[Federal Register Volume 86, Number 113 (Tuesday, June 15, 2021)]
[Rules and Regulations]
[Pages 31604-31609]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2021-12436]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2020-0341; Project Identifier 2020-NM-017-AD; Amendment 
39-21586; AD 2021-11-24]
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 adopting a new airworthiness directive (AD) for 
certain The Boeing Company Model 737-600, -700, -700C, -800, -900, and 
-900ER series airplanes. This AD was prompted by significant changes 
made to the airworthiness limitations (AWLs) related to fuel tank 
ignition prevention and the nitrogen generation system (NGS). This AD 
requires revising the existing maintenance or inspection program, as 
applicable, to incorporate the April 2019 or November 2020 revision of 
the airworthiness limitations document. The FAA is issuing this AD to 
address the unsafe condition on these products.

DATES: This AD is effective July 20, 2021.
    The Director of the Federal Register approved the incorporation by 
reference of a certain publication listed in this AD as of July 20, 
2021.

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-2020-0341.

Examining the AD Docket

    You may examine the AD docket on the internet at https://www.regulations.gov by searching for and locating Docket No. FAA-2020-
0341; 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: Christopher Baker, Aerospace Engineer, 
Propulsion Section, FAA, Seattle ACO Branch, 2200 South 216th St., Des 
Moines, WA 98198; phone and fax: 206-231-3552; email: 
[email protected].

SUPPLEMENTARY INFORMATION:

Background

    The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 
CFR part 39 by adding an AD that would apply to certain The Boeing 
Company Model 737-600, -700, -700C, -800, -900, and -900ER series 
airplanes. The NPRM published in the Federal Register on May 6, 2020 
(85 FR 26888). The NPRM was prompted by significant changes made to the 
AWLs related to fuel tank ignition prevention and the NGS. The NPRM 
proposed to require revising the existing maintenance or inspection 
program, as applicable, to incorporate the April 2019 revision of the 
airworthiness limitations document.
    The FAA is issuing this AD to prevent the potential for ignition 
sources inside the fuel tanks and also to prevent increasing the 
flammability exposure of the center fuel tank caused by latent 
failures, alterations, repairs, or maintenance actions, which could 
result in a fuel tank explosion and consequent loss of an airplane. In 
addition, the FAA is issuing this AD to address the potential loss of 
engine fuel suction feed

[[Page 31605]]

capability, which could result in dual engine flameouts, inability to 
restart engines, and consequent forced landing of the airplane.

Comments

    The FAA gave the public the opportunity to participate in 
developing this final rule. The following presents the comments 
received on the NPRM and the FAA's response to each comment.

Support for the NPRM

    The Air Line Pilots Association, International (ALPA), United 
Airlines, and an individual stated support for the NPRM.

Request To Delay Issuance of Final Rule

    All Nippon Airways (ANA) requested that the FAA delay issuance of 
the final rule until Boeing releases the next revision of Boeing 737-
600/700/700C/800/900/900ER Special Compliance Items/Airworthiness 
Limitations, D626A001-9-04. The commenter explained that in the current 
revision of Boeing 737-600/700/700C/800/900/900ER Special Compliance 
Items/Airworthiness Limitations, D626A001-9-04, dated April 2019 
(Boeing D626A001-9-04, dated April 2019), for Airworthiness Limitation 
(AWL) No. 47-AWL-09, ``Nitrogen Generation System--Oxygen Sensor,'' 
operators must replace the NGS oxygen sensor with a new oxygen sensor 
because installation of an overhauled part is not approved at this 
time. The commenter stated that Boeing is working with the FAA to allow 
testing of the oxygen sensor using the component maintenance manual, 
and on-wing testing using the aircraft maintenance manual, and Boeing 
plans to revise AWL No. 47-AWL-09 to allow testing and installation of 
an overhauled part. The commenter anticipated that the next revision of 
Boeing 737-600/700/700C/800/900/900ER Special Compliance Items/
Airworthiness Limitations, D626A001-9-04, would include the revised AWL 
No. 47-AWL-09.
    The FAA acknowledges the commenter's request. Since publication of 
the NPRM, Boeing has issued Boeing 737-600/700/700C/800/900/900ER 
Special Compliance Items/Airworthiness Limitations, D626A001-9-04, 
dated November 2020 (Boeing D626A001-9-04, dated November 2020), which 
limits the applicability of AWL No. 47-AWL-09 to certain Model 737 
airplanes and states that an oxygen sensor can be replaced with a new 
oxygen sensor or an oxygen sensor repaired per AWL No. 47-AWL-10 
``Nitrogen Generation System--Oxygen Sensor Repair.'' This AD requires 
updating the existing maintenance/inspection program to incorporate 
Boeing D626A001-9-04, dated April 2019; or Boeing D626A001-9-04, dated 
November 2020. In addition, based upon requests from other commenters, 
the FAA has extended the initial compliance time for replacement of the 
oxygen sensor due to a parts availability issue. The FAA has revised 
the grace period for the initial ALI task in paragraph (g)(13) of this 
AD from 12 months to 36 months. This extended grace period will still 
provide an acceptable level of safety.

