[Federal Register Volume 89, Number 66 (Thursday, April 4, 2024)]
[Rules and Regulations]
[Pages 23507-23510]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-07139]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2021-1034; Special Conditions No. 25-857-SC]


Special Conditions: Airbus Model A321neo XLR Airplane; Electronic 
Flight-Control System: Lateral-Directional and Longitudinal Stability, 
and Low-Energy Awareness

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final special conditions.

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SUMMARY: These special conditions are issued for the Airbus Model 
A321neo XLR airplane. This airplane will have a novel or unusual design 
feature when compared to the state of technology envisioned in the 
applicable airworthiness standards. This design feature is an 
electronic flight-control system (EFCS) associated with lateral-
directional and longitudinal stability, and low-energy awareness. The 
applicable airworthiness regulations do not contain adequate or 
appropriate safety standards for this design feature. These special 
conditions contain the additional safety standards that the 
Administrator considers necessary to establish a level of safety 
equivalent to that established by the existing airworthiness standards.

DATES: Effective April 4, 2024.

FOR FURTHER INFORMATION CONTACT: Troy Brown, Performance and 
Environment Unit, AIR-621A, Technical Policy Branch, Policy and 
Standards Division, Aircraft Certification Service, Federal Aviation 
Administration, 1801 S Airport Rd., Wichita, KS 67209-2190; telephone 
and fax 405-666-1050; email [email protected].

SUPPLEMENTARY INFORMATION:

Background

    On September 16, 2019, Airbus applied for an amendment to Type 
Certificate No. A28NM to include the new Model A321neo XLR airplane. 
This airplane is a twin-engine, transport-category airplane, with 
seating for 244 passengers, and a maximum takeoff weight of 222,000 
pounds.

Type Certification Basis

    Under the provisions of 14 CFR 21.101, Airbus must show that the 
Model A321neo XLR airplane meets the applicable provisions of the 
regulations listed in Type Certificate No. A28NM, or the applicable 
regulations in effect on the date of application for the change, except 
for earlier amendments as agreed upon by the FAA.
    If the Administrator finds that the applicable airworthiness 
regulations (e.g., 14 CFR part 25) do not contain adequate or 
appropriate safety standards for the Airbus Model A321neo XLR airplane 
because of a novel or unusual design feature, special conditions are 
prescribed under the provisions of Sec.  21.16.
    Special conditions are initially applicable to the model for which 
they are issued. Should the type certificate for that model be amended 
later to include any other model that incorporates the same novel or 
unusual design feature, or should any other model already included on 
the same type certificate be modified to incorporate the same novel or 
unusual design feature, these special conditions would also apply to 
the other model under Sec.  21.101.
    In addition to the applicable airworthiness regulations and special 
conditions, the Airbus Model A321neo XLR airplane must comply with the 
fuel-vent and exhaust-emission requirements of 14 CFR part 34, and the 
noise-certification requirements of 14 CFR part 36.
    The FAA issues special conditions, as defined in Sec.  11.19, in 
accordance with Sec.  11.38, and they become part of the type 
certification basis under Sec.  21.101.

[[Page 23508]]

Novel or Unusual Design Feature

    The Airbus Model A321neo XLR airplane will incorporate the 
following novel or unusual design feature:
    An EFCS associated with lateral-directional and longitudinal 
stability, and low-energy awareness.

