Document ID: FAA-2021-1034-0001
Agency: faa
Document Type: Proposed Rule
Title: Special Conditions: Airbus Model A321neo XLR Airplane; Electronic Flight-Control System: Lateral-Directional and Longitudinal Stability, and Low-Energy Awareness
Posted Date: 2023-11-03T04:00Z

[Federal Register Volume 88, Number 212 (Friday, November 3, 2023)]
[Proposed Rules]
[Pages 75517-75520]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2023-24312]

-----------------------------------------------------------------------

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2021-1034; Notice No. 25-23-02-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: Notice of proposed special conditions.

-----------------------------------------------------------------------

SUMMARY: This action proposes special conditions for the Airbus Model 
A321neo XLR airplane. The airplane will have a novel or unusual design 
feature when compared to the state of technology envisioned in the 
airworthiness standards for transport-category airplanes. 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: Send comments on or before December 4, 2023.

ADDRESSES: Send comments identified by Docket No. FAA-2021-1034 using 
any of the following methods:
    Federal eRegulations Portal: Go to https://www.regulations.gov/ and 
follow the online instructions for sending your comments 
electronically.
    Mail: Send comments to Docket Operations, M-30, U.S. Department of 
Transportation (DOT), 1200 New Jersey Avenue SE, Room W12-140, West 
Building Ground Floor, Washington, DC 20590-0001.
    Hand Delivery or Courier: Take comments to Docket Operations in 
Room W12-140 of the West Building Ground Floor at 1200 New Jersey 
Avenue SE, Washington, DC, between 9 a.m. and 5 p.m., Monday through 
Friday, except Federal holidays.
    Fax: Fax comments to Docket Operations at 202-493-2251.
    Docket: Background documents or comments received may be read at 
https://www.regulations.gov/ at any time. Follow the online 
instructions for accessing the docket or go to Docket Operations in 
Room W12-140 of the West Building Ground Floor at 1200 New Jersey 
Avenue SE, Washington, DC, between 9 a.m. and 5 p.m., Monday through 
Friday, except Federal holidays.

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: 

Comments Invited

    The FAA invites interested people to take part in this rulemaking 
by sending written comments, data, or views. The most helpful comments 
reference a specific portion of the proposed special conditions, 
explain the reason for any recommended change, and include supporting 
data.
    Certification of the Airbus Model A321neo XLR airplane is currently 
scheduled for December 2023. The substance of these special conditions, 
in all material respects, has been subject to the notice and public-
comment procedure in several prior instances. Therefore, because a 
delay would significantly affect the applicant's installation of the 
new or unusual feature, and delay certification of the airplane, the 
FAA is reducing the public-comment period to 30 days.
    The FAA will consider all comments received by the closing date for 
comments, and will consider comments filed late if it is possible to do 
so without incurring delay. The FAA may change these special conditions 
based on the comments received.

Privacy

    Except for Confidential Business Information (CBI) as described in 
the following paragraph, and other information as described in title 
14, Code of Federal Regulations (14 CFR) 11.35, the FAA will post all 
comments received without change to https://www.regulations.gov/, 
including any personal information you provide. The FAA will also post 
a report summarizing each substantive verbal contact received about 
these special conditions.

Confidential Business Information

    Confidential Business Information (CBI) is commercial or financial 
information that is both customarily and actually treated as private by 
its owner. Under the Freedom of Information Act (FOIA) (5 U.S.C. 552), 
CBI is exempt from public disclosure. If your

[[Page 75518]]

comments responsive to these special conditions contain commercial or 
financial information that is customarily treated as private, that you 
actually treat as private, and that is relevant or responsive to these 
special conditions, it is important that you clearly designate the 
submitted comments as CBI. Please mark each page of your submission 
containing CBI as ``PROPIN.'' The FAA will treat such marked 
submissions as confidential under the FOIA, and the indicated comments 
will not be placed in the public docket of these special conditions. 
Send submissions containing CBI to the individual listed in the For 
Further Information Contact section below. Comments the FAA receives, 
which are not specifically designated as CBI, will be placed in the 
public docket for these special conditions.