Request To Extend the Compliance Time

    Air China, ANA, China Eastern Airlines (CEA), Delta Air lines 
(DAL), Japan Airlines (JAL), Okay Airlines (OKY), Southwest Airlines 
(SWA), and Turkish Airlines (THY) requested that the initial compliance 
time for the replacement of the oxygen sensor as specified in paragraph 
(g)(13) of the proposed AD be extended because of a parts availability 
issue. The commenters contacted Boeing and determined that there would 
be a shortage of parts, which would prevent them from complying with 
the proposed requirement specified in paragraph (g)(13) of the proposed 
AD within the compliance time specified in the proposed AD.
    The FAA agrees with the commenters' requests based on the 
information provided by the commenters. The FAA contacted Boeing and 
confirmed that there is a parts availability issue, which could create 
an undue burden on operators because it could prevent them from 
complying with the requirement specified in paragraph (g)(13) of this 
AD, subsequently grounding the affected airplanes. The FAA has revised 
the initial compliance time in paragraph (g)(13) of this AD from 12 
months to 36 months.

Request To Add an Off-Wing Inspection for AWL No. 47-AWL-09

    SWA requested that the FAA consider coordinating this AD with 
Boeing to allow for an off-wing inspection of the oxygen sensor, and 
repair if necessary, as opposed to a mandated time-limited replacement. 
SWA stated that this would mitigate the parts availability impact 
across the industry as well the cost of complying with AWL No. 47-AWL-
09.
    The FAA agrees that an off-wing inspection would reduce the burden 
on operators. This AD allows operators to optionally incorporate Boeing 
D626A001-9-04, dated November 2020, which includes an off-wing 
inspection as specified in AWL No. 47-AWL-10. Boeing modified the 
requirement of AWL No. 47-AWL-09 to allow the use of repaired parts and 
added AWL No. 47-AWL-10 to describe the requirements for the method of 
inspection/repair in Boeing D626A001-9-04, dated November 2020. This AD 
still requires operators to use Boeing D626A001-9-04, dated April 2019, 
but provides an option for operators to incorporate Boeing D626A001-9-
04, dated November 2020, into their maintenance program. In addition, 
the FAA has determined that an on-wing inspection is currently being 
developed by Boeing and that subject on-wing inspection is expected to 
be implemented in a future revision of the Boeing D626A001-9-04 
document. Under the provisions of paragraph (k) of this AD, the FAA 
will consider requests for approval of an alternative method of 
compliance (AMOC) if sufficient data are submitted to substantiate that 
the change would provide an acceptable level of safety.

Request To Clarify the Applicability of a Certain AWL Item

    CEA, DAL, JAL, OKY, Ryan Air, and THY requested clarification of 
the applicability specified in AWL No. 47-AWL-09, which addresses 
replacement of the oxygen sensor in the NGS. The commenters noted that 
in Boeing D626A001-9-04, dated April 2019, the other AWLs specific to 
the NGS show the airplane applicability in terms of the airplanes that 
have the NGS installed, or incorporation of the actions in Boeing 
Service Bulletin 737-47-1002 or Boeing Service Bulletin 737-47-1003. 
The commenters noted that all airplanes on the U.S. registry are 
required to have the NGS system installed, but the commenters expected 
the applicability of AWL No. 47-AWL-09 to be similar to the other AWLs 
included in Boeing D626A001-9-04, dated April 2019. In addition, OKY 
noted that the applicability in AWL No. 47-AWL-09 was listed as all 
airplanes, but in Boeing Fleet Team Digest Document 737NG-FTD-47-19003, 
created on May 8, 2020, the applicability is Model 737NG airplanes 
delivered after June 2019 or modified with a new, improved ASM (air 
separation module) using the procedures in Boeing Service Bulletin 737-
47-1015.
    DAL and the other commenters explained that they contacted Boeing 
regarding this applicability issue. The commenters stated that Boeing 
confirmed that the applicability

