[Federal Register Volume 79, Number 143 (Friday, July 25, 2014)]
[Rules and Regulations]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2014-17575]
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.
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.
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-
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
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
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
As a result of neutral static stability, the A350 airplane does not
meet the requirements of 14 CFR part 25 for static longitudinal
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-
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-
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
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.
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
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
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
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.
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.
This action affects only certain novel or unusual design features
on the Airbus Model A350-900 series airplanes. It is not a rule of
List of Subjects in 14 CFR Part 25
Aircraft, Aviation safety, Reporting and recordkeeping
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
(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
(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-
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
Issued in Renton, Washington, on July 9, 2014.
Jeffrey E. Duven,
Manager, Transport Airplane Directorate, Aircraft Certification
[FR Doc. 2014-17575 Filed 7-24-14; 8:45 am]
BILLING CODE 4910-13-P