Document ID: FAA-2013-0904-0003
Agency: faa
Document Type: Rule
Title: Special Conditions: Airbus Model A350-900 Series Airplane; Electronic Flight-Control System; Lateral-Directional and Longitudinal Stability, and Low-Energy Awareness
Posted Date: 2014-07-25T04:00Z

[Federal Register Volume 79, Number 143 (Friday, July 25, 2014)]
[Rules and Regulations]
[Pages 43233-43236]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2014-17575]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2013-0904; Special Conditions No. 25-542-SC]

Special Conditions: Airbus Model A350-900 Series 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 Airbus Model A350-900 
series airplanes. These airplanes will

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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.

DATES: Effective Date: August 25, 2014.

FOR FURTHER INFORMATION CONTACT: Joe Jacobsen, FAA, Airplane and 
Flightcrew Interface Branch, ANM-111, Transport Airplane Directorate, 
Aircraft Certification Service, 1601 Lind Avenue SW., Renton, 
Washington 98057-3356; telephone (425) 227-2011; facsimile (425) 227-
1320.

SUPPLEMENTARY INFORMATION:

Background

    On August 25, 2008, Airbus applied for a type certificate for their 
new Model A350-900 series airplane. Later, Airbus requested, and the 
FAA approved, an extension to the application for FAA type 
certification to November 15, 2009. The Model A350-900 series airplane 
has a conventional layout with twin wing-mounted Rolls-Royce Trent XWB 
engines. It features a twin-aisle, 9-abreast, economy-class layout, and 
accommodates side-by-side placement of LD-3 containers in the cargo 
compartment. The basic Model A350-900 airplane configuration 
accommodates 315 passengers in a standard two-class arrangement. The 
design cruise speed is Mach 0.85 with a maximum take-off weight of 
602,000 lbs.

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 Sec.  25.177, static lateral-
directional stability. This harmonized text will form the basis for 
these special conditions.

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, Airbus 
must show that the Model A350-900 series airplane meets the applicable 
provisions of 14 CFR part 25, as amended by Amendments 25-1 through 25-
129.
    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 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 or similar 
novel or unusual design feature, the special conditions would also 
apply to the other model under Sec.  21.101.
    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 FAA must issue 
a finding of regulatory adequacy under Sec.  611 of Public Law 92-574, 
the ``Noise Control Act of 1972.''
    The FAA issues special conditions, as defined in 14 CFR 11.19, 
under Sec.  11.38, and they become part of the type-certification basis 
under Sec.  21.17(a)(2).

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

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design that does not comply with the static longitudinal stability 
requirements of Sec. Sec.  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.

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.
    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. Airbus may accomplish this by 
using the Handling Qualities Rating Method presented in Appendix 5 of 
the Flight Test Guide, AC 25-7C, or an acceptable alternative method. 
Important considerations are as follows:
    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 Federal Register 
on January 14, 2014 (79 FR 2384). No comments were received, and the 
special conditions are adopted as proposed.

Applicability

    As discussed above, these special conditions apply to Airbus Model 
A350-900 series airplanes. 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 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 Sec. Sec.  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 -
    (1) Gradual;
    (2) Easily recognizable by the pilot; and
    (3) Easily controllable by the pilot.

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    (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.

    Issued in Renton, Washington, on July 9, 2014.
Jeffrey E. Duven,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 2014-17575 Filed 7-24-14; 8:45 am]
BILLING CODE 4910-13-P