[[Page 31606]]

specified in AWL No. 47-AWL-09 is incorrect and Boeing advised that in 
the next revision of Boeing D626A001-9-04, dated April 2019, the 
applicability specified in AWL No. 47-AWL-09 will be updated to include 
airplanes having L/Ns 1820, 1831, 2517, 2620, and subsequent, and all 
airplanes that have incorporated the actions specified in Boeing 
Service Bulletin 737-47-1003. Several of the commenters stated that 
paragraph (g)(13) of the proposed AD should include additional text to 
clarify the applicability of AWL No. 47-AWL-09. The commenters stated 
that by incorporating this information into the final rule, the FAA 
would prevent the need for operators to apply for an AMOC in the 
future.
    The FAA acknowledges the commenters' requests. As mentioned 
previously, Boeing has published Boeing D626A001-9-04, dated November 
2020, and in this revision the applicability specified in AWL No. 47-
AWL-09 was updated and limited to specify only airplanes having L/Ns 
1820, 1831, 2517, 2620, and subsequent, and all airplanes that have 
incorporated the actions specified in Boeing Service Bulletin 737-47-
1003. The FAA has changed the AD to allow the use of Boeing D626A001-9-
04, dated November 2020, which correctly identifies the applicability 
for AWL No. 47-AWL-09.

Request To Clarify the Unsafe Condition

    Boeing, DAL, and CEA requested clarification of the unsafe 
condition specified in the Discussion section of the NPRM and paragraph 
(e) of the proposed AD. The commenters remarked that the wording 
implied that increased flammability in the center fuel tank leads to 
fuel tank explosion. The commenters explained that increased 
flammability alone does not lead to a fuel tank explosion; there must 
also be a concurrent ignition source in the center fuel tank. The 
commenters suggested that it would be more accurate to state that the 
increased flammability combined with an ignition source could lead to a 
fuel tank explosion.
    In addition, the commenters requested that the unsafe condition be 
revised to address the potential loss of engine fuel suction feed 
capability. The commenters pointed out that paragraph (g)(14) of the 
proposed AD specified AWL No. 28-AWL-101, ``Engine Fuel Suction Feed 
Operational Test,'' which would be required to protect the airplane 
from engine flameout during suction feed operations. The commenters 
observed that this proposed action is not associated with the 
prevention of fuel tank ignition.
    The FAA agrees with the commenters' requests based on the reasons 
provided by the commenters. The FAA has revised the Background section 
in this final rule and paragraph (e) of this AD accordingly.

Request To Account for Indirect Costs

    ANA, CEA, DAL, OKY, SWA, and THY expressed concern regarding the 
indirect financial burden associated with accomplishing the actual on-
wing work resulting from the maintenance program or inspection program 
changes that would be required by the proposed AD. DAL specifically 
noted that accomplishment of the oxygen sensor replacement specified in 
AWL No. 47-AWL-09, would require replacement of approximately 130 
oxygen sensors with new oxygen sensors, and based on price quotes from 
the manufacturer of the oxygen sensors, it would cost an additional 
$7,000,000 over the estimated $7,560 per operator specified in the 
Costs of Compliance section of the NPRM. DAL stated that once all 
airplanes have reached the threshold for replacing the oxygen sensor 
the recurring cost would be approximately $2,000,000 per year for this 
task.
    The FAA disagrees with changing the cost of compliance information 
for this AD. The cost information provided in this AD describes only 
the direct costs of the specific actions required by this AD. The FAA 
recognizes that, in doing the actions required by an AD, operators 
might incur incidental costs in addition to the direct costs. The 
extended compliance time previously mentioned could alleviate some of 
these indirect costs, as they would be spread out over 36 months 
instead of occurring within 12 months. Also, Boeing D626A001-9-04, 
dated November 2020, was revised to allow operators to use a repaired 
oxygen sensor in AWL No. 47-AWL-09 and added AWL No. 47-AWL-10 to 
describe the requirements for the method of inspection/repair. As 
identified above in a previous comment, operators now have the option 
to incorporate Boeing D626A001-9-04, dated November 2020, into their 
maintenance and inspection program, thereby reducing the financial 
impact of these requirements. The AD has not been changed in this 
regard.