Background

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

Static Lateral-Directional Stability

    The EFCS on the Airbus Model A321neo XLR airplane contains fly-by-
wire control laws that can result in neutral static lateral-directional 
stability. Therefore, the airplane does not meet the conventional 
requirements in the regulations which require positive static lateral-
directional stability.
    Positive static directional stability is defined as the tendency to 
recover from a skid with the rudder free. Positive static lateral 
stability is defined as the tendency to raise the low wing in a 
sideslip with the aileron controls free. These control criteria are 
intended to accomplish the following:
    (a) Provide additional cues of inadvertent sideslips and skids 
through control-force changes.
    (b) Ensure that short periods of unattended operation do not result 
in any significant changes in yaw or bank angle.
    (c) Provide predictable roll and yaw response.
    (d) Provide an acceptable level of pilot attention (workload) to 
attain and maintain a coordinated turn.
    Neutral static lateral-directional stability, conversely, means 
that the airplane will stay in its new attitude when disturbed by an 
external force (e.g., crosswind). Therefore, the regulations under 14 
CFR 25.171 for the Airbus Model A321neo XLR airplane are inadequate.

Static Longitudinal Stability

    Static longitudinal stability on airplanes with mechanical links to 
the pitch-control surface means that a pull force on the controller 
results in a reduction in speed relative to the trim speed, and a push 
force results in higher than trim speed. Longitudinal stability is 
required by the regulations for the following reasons:
    (a) Speed-change cues are provided to the pilot through increased 
and decreased forces on the controller.
    (b) Short periods of unattended control of the airplane do not 
result in significant changes in attitude, airspeed, or load factor.
    (c) A predictable pitch response is provided to the pilot.
    (d) An acceptable level of pilot attention (workload) to attain and 
maintain trim speed and altitude is provided to the pilot.
    (e) Longitudinal stability provides gust stability.
    The pitch-control movement of the side stick on the Airbus Model 
A321neo XLR airplane is designed to be a normal load factor, or ``g'' 
command, that results in an initial movement of the elevator surface to 
attain the commanded load factor that is then followed by integrated 
movement of the stabilizer and elevator to automatically trim the 
airplane to a neutral, 1g, stick-free stability. The flight path 
commanded by the initial side-stick input will remain stick-free until 
the pilot provides another command. This control function is applied 
during ``normal'' control law within the speed range, from initiation 
of the angle-of-attack protection limit, V[alpha]prot, to 
VMO/MMO. Once outside this speed range, the 
control laws introduce the conventional longitudinal static stability 
as described above.
    As a result of neutral static stability, the Airbus Model A321neo 
XLR airplane does not meet the regulatory requirements for static 
longitudinal stability during normal operations.

Low Energy Awareness

    Past experience on airplanes fitted with a flight-control system 
providing neutral longitudinal stability reveals insufficient feedback 
cues to the pilot of excursion below normal operational speeds. The 
maximum angle-of-attack protection system limits the airplane angle of 
attack and prevents stall during normal operating speeds, but this EFCS 
is not sufficient to prevent stall at low-speed excursions below normal 
operational speeds. Until intervention, there are no stability cues 
because the aircraft remains trimmed. Additionally, feedback from the 
pitching moment due to thrust variation is reduced by the flight-
control laws. Low-speed excursions may become more hazardous without 
the typical longitudinal stability, and recovery is more difficult when 
the low-speed situation is associated with a low altitude, and with the 
engines at low thrust or with performance-limiting conditions.

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.

Novel or Unusual Design Features

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

[[Page 75519]]

Discussion

    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.
    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, there is recognition that 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 
and the 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.
    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.

Applicability

    As discussed above, these special conditions apply to Airbus Model 
A321neo XLR airplane. Should Airbus apply later for a change to the 
type certificate to include another model incorporating the same novel 
or unusual design feature, the special conditions would apply to that 
model as well.

Conclusion

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

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, and 
44704.

The Proposed Special Conditions

    Accordingly, the Federal Aviation Administration (FAA) proposes the 
following special conditions as part of the type certification basis 
for Airbus Model A321neo XLR airplane. These special conditions are 
issued in lieu of the paragraphs of 14 CFR part 25 referenced below.

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:
    (2) 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.
    (3) 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.

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

[[Page 75520]]

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 should 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 should be provided by an appropriate 
warning with the following characteristics. The low-energy awareness 
should:
    (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 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 in Kansas City, Missouri, on October 27, 2023.
Patrick R. Mullen,
Manager, Technical Policy Branch, Policy and Standards Division, 
Aircraft Certification Service.
[FR Doc. 2023-24312 Filed 11-2-23; 8:45 am]
BILLING CODE 4910-13-P