Rule2024-07139
Special Conditions: Airbus Model A321neo XLR Airplane; Electronic Flight-Control System: Lateral-Directional and Longitudinal Stability, and Low-Energy Awareness
Primary source
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Published
April 4, 2024
Effective
April 4, 2024
Issuing agencies
Transportation DepartmentFederal Aviation Administration
Abstract
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.
Full Text
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<title>Federal Register, Volume 89 Issue 66 (Thursday, April 4, 2024)</title>
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[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 [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[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 <a href="/cdn-cgi/l/email-protection#2a5e584553044b044858455d446a4c4b4b044d455c"><span class="__cf_email__" data-cfemail="ff8b8d9086d19ed19d8d908891bf999e9ed1989089">[email protected]</span></a>.
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 V<INF>mo</INF>/M<INF>mo</INF> 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
V<INF>mo</INF>/M<INF>mo</INF> 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 V<INF>mo</INF>/
M<INF>mo</INF>, 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
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