Request for Clarification of Compliance Time Interval Units

    DAL and CEA requested that the FAA explain why paragraph (g) of the 
proposed AD expressed compliance time intervals in terms of months when 
Boeing D626A001-9-04, dated April 2019, specifies these intervals in 
terms of years. The commenter mentioned it was unclear if an AMOC would 
be required if an operator incorporated the compliance intervals in 
years, as specified in Boeing D626A001-9-04, dated April 2019, instead 
of months, as specified in the proposed AD.
    The FAA disagrees that there is a difference in the calculated 
compliance times described by the commenters. The FAA uses compliance 
times defined in months for required AD actions. Operators have the 
capability of converting years to months when comparing the compliance 
times of required AD actions and the times specified in a 
manufacturer's document. The FAA has determined that operators should 
not be confused when these conversions take place and an operator does 
not need to request an AMOC. Regardless of how an operator records the 
compliance time, within 36 months or within 3 years after the effective 
date of this AD, the compliance times in this AD, Boeing D626A001-9-04, 
dated April 2019, and Boeing D626A001-9-04, dated November 2020, end at 
the same time. The FAA has not changed this AD in regard to this issue.

Request To Include Additional Affected AD in Paragraph (b) of Proposed 
AD

    DAL and CEA requested that AD 2018-20-13, Amendment 39-19447 (83 FR 
52305, October 17, 2018) (AD 2018-20-13), be included in the list of 
affected ADs in paragraph (b) of the proposed AD. DAL and CEA noted 
that paragraphs (i)(1)(i) through (iii) of AD 2018-20-13 require 
incorporation of AWL Nos. 28-AWL-21, 28-AWL-22, and 28-AWL-24 that are 
included in Boeing 737-600/700/700C/800/900/900ER Special Compliance 
Items/Airworthiness Limitations, D626A001-9-04, dated June 2018, into 
an operator's maintenance or inspection program as applicable. DAL and 
CEA suggested that only the latter revision of these AWLs that are 
included in Boeing D626A001-9-04, dated April 2019, should be mandated. 
DAL and CEA stated that the requirements of paragraphs (i)(1)(i) 
through (iii) of AD 2018-20-13 should be terminated when the 
requirements of the proposed AD become effective.
    The FAA agrees with the commenter's request; paragraph (b) of this 
AD has been revised to include AD 2018-20-13 in paragraph (b)(6) of 
this AD, and the subsequent paragraph has been redesignated as 
paragraph (b)(7) of this AD. In addition, this AD has been revised to 
include a new paragraph (j)(6) of this AD to terminate the requirements 
specified in paragraphs (i)(1)(i) through

[[Page 31607]]

(iii) of AD 2018-20-13, and the subsequent paragraph has been 
redesignated as paragraph (j)(7) of this AD.

Request To Change the Subject in Paragraph (d) of the Proposed AD

    DAL and CEA requested that the Subject in paragraph (d) of the 
proposed AD be changed from Air Transport Association (ATA) of America 
Code 71, Powerplant, to ATA Code 28, Fuel. DAL and CEA noted that the 
unsafe condition specified in the proposed AD is associated with 
ignition sources in the fuel tanks and flammability exposure of the 
center fuel tank. The fuel system is the subject of ATA Code 28. 
Previously issued ADs identified in paragraph (b) of the proposed AD, 
which are similar to the proposed AD, have specified ATA Code 28 as the 
subject in paragraph (d).
    The FAA agrees with the commenter's request based on the reasons 
provided by the commenter. Paragraph (d) of this AD has been changed to 
ATA Code 28, Fuel.

Effects of Winglets on Accomplishment of the Proposed Actions

    Aviation Partners Boeing stated that the installation of blended or 
split scimitar winglets per Supplemental Type Certificated (STC) 
ST008300SE does not affect compliance with the proposed actions.
    The FAA agrees that STC ST008300SE does not affect compliance with 
the actions required by this AD. The FAA has not changed this AD in 
this regard.

