Special Conditions: Airbus Model A350-900 Series Airplane; Electronic Flight-Control System: Lateral-Directional and Longitudinal Stability, and Low-Energy Awareness
Final special conditions.
CFR Part: "14 CFR Part 25"
Citation: "79 FR 43233"
Document Number: "Docket No.
"Rules and Regulations"
SUMMARY: These special conditions are issued for Airbus Model A350-900 series airplanes. These airplanes will have a novel or unusual design feature 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.
EFFECTIVE DATE: Effective Date:
FOR FURTHER INFORMATION CONTACT:
SUPPLEMENTARY INFORMATION: On
Lateral-Directional Static Stability
The electronic flight-control system (EFCS) on the A350 airplane, like its predecessors the A320, A330, A340, and A380 airplanes, contains fly-by-wire control laws that can result in neutral lateral-directional static stability; therefore, the conventional requirements in the regulations are not met.
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:
1. Provide additional cues of inadvertent sideslips and skids through control-force changes.
2. Ensure that short periods of unattended operation do not result in any significant changes in yaw or bank angle.
3. Provide predictable roll and yaw response.
4. Provide an acceptable level of pilot attention (workload) to attain and maintain a coordinated turn.
The Flight Test Harmonization Working Group has recommended a rule and advisory-material change for
Longitudinal Static Stability
Static longitudinal stability on airplanes with mechanical links to the pitch-control surface means that a pull force on the controller will result in a reduction in speed relative to the trim speed, and a push force will result in a higher speed than the trim speed. Longitudinal stability is required by the regulations for the following reasons:
1. Speed-change cues are provided to the pilot through increased and decreased forces on the controller.
2. Short periods of unattended control of the airplane do not result in significant changes in attitude, airspeed, or load factor.
3. A predictable pitch response is provided to the pilot.
4. An acceptable level of pilot attention (workload) to attain and maintain trim speed and altitude is provided to the pilot.
5. Longitudinal stability provides gust stability.
The pitch-control movement of the sidestick on the A350 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, which 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 sidestick 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 A350 airplane does not meet the requirements of 14 CFR part 25 for static longitudinal stability.
Low Energy Awareness
Past experience on airplanes fitted with a flight-control system providing neutral longitudinal stability shows 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 system is not sufficient to prevent stall at low-speed excursions below normal operational speeds. Until intervention, the pilot has 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. Recovery from a low-speed excursion may become hazardous 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 Title 14, Code of Federal Regulations (14 CFR) 21.17,
If the Administrator finds that the applicable airworthiness regulations (i.e., 14 CFR part 25) do not contain adequate or appropriate safety standards for the Model A350-900 series airplane because of a novel or unusual design feature, special conditions are prescribed under
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 or similar novel or unusual design feature, the special conditions would also apply to the other model under
In addition to the applicable airworthiness regulations and special conditions, the Model A350-900 series 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
The FAA issues special conditions, as defined in 14 CFR 11.19, under
Novel or Unusual Design Features
The Airbus Model A350-900 series airplane will incorporate the following novel or unusual design features: A flight-control design feature within the normal operational envelope in which sidestick deflection in the roll axis commands roll rate; an operational design that does not comply with the static longitudinal stability requirements of SUBSEC 25.171, 25.173, and 25.175; and a low-energy state where recovery may become hazardous when associated with both a low altitude and performance-limiting conditions.
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.
Since conventional relationships between stick forces and control-surface displacements do not apply to the "load-factor command" flight-control system on the A350 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.
a. Adequate speed control without excessive pilot workload,
b. Acceptable high- and low-speed protection, and
c. Provision for adequate cues to the pilot of significant speed excursions beyond VMO /MMO, and low-speed-awareness flight conditions.
The airplane should 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 may be accomplished as follows:
a. Adequate low-speed/low-thrust cues at low altitude may 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 may be provided by an appropriate warning with the following characteristics:
i. It should be unique, unambiguous, and unmistakable.
ii. It should be active at appropriate altitudes and in appropriate configurations (i.e., at low altitude, in the approach and landing configurations).
iii. It should 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.
iv. It should not be triggered during normal operation, including operation in moderate turbulence for recommended maneuvers at recommended speeds.
v. It should not be cancelable by the pilot other than by achieving a higher-energy state.
vi. There should be 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 of low-energy cues should be evaluated by simulator and flight tests in the whole take-off and landing altitude range for which certification is requested. This would include all relevant combinations of weight, center-of-gravity position, configuration, airbrakes position, and available thrust, including reduced and de-rated take-off thrust operations and engine-failure cases. A sufficient number of tests should be conducted, allowing 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.
Discussion of Comments
Notice of Proposed Special Conditions No. 25-13-14-SC for Airbus Model A350-900 series airplanes was published in the
As discussed above, these special conditions apply to Airbus Model A350-900 series airplanes. Should
This action affects only certain novel or unusual design features on the Airbus Model A350-900 series airplanes. It is not a rule of general applicability.
List of Subjects in 14 CFR Part 25
Aircraft, Aviation safety, Reporting and recordkeeping requirements.
The authority citation for these special conditions is as follows:
Authority: 49 U.S.C. 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 Airbus Model A350-900 series airplanes.
(1) Electronic Flight-Control System: Lateral-Directional and Longitudinal Stability, and Low-Energy Awareness. In lieu of the requirements of SUBSEC 25.171, 25.173, 25.175 and 25.177, the following special conditions apply:
(a) The airplane must be shown to have suitable static lateral, directional, and longitudinal stability in any condition normally encountered in service, including the effects of atmospheric disturbance. The showing of suitable static lateral, directional, and longitudinal stability must be based on the airplane handling qualities, including pilot workload and pilot compensation, for specific test procedures during the flight-test evaluations.
(b) The airplane must provide adequate awareness to the pilot of a low-energy (low speed/low thrust/low height) state when fitted with flight-control laws presenting neutral longitudinal stability significantly below the normal operating speeds. "Adequate awareness" means warning information must be provided to alert the crew of unsafe operating conditions and to enable them to take appropriate corrective action.
(c) The static directional stability (as shown by the tendency to recover from a skid with the rudder free) must be positive for any landing gear and flap position, and symmetrical power condition, at speeds from 1.13 VSR1, up to VFE, VLE, or VFC /MFC (as appropriate).
(d) The static lateral stability (as shown by the tendency to raise the low wing in a sideslip with the aileron controls free) for any landing gear and wing-flap position, and symmetric power condition, may not be negative at any airspeed (except that speeds higher than VFE need not be considered for wing-flaps-extended configurations, nor speeds higher than VLE for landing-gear-extended configurations) in the following airspeed ranges:
(i) From 1.13 VSR1 to VMO /MMO.
(ii) From VMO /MMO to VFC /MFC, unless the divergence is -
(2) Easily recognizable by the pilot; and
(3) Easily controllable by the pilot.
(e) In straight, steady sideslips over the range of sideslip angles appropriate to the operation of the airplane, but not less than those obtained with one-half of the available rudder-control movement (but not exceeding a rudder-control force of 180 pounds), rudder-control movements and forces must be substantially proportional to the angle of sideslip in a stable sense; and the factor of proportionality must lie between limits found necessary for safe operation. This requirement must be met for the configurations and speeds specified in paragraph (c) of this section.
(f) For sideslip angles greater than those prescribed by paragraph (e) of this section, 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 wing-flap positions for the range of operating speeds and power conditions appropriate to each landing-gear and wing-flap position with all engines operating.
[FR Doc. 2014-17575 Filed 7-24-14;
BILLING CODE 4910-13-P
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