# 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.
**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 § 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 § 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 § 11.19, in accordance with § 11.38, and they become part of the type certification basis under § 21.101.
**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 § 25.21(g) regarding § 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 § 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.
[^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.”
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.
**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**
▪ 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 § 25.171, the airplane must have lateral and directional stability characteristics in accordance with § 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 § 25.177(c), the following requirement must be met for the configurations and speed specified in § 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 <sub>mo</sub> /M <sub>mo</sub> per § 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 § 25.177(d), the following requirements must be met:
(1) In non-icing conditions, for sideslip angles greater than those prescribed by § 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 §§ 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 <sub>mo</sub> /M <sub>mo</sub> 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 <sub>mo</sub> /M <sub>mo</sub> , 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 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.