Reduction of Applicable Airplanes Since NPRM Was Issued

    The FAA has updated the applicability, paragraph (c) of this AD, to 
remove airplanes that were delivered with Boeing 737-600/700/700C/800/
900/900ER Special Compliance Items/Airworthiness Limitations, D626A001-
9-04, dated April 2019, thereby reducing the scope of the affected 
airplanes in this final rule.

Conclusion

    The FAA reviewed the relevant data, considered the comments 
received, and determined that air safety and the public interest 
require adopting this final rule 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 final 
rule.

Related Service Information Under 1 CFR Part 51

    The FAA reviewed Boeing 737-600/700/700C/800/900/900ER Special 
Compliance Items/Airworthiness Limitations, D626A001-9-04, dated April 
2019; and Boeing 737-600/700/700C/800/900/900ER Special Compliance 
Items/Airworthiness Limitations, D626A001-9-04, dated November 2020. 
This service information describes AWLs that include airworthiness 
limitation instructions (ALIs) and critical design configuration 
control limitations (CDCCLs) tasks related to fuel tank ignition 
prevention and the NGS. This service information is reasonably 
available because the interested parties have access to it through 
their normal course of business or by the means identified in the 
ADDRESSES section.

Differences Between This AD and the Service Information

    The ``Description'' column of AWL No. 28-AWL-20 identifies certain 
operational tests. However, the operational test for left center tank 
fuel boost pump relay R54 and right center tank fuel boost pump relay 
R55 is not required for airplanes on which the actions specified in 
paragraph (g)(2)(ii) of AD 2011-20-07, Amendment 39-16818 (76 FR 60710, 
September 30, 2011), have been done, or airplanes that have installed 
STC ST02076LA.
    Paragraph (g) of this AD requires operators to revise their 
existing maintenance or inspection program by incorporating, in part, 
AWL No. 28-AWL-05, ``Wire Separation Requirements for New Wiring 
Installed in Proximity to Wiring That Goes Into the Fuel Tanks'' in 
Boeing D626A001-9-04, dated November 2020. Paragraph (h) of this AD 
allows certain changes to be made to the requirements specified in AWL 
No. 28-AWL-05 as an option.

Costs of Compliance

    The FAA estimates that this AD affects 2,057 airplanes of U.S. 
registry. The FAA estimates the following costs to comply with this AD:
    The FAA determined that revising the existing maintenance or 
inspection program takes an average of 90 work-hours per operator, 
although the agency recognizes that this number may vary from operator 
to operator. In the past, the FAA estimated that this action takes 1 
work-hour per airplane. Since operators incorporate maintenance or 
inspection program changes for their affected fleet(s), the FAA 
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).

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

    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:

[[Page 31608]]

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 adding the following new airworthiness 
directive:

2021-11-24 The Boeing Company: Amendment 39-21586; Docket No. FAA-
2020-0341; Project Identifier 2020-NM-017-AD.

(a) Effective Date

    This airworthiness directive (AD) is effective July 20, 2021.

(b) Affected (Airworthiness Directives) ADs

    This AD affects the ADs specified in paragraphs (b)(1) through 
(7) of this AD.
    (1) AD 2008-06-03, Amendment 39-15415 (73 FR 13081, March 12, 
2008) (AD 2008-06-03).
    (2) AD 2008-10-10 R1, Amendment 39-16164 (75 FR 1529, January 
12, 2010) (AD 2008-10-10 R1).
    (3) AD 2008-17-15, Amendment 39-15653 (73 FR 50714, August 28, 
2008) (AD 2008-17-15).
    (4) AD 2011-18-03, Amendment 39-16785 (76 FR 53317, August 26, 
2011) (AD 2011-18-03).
    (5) AD 2013-15-17, Amendment 39-17533 (78 FR 52838, August 27, 
2013) (AD 2013-15-17).
    (6) AD 2018-20-13, Amendment 39-19447 (83 FR 52305, October 17, 
2018) (AD 2018-20-13).
    (7) AD 2018-20-24, Amendment 39-19458 (83 FR 51815, October 15, 
2018) (AD 2018-20-24).

