Document ID: FAA-2013-0904-0001
Agency: faa
Document Type: Proposed 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: 2014-01-14T05:00Z

[Federal Register Volume 79, Number 9 (Tuesday, January 14, 2014)]
[Proposed Rules]
[Pages 2384-2387]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2014-00449]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2013-0904; Notice No. 25-13-14-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: Notice of proposed special conditions.

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SUMMARY: This action proposes special conditions for the Airbus Model 
A350-900 series airplanes. These airplanes will have a novel or unusual 
design feature(s) 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 proposed 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: Send your comments on or before February 28, 2014.

ADDRESSES: Send comments identified by docket number FAA-2013-0904 
using any of the following methods:
     Federal eRegulations Portal: Go to http://www.regulations.gov/ and follow

[[Page 2385]]

the online instructions for sending your comments electronically.
     Mail: Send comments to Docket Operations, M-30, U.S. 
Department of Transportation (DOT), 1200 New Jersey Avenue SE., Room 
W12-140, West Building Ground Floor, Washington, DC 20590-0001.
     Hand Delivery or Courier: Take comments to Docket 
Operations in Room W12-140 of the West Building Ground Floor at 1200 
New Jersey Avenue SE., Washington, DC, between 9 a.m. and 5 p.m., 
Monday through Friday, except federal holidays.
     Fax: Fax comments to Docket Operations at 202-493-2251.
    Privacy: The FAA will post all comments it receives, without 
change, to http://www.regulations.gov/, including any personal 
information the commenter provides. Using the search function of the 
docket Web site, anyone can find and read the electronic form of all 
comments received into any FAA docket, including the name of the 
individual sending the comment (or signing the comment for an 
association, business, labor union, etc.). DOT's complete Privacy Act 
Statement can be found in the Federal Register published on April 11, 
2000 (65 FR 19477-19478), as well as at http://DocketsInfo.dot.gov/.
    Docket: Background documents or comments received may be read at 
http://www.regulations.gov/ at any time. Follow the online instructions 
for accessing the docket or go to the Docket Operations in Room W12-140 
of the West Building Ground Floor at 1200 New Jersey Avenue SE., 
Washington, DC, between 9 a.m. and 5 p.m., Monday through Friday, 
except federal holidays.

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: 

Comments Invited

    We invite interested people to take part in this rulemaking by 
sending written comments, data, or views. The most helpful comments 
reference a specific portion of the special conditions, explain the 
reason for any recommended change, and include supporting data.
    We will consider all comments we receive on or before the closing 
date for comments. We may change these proposed special conditions 
based on the comments we receive.

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 
June 28, 2009. The Model A350-900 series 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 series configuration will accommodate 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. Airbus proposes the 
Model A350-900 series to be certified for extended operations (ETOPS) 
beyond 180 minutes at entry into service for up to a 420-minute maximum 
diversion time.

Lateral-Directional Static Stability

    The electronic flight control system (EFCS) on the A350, like its 
predecessors the A320, A330, A340, and A380, 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:
    (a) Provide additional cues of inadvertent sideslips and skids 
through control force changes.
    (b) Ensure that short periods of unattended operation do not result 
in any significant changes in yaw or bank angle.
    (c) Provide predictable roll and yaw response.
    (d) Provide 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 proposed 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:
    (a) Speed change cues are provided to the pilot through increased 
and decreased forces on the controller.
    (b) Short periods of unattended control of the airplane do not 
result in significant changes in attitude, airspeed or load factor.
    (c) A predictable pitch response is provided to the pilot.
    (d) An acceptable level of pilot attention (workload) to attain and 
maintain trim speed and altitude is provided to the pilot.
    (e) Longitudinal stability provides gust stability.
    The pitch control movement of the sidestick on the A350 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 
that's 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 another command is given by the 
pilot. This control function is applied during ``normal'' control law 
within the speed range from initiation of the angle of attack 
protection limit, Vaprot, 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 does not meet the 
requirements in 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 there is 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, there 
are no stability cues since 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

[[Page 2386]]

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 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 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 proposed special conditions would 
also apply to the other model under Sec.  21.101.
    In addition to the applicable airworthiness regulations and 
proposed special conditions, the Model A350-900 series 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 and 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 will incorporate the following 
novel or unusual design features: A flight control design feature 
within the normal operational envelope in which side stick deflection 
in the roll axis commands roll rate; an operational design which 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 a low altitude and 
performance limiting conditions.

Discussion

    1. 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.
    2. Since conventional relationships between stick forces and 
control surface displacements do not apply to the ``load factor 
command'' flight control system on the A350, longitudinal stability 
characteristics should be evaluated by assessing the airplane handling 
qualities during simulator and flight test maneuvers appropriate to 
operation of the airplane. This may be accomplished by using the 
Handling Qualities Rating Method presented in Appendix 7 of the Flight 
Test Guide, AC 25-7A, or an acceptable alternative method proposed by 
Airbus. 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.
    3. 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.
    (c) 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 derated 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.

Applicability

    As discussed above, these proposed 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 proposed 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.

0
The authority citation for these special conditions is as follows:

    Authority:  49 U.S.C. 106(g), 40113, 44701, 44702, 44704.

The Proposed Special Conditions

    Accordingly, the Federal Aviation Administration (FAA) proposes the 
following special conditions 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

[[Page 2387]]

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:
    (1) From 1.13 VSR1 to VMO /
MMO.
    (2) From VMO/MMO to VFC/
MFC, unless the divergence is--
    (i) Gradual;
    (ii) Easily recognizable by the pilot; and
    (iii) 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.

    Issued in Renton, Washington, on October 22, 2013.
Stephen P. Boyd,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 2014-00449 Filed 1-13-14; 8:45 am]
BILLING CODE 4910-13-P