Proposed Special Conditions

    The FAA issued Notice of Proposed Special Conditions No. FAA-2021-
1034, which was published in the Federal Register on November 3, 2023 
(88 FR 75517).
    In that document, the FAA explained that the Airbus' proposed 
A321neo XLR includes an EFCS, and that the control laws of that system 
can result in neutral static lateral-directional stability and neutral 
static longitudinal stability, insufficient feedback to the flightcrew 
from the pitching moment, and insufficient awareness that the airplane 
is in a low-energy state. The FAA therefore proposed that the 
applicable airworthiness regulations are inadequate or inappropriate to 
address these issues and proposed special conditions to address them.
    The FAA proposed that in the absence of positive lateral stability, 
the curve of lateral control-surface deflections against sideslip angle 
should be, in a conventional sense and reasonably in harmony with, 
rudder deflection during steady-heading sideslip maneuvers.
    The FAA further proposed that because conventional relationships 
between stick forces and control-surface displacements do not apply to 
the ``load-factor command'' flight-control system on the Airbus Model 
A321neo XLR airplane, longitudinal stability characteristics should be 
evaluated by assessing the airplane's handling qualities during 
simulator and flight-test maneuvers appropriate to operation of the 
airplane. Additionally, under icing and non-icing conditions there may 
be a difference in full pedal deflection. This difference may result in 
changes to testing before reaching full pedal deflection, and these 
special conditions account for these differences.
    The airplane must provide adequate awareness cues to the pilot of a 
low-energy (low-speed/low-thrust/low-height) state to ensure that the 
airplane retains sufficient energy to recover when flight-control laws 
provide neutral longitudinal stability significantly below the normal 
operating speeds. ``Adequate awareness'' means that information must be 
provided to alert the crew of unsafe operating conditions and to enable 
them to take appropriate corrective action. Testing of these awareness 
cues should occur by simulator and flight test in the operational 
flight envelope for which certification is requested. Testing should 
include a sufficient number of tests to allow the level of energy 
awareness, and the effects of energy-management errors, to be assessed.

Discussion of Comments and Final Special Conditions

    Airbus Commercial Aircraft (Airbus) and The Boeing Company (Boeing) 
submitted comments on the same provision of the proposed special 
conditions.
    The Static Lateral-Directional Stability section of the proposed 
special conditions required the applicant to conduct, in icing 
conditions, steady heading sideslip maneuvers in several 
configurations. The proposed conditions would have required these 
sideslip maneuvers to be conducted ``over the range of sideslip angles 
appropriate to the operation of the airplane, but not less than those 
obtained with one half of available rudder control input.''
    Airbus and Boeing each recommended that these maneuvers be 
conducted with full pedal deflection but recommended different 
approaches to implement that change.
    Airbus requested that the FAA add a note stating that these 
maneuvers will be continued beyond the sideslip angles appropriate for 
normal operation of the airplane and demonstrate that full pedal travel 
can be safely applied. Airbus stated that deflecting the pedals as much 
as practicable in icing conditions would provide a better coverage of 
the intent of Sec.  25.21(g) regarding Sec.  25.177. Further, Airbus 
stated that the addition of this note would align FAA and EASA 
standards.
    Boeing recommended that the FAA revise the special conditions to 
require Airbus to conduct these sideslips ``up to the angle at which 
full rudder control is used or a rudder control force of 180 pounds is 
obtained.'' Boeing said this change would be consistent with the 
language of paragraph 4.15.2.3 of AC 25-25A, Performance and Handling 
Characteristics in Icing Conditions.
    AC 25-25A provides an acceptable means of showing compliance with 
certain requirements of part 25 of 14 CFR related to airplane 
performance and handling characteristics in icing conditions. To 
address static lateral directional stability, the AC provides, as 
examples of an acceptable test program, that the applicant may conduct 
steady heading sideslips, in certain configurations, including ``to 
full rudder authority, 180 pounds of rudder pedal force, or full 
lateral control authority.'' Paragraph 4.15.2.3.
    The FAA agrees with the commenters that full-pedal deflection meets 
the intent of Sec.  25.21(g) and aligns with guidance in the referenced 
AC. The FAA also agrees that this approach is harmonized with EASA's 
certification approach \2\ to this issue. The FAA finds that it is 
unnecessary to revise the condition as suggested by Boeing, and that 
the language provided by Airbus, with minor revision by the FAA,\3\ is 
sufficient to address this issue.
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    \2\ EASA Certification Review Item (CRI) B-06, ``Flight in Icing 
Conditions'', issue 2, April 11, 2013.
    \3\ Under the U.S. regulatory system, notes are explanatory 
rather than mandatory. See, e.g., section 7.5 of the Document 
Drafting Handbook (Aug. 2018 Edition, Rev. 2.1, dated Oct. 2023). 
Therefore, in the final special conditions, the recommended language 
is no longer a ``note,'' and the commenter's ``will'' is a ``must.''
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    These final special conditions correct minor discrepancies in the 
numbering of the proposed special conditions. Also, the proposed 
special conditions related to low energy awareness contained three 
instances of ``should.'' The FAA has revised these to ``must'' in these 
final special conditions, for enforceability and for consistency with 
the expectations of the FAA and the applicant.
    Other than these foregoing changes, these special conditions are 
adopted as proposed. The special conditions contain the additional 
safety standards that the Administrator considers necessary to 
establish a level of safety equivalent to that established by the 
existing airworthiness standards.