(c) Applicability

    This AD applies to The Boeing Company Model 737-600, -700, -
700C, -800, -900, and -900ER series airplanes, certificated in any 
category, Line Numbers (L/Ns) 1 through 7596 inclusive, except L/Ns 
7352, 7362, 7377, 7417, 7457, 7522, 7587, and 7592.

(d) Subject

    Air Transport Association (ATA) of America Code 28, Fuel.

(e) Unsafe Condition

    This AD was prompted by significant changes made to the 
airworthiness limitations (AWLs) related to fuel tank ignition 
prevention and the nitrogen generation system (NGS). The FAA is 
issuing this AD to address the development of an ignition source 
inside the fuel tanks and also to prevent increasing the 
flammability exposure of the center fuel tank, which could lead to 
fuel tank explosion and consequent loss of the airplane. The FAA is 
also issuing this AD to prevent increasing the flammability exposure 
of the center fuel tank, which together with an ignition source in 
the fuel tank, could lead to a fuel tank explosion and consequent 
loss of the airplane. In addition, the FAA is issuing this AD to 
address the potential loss of engine fuel suction feed capability, 
which could result in dual engine flameouts, inability to restart 
engines, and consequent forced landing of the airplane.

(f) Compliance

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

(g) Maintenance or Inspection Program Revision

    Within 60 days after the effective date of this AD, revise the 
existing maintenance or inspection program, as applicable, to 
incorporate the applicable information specified in Section A, 
including Subsections A.1., A.2., and A.3, of Boeing 737-600/700/
700C/800/900/900ER Special Compliance Items/Airworthiness 
Limitations, D626A001-9-04, dated April 2019; or November 2020; 
except as provided by paragraph (h) of this AD. The initial 
compliance times for the airworthiness limitation instruction (ALI) 
tasks are within the applicable compliance times specified in in 
paragraphs (g)(1) through (14) of this AD.
    (1) For AWL No. 28-AWL-01, ``External Wires Over Center Fuel 
Tank'': Within 120 months after the date of issuance of the original 
airworthiness certificate or the original export certificate of 
airworthiness, or within 120 months after the most recent inspection 
was performed as specified in AWL No. 28-AWL-01, whichever is later.
    (2) For AWL No. 28-AWL-03, ``Fuel Quantity Indicating System 
(FQIS)--Out Tank Wiring Lightning Shield to Ground Termination'': 
Within 120 months after the date of issuance of the original 
airworthiness certificate or the original export certificate of 
airworthiness, or within 120 months after the most recent inspection 
was performed as specified in AWL No. 28-AWL-03, whichever is later.
    (3) For AWL No. 28-AWL-19, ``Center Tank Fuel Boost Pump 
Automatic Shutoff System'': Within 12 months after the date of 
issuance of the original airworthiness certificate or the original 
export certificate of airworthiness, within 12 months after 
accomplishment of the actions specified in Boeing Service Bulletin 
737-28A1206, or within 12 months after the most recent inspection 
was performed as specified in AWL No. 28-AWL-19, whichever is 
latest. This AWL does not apply to airplanes that have complied with 
paragraph (s) of AD 2011-18-03.
    (4) For AWL No. 28-AWL-20, ``Over-Current and Arcing Protection 
Electrical Design Features Operation--Boost Pump Ground Fault 
Interrupter (GFI)'': Within 12 months after the date of issuance of 
the original airworthiness certificate or the original export 
certificate of airworthiness, within 12 months after accomplishment 
of the actions specified in Boeing Service Bulletin 737-28A1201, or 
within 12 months after the most recent inspection was performed as 
specified in AWL No. 28-AWL-20, whichever is latest.
    (5) For AWL No. 28-AWL-23, ``Center Tank Fuel Boost Pump Power 
Failed On Protection System'': Within 12 months after the date of 
issuance of the original airworthiness certificate or the original 
export certificate of airworthiness, within 12 months after 
accomplishment of the actions specified in Boeing Service Bulletin 
737-28A1248, or within 12 months after the most recent inspection 
was performed as specified in AWL No. 28-AWL-23, whichever is 
latest. This AWL does not apply to airplanes that have complied with 
paragraph (s) of AD 2011-18-03.
    (6) For AWL No. 28-AWL-24, ``Spar Valve Motor Operated Valve 
(MOV) Actuator--Lightning and Fault Current Protection Electrical 
Bond'': Within 72 months after accomplishment of the actions 
specified in Boeing Service Bulletin 737-28A1207, or within 72 
months after the most recent inspection was performed as specified 
in AWL No. 28-AWL-24, whichever is later.
    (7) For AWL No. 28-AWL-29, ``Full Cushion Clamps and Teflon 
Sleeving (If Installed) Installed on Out-of-Tank Wire Bundles 
Installed on Brackets that are Mounted Directly on the Fuel Tanks'': 
For airplanes having line numbers (L/N) 1 through 1754 inclusive, 
within 120 months after accomplishment of the actions specified in 
Boeing Service Bulletin 737-57A1279. For airplanes having L/N 1755 
and subsequent, within 120 months after the date of issuance of the 
original airworthiness certificate or the original export 
certificate of airworthiness, or within 48 months after the 
effective date of this AD, whichever is later.
    (8) For AWL No. 28-AWL-35, ``Fuel Quantity Indicating System 
(FQIS)--Center Fuel Tank In-Tank Component and Wire Harness 
Protection Features-Separation from Center Tank Internal 
Structure'': For airplanes that have incorporated Boeing Service 
Bulletin 737-28-1356, within 120 months after accomplishment of the 
actions specified in Boeing Service Bulletin 737-28-1356, or within 
120 months after the most recent inspection was performed as 
specified in AWL No. 28-AWL-35, whichever is later.
    (9) For AWL No. 28-AWL-37, ``Fuel Quantity Indicating System 
(FQIS)--Built in Test Equipment (BITE) Test'': For airplane L/Ns 
6987 and 7000 and subsequent, within 750 flight hours since the date 
the most recent BITE test was accomplished as specified in AWL No. 
28-AWL-37, or within 750 flight hours after the effective date of 
this AD, whichever is later.
    (10) For AWL No. 47-AWL-04, ``Nitrogen Generation System--
Thermal Switch'': Within 22,500 flight hours after the date of 
issuance of the original airworthiness certificate or the original 
export certificate of airworthiness, within 22,500 flight hours 
after accomplishment of the actions specified in Boeing Service 
Bulletin 737-47-1003, or within 22,500 flight hours after the most 
recent inspection was performed as specified in AWL No. 47-AWL-04, 
whichever is latest.
    (11) For AWL No. 47-AWL-06, ``Nitrogen Generation System (NGS)--
Cross Vent Check Valve'': Within 13,000 flight hours after the date 
of issuance of the original airworthiness certificate or the 
original export certificate of airworthiness, within 13,000 flight 
hours after accomplishment of the actions specified in Boeing 
Service Bulletin 737-47-1003, or within 13,000 flight hours after 
the most

