2020, have been incorporated on the airplane. Accomplishment of 
FAA-approved Boeing IDRs not identified in paragraph 1.D., 
``Description,'' of Boeing Special Attention Service Bulletin 737-
27-1318, Revision 2, dated November 10, 2020, before the effective 
date of this AD, is acceptable for compliance with the corresponding 
RC steps specified in Special Attention Service Bulletin 737-27-
1318, Revision 1, dated June 10, 2020, provided those IDRs reference 
Boeing Special Attention Service Bulletin 737-27-1318, Revision 1, 
dated June 10, 2020.

[[Page 74593]]

(p) Alternative Methods of Compliance (AMOCs)

    (1) The Manager, Seattle ACO Branch, FAA, has the authority to 
approve AMOCs for this AD, if requested using the procedures found 
in 14 CFR 39.19. In accordance with 14 CFR 39.19, send your request 
to your principal inspector or responsible Flight Standards Office, 
as appropriate. If sending information directly to the manager of 
the certification office, send it to the attention of the person 
identified in paragraph (q)(1) of this AD. Information may be 
emailed to: [email protected].
    (2) Before using any approved AMOC, notify your appropriate 
principal inspector, or lacking a principal inspector, the manager 
of the responsible Flight Standards Office.
    (3) AMOCs approved previously for AD 2018-23-51 are not approved 
as AMOCs for this AD.
    (4) For service information that contains steps that are labeled 
as RC, the provisions of paragraphs (p)(4)(i) and (ii) of this AD 
apply.
    (i) The steps labeled as RC, including substeps under an RC step 
and any figures identified in an RC step, must be done to comply 
with the AD. If a step or substep is labeled ``RC Exempt,'' then the 
RC requirement is removed from that step or substep. An AMOC is 
required for any deviations to RC steps, including substeps and 
identified figures.
    (ii) Steps not labeled as RC may be deviated from using accepted 
methods in accordance with the operator's maintenance or inspection 
program without obtaining approval of an AMOC, provided the RC 
steps, including substeps and identified figures, can still be done 
as specified, and the airplane can be put back in an airworthy 
condition.

(q) Related Information

    (1) For more information about this AD, contact Ian Won, 
Manager, Seattle ACO Branch, FAA, 2200 South 216th St., Des Moines, 
WA 98198; phone and fax: 206-231-3500; email: [email protected].
    (2) Service information identified in this AD that is not 
incorporated by reference is available at the addresses specified in 
paragraphs (r)(3) and (4) of this AD.

(r) Material Incorporated by Reference

    (1) The Director of the Federal Register approved the 
incorporation by reference (IBR) of the service information listed 
in this paragraph under 5 U.S.C. 552(a) and 1 CFR part 51.
    (2) You must use this service information as applicable to do 
the actions required by this AD, unless the AD specifies otherwise.
    (i) Boeing Alert Requirements Bulletin 737-22A1342 RB, dated 
November 17, 2020.
    (ii) Boeing Special Attention Service Bulletin 737-00-1028, 
dated July 20, 2020.
    (iii) Boeing Special Attention Service Bulletin 737-27-1318, 
Revision 2, dated November 10, 2020.
    (iv) Boeing Special Attention Service Bulletin 737-31-1860, 
Revision 1, dated July 2, 2020.
    (3) For service information identified in this AD, contact 
Boeing Commercial Airplanes, Attention: Contractual & Data Services 
(C&DS), 2600 Westminster Blvd., MC 110-SK57, Seal Beach, CA 90740-
5600; telephone 562-797-1717; internet https://www.myboeingfleet.com.
    (4) You may view this service information at the FAA, 
Airworthiness Products Section, Operational Safety Branch, 2200 
South 216th St., Des Moines, WA. For information on the availability 
of this material at the FAA, call 206-231-3195.
    (5) You may view this service information that is incorporated 
by reference at the National Archives and Records Administration 
(NARA). For information on the availability of this material at 
NARA, email [email protected], or go to: https://www.archives.gov/federal-register/cfr/ibr-locations.html.

    Issued on November 18, 2020.
Lance T. Gant,
Director, Compliance & Airworthiness Division, Aircraft Certification 
Service.
[FR Doc. 2020-25844 Filed 11-18-20; 4:15 pm]
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