Applicability

    As discussed above, these special conditions are applicable to the 
Airbus Model A321neo XLR airplane. Should Airbus apply at a later date 
for a change to the type certificate to include another model 
incorporating the same novel or unusual design feature, these special 
conditions would apply to that model as well.
    Under standard practice, the effective date of final special 
conditions would be 30 days after the date of publication in the 
Federal Register. However, as the certification date for the Airbus 
Model A321neo XLR is imminent, the FAA finds that good cause exists to 
make these special conditions effective upon publication.

Conclusion

    This action affects only certain novel or unusual design features 
on one model series of airplane. It is not a rule of general 
applicability.

[[Page 23509]]

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

Authority Citation

    The authority citation for these special conditions is as follows:

    Authority:  49 U.S.C. 106(f), 106(g), 40113, 44701, 44702, 
44704.

The Special Conditions

    [squf] Accordingly, pursuant to the authority delegated to me by 
the Administrator, the following special conditions are issued as part 
of the type certification basis for the Airbus Model A321neo XLR 
airplane.

Static Lateral-Directional Stability

    (a) In lieu of compliance with Sec.  25.171, the airplane must have 
lateral and directional stability characteristics in accordance with 
Sec.  25.177. In addition, both suitable stability and suitable control 
feel are required in any condition normally encountered in service.
    (b) In lieu of compliance with Sec.  25.177(c), the following 
requirement must be met for the configurations and speed specified in 
Sec.  25.177(a):
    (1) In straight, steady sideslips over the range of sideslip angles 
appropriate to the operation of the airplane, the directional control 
movements and forces must be substantially proportional to the angle of 
sideslip in a stable sense. The factor of proportionality must lie 
between limits found necessary for safe operation. During these 
straight, steady sideslips, necessary lateral control movements and 
forces must not be in the unstable sense with the exception of speeds 
above Vmo/Mmo per Sec.  25.177(b)(2). The range 
of sideslip angles evaluated must include those sideslip angles 
resulting from the lesser of:
    (i) One-half of the available directional (pedal) control input; 
and
    (ii) A directional (pedal) control force of 180 pounds.
    (c) In lieu of compliance with Sec.  25.177(d), the following 
requirements must be met:
    (1) In non-icing conditions, for sideslip angles greater than those 
prescribed by Sec.  25.177(a), up to the angle at which full rudder 
control is used or a rudder control force of 180 pounds is obtained, 
the rudder control forces may not reverse, and increased rudder 
deflection must be needed for increased angles of sideslip. Compliance 
with this requirement must be shown using straight, steady sideslips, 
unless full lateral control input is achieved before reaching either 
full rudder control input or a rudder control force of 180 pounds; a 
straight, steady sideslip need not be maintained after achieving full 
lateral control input. This requirement must be met at all approved 
landing gear and flap positions for the range of operating speeds and 
power conditions appropriate to each landing gear and flap position 
with all engines operating.
    (2) In icing conditions, in the configurations listed below, trim 
the airplane at the specified speed and conduct steady heading 
sideslips over the range of sideslip angles appropriate to the 
operation of the airplane but not less than those obtained with one-
half of available rudder control input.
    (i) High lift devices retracted configuration: trim at best rate of 
climb speed but not less than minimum all engines operating climb speed 
defined for icing conditions.
    (ii) Lowest lift take-off configuration: trim at the all-engines 
operating initial climb speed defined for icing conditions.
    (iii) Landing configurations: trim at minimum landing speed defined 
for icing conditions.
    The steady heading sideslip maneuver must be continued beyond 
sideslip angles appropriate for normal operation of the airplane to 
demonstrate full pedal can be safely applied unless justification for 
smaller input is provided (e.g., heavy buffet that would deter the 
pilot from further deflecting the pedals and would make investigations 
to full pedal a potential flight test safety concern, or pedal input 
required for normal operations significantly smaller than full pedal).