[[Page 31609]]

recent inspection was performed as specified in AWL No. 47-AWL-06, 
whichever is latest.
    (12) For AWL No. 47-AWL-07, ``Nitrogen Generation System (NGS)--
Nitrogen Enriched Air (NEA) Distribution Ducting Integrity'': Within 
6,500 flight hours after the date of issuance of the original 
airworthiness certificate or the original export certificate of 
airworthiness, within 6,500 flight hours after accomplishment of the 
actions specified in Boeing Service Bulletin 737-47-1003, or within 
6,500 flight hours after the most recent inspection was performed as 
specified in AWL No. 47-AWL-07, whichever is latest.
    (13) For AWL No. 47-AWL-09, ``Nitrogen Generation System--Oxygen 
Sensor'': Within 18,000 flight hours after the date of issuance of 
the original airworthiness certificate or the original export 
certificate of airworthiness, or within 18,000 flight hours after 
the most recent replacement was performed as specified in AWL No. 
47-AWL-09, or within 36 months after the effective date of this AD, 
whichever is latest.
    (14) For AWL No. 28-AWL-101, ``Engine Fuel Suction Feed 
Operational Test'': Within 7,500 flight hours or 36 months, 
whichever occurs first, after the date of issuance of the original 
airworthiness certificate or the original export certificate of 
airworthiness; or within 7,500 flight hours or 36 months, whichever 
occurs first, after the most recent inspection was performed as 
specified in AWL No. 28-AWL-101; whichever is later.