Longitudinal Stability

    In lieu of compliance with the requirements of Sec. Sec.  25.171, 
25.173, and 25.175, the airplane must be shown to have longitudinal 
stability characteristics in accordance with the following conditions. 
In addition, both suitable stability and suitable control feel are 
required in any condition normally encountered in service, including 
the effects of atmospheric disturbance.
    (a) Strong positive static longitudinal stability (1 pound per 6 
knots applied through the sidestick) must be present which provides 
adequate awareness cues to the crew that the speed is above 
Vmo/Mmo or below the minimum speed for hands-free 
stabilized flight. Static longitudinal characteristics must be shown to 
be suitable based on the airplane handling qualities, including an 
evaluation of pilot workload and pilot compensation, for specific test 
procedures during the flight-test evaluations. These characteristics 
must be shown for appropriate combinations of airplane configuration 
(i.e., flaps extended or retracted, gear deployed or stowed) and thrust 
for climb, cruise, approach, landing, and go-around.
    (1) Release of the controller at speeds above Vmo/
Mmo, or below the minimum speed for hands-free stabilized 
flight, must produce a prompt recovery towards normal operating speeds 
without resulting in a hazardous condition.
    (2) The design must not allow a pilot to re-trim the controller 
forces resulting from this stability.

Low Energy Awareness

    The airplane must provide adequate awareness cues to the pilot of a 
low-energy (low-speed/low-thrust/low-height) state to ensure that the 
airplane retains sufficient energy to recover when flight-control laws 
provide neutral longitudinal stability significantly below the normal 
operating speeds. This must be accomplished as follows:
    (a) Adequate low speed/low thrust cues at low altitude should be 
provided by a strong positive static stability force gradient (1 pound 
per 6 knots applied through the sidestick), or
    (b) The low energy awareness must be provided by an appropriate 
warning with the following characteristics. The low-energy awareness 
must:
    (1) Be unique, unambiguous, and unmistakable.
    (2) Be active at appropriate altitudes and in appropriate 
configurations (i.e., at low altitude, in the approach and landing 
configurations).
    (3) Be sufficiently timely to allow recovery to a stabilized flight 
condition inside the normal flight envelope while maintaining the 
desired flight path and without entering the flight controls angle-of-
attack protection mode.
    (4) Not be triggered during normal operation, including operation 
in moderate turbulence for recommended maneuvers at recommended speeds.
    (5) Not be cancelable by the pilot other than by achieving a higher 
energy state.
    (6) Have an adequate hierarchy among the various warnings so that 
the pilot is not confused and led to take inappropriate recovery action 
if multiple warnings occur.
    Global energy awareness and non-nuisance on low-energy cues must be 
evaluated by simulator and flight tests in the whole take-off and 
landing altitude range for which certification is requested. This 
includes all relevant combinations of weight, center-of-gravity 
position, configuration, airbrakes position, and available thrust, 
including

[[Page 23510]]

reduced and derated take-off thrust operations and engine-failure 
cases. The tests must assess the level of energy awareness, and the 
effects of energy-management errors.

    Issued in Kansas City, Missouri, on March 28, 2024.
Patrick R. Mullen,
Manager, Technical Innovation Policy Branch, Policy and Innovation 
Division, Aircraft Certification Service.
[FR Doc. 2024-07139 Filed 4-3-24; 8:45 am]
BILLING CODE 4910-13-P