(h) Additional Acceptable Exceptions to the AWLs

    As an option, when accomplishing the actions required by 
paragraph (g) of this AD, the changes specified in paragraphs (h)(1) 
through (3) of this AD are acceptable.
    (1) Where AWL No. 28-AWL-05 identifies wire types BMS 13-48, BMS 
13-58, and BMS 13-60, the following wire types are acceptable: MIL-
W-22759/16, SAE AS22759/16 (M22759/16), MIL-W-22759/32, SAE AS22759/
32 (M22759/32), MIL-W-22759/34, SAE AS22759/34 (M22759/34), MIL-W-
22759/41, SAE AS22759/41 (M22759/41), MIL-W-22759/86, SAE AS22759/86 
(M22759/86), MIL-W-22759/87, SAE AS22759/87 (M22759/87), MIL-W-
22759/92, and SAE AS22759/92 (M22759/92); and MIL-C-27500 and NEMA 
WC 27500 cables constructed from these military or SAE specification 
wire types, as applicable.
    (2) Where AWL No. 28-AWL-05 identifies TFE-2X Standard wall for 
wire sleeving, the following sleeving materials are acceptable: 
Roundit 2000NX and Varglas Type HO, HP, or HM.
    (3) Where AWL No. 28-AWL-20 specifies the operational test for 
left center tank fuel boost pump relay R54 and right center tank 
fuel boost pump relay R55, for airplanes that have complied with 
paragraph (g)(2)(ii) of AD 2011-20-07, Amendment 39-16818 (76 FR 
60710, September 30, 2011), or that have installed Supplemental Type 
Certificate (STC) ST02076LA, that action does not apply.

(i) No Alternative Actions, Intervals, or Critical Design Configuration 
Control Limitations (CDCCLs)

    Except as provided in paragraph (h) of this AD, after the 
existing maintenance or inspection program has been revised as 
required by paragraph (g) of this AD, no alternative actions (e.g., 
inspections), intervals, or CDCCLs may be used unless the actions, 
intervals, and CDCCLs are approved as an alternative method of 
compliance (AMOC) in accordance with the procedures specified in 
paragraph (k) of this AD.

(j) Terminating Action for Certain AD Requirements

    Accomplishment of the revision required by paragraph (g) of this 
AD terminates the requirements specified in paragraphs (j)(1) 
through (7) of this AD for that airplane.
    (1) The revision required by the introductory text to paragraph 
(h) and paragraph (h)(1) of AD 2008-06-03.
    (2) All requirements of AD 2008-10-10 R1.
    (3) The revision required by paragraph (g) of AD 2008-17-15.
    (4) The revision required by paragraph (k) of AD 2011-18-03.
    (5) All requirements of AD 2013-15-17.
    (6) The revisions required by paragraphs (i)(1)(i) through (iii) 
of AD 2018-20-13.
    (7) All requirements of AD 2018-20-24.

(k) 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 local Flight Standards District 
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 (l) 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 local flight standards district office/certificate holding 
district office.
    (3) An AMOC that provides an acceptable level of safety may be 
used for any repair, modification, or alteration required by this AD 
if it is approved by The Boeing Company Organization Designation 
Authorization (ODA) that has been authorized by the Manager, Seattle 
ACO Branch, FAA, to make those findings. To be approved, the repair 
method, modification deviation, or alteration deviation must meet 
the certification basis of the airplane, and the approval must 
specifically refer to this AD.

(l) Related Information

    For more information about this AD, contact Christopher Baker, 
Aerospace Engineer, Propulsion Section, FAA, Seattle ACO Branch, 
2200 South 216th St., Des Moines, WA 98198; phone and fax: 206-231-
3552; email: [email protected].

(m) 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 737-600/700/700C/800/900/900ER Special Compliance 
Items/Airworthiness Limitations, D626A001-9-04, dated April 2019.
    (ii) Boeing 737-600/700/700C/800/900/900ER Special Compliance 
Items/Airworthiness Limitations, D626A001-9-04, dated November 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 May 21, 2021.
Lance T. Gant,
Director, Compliance & Airworthiness Division, Aircraft Certification 
Service.
[FR Doc. 2021-12436 Filed 6-14-21; 8:45 am]
BILLING CODE 4910-13-P