Special Conditions: Airbus Model A350-900 Airplanes; Lateral-Trim Function Through Differential Flap Setting, 43236-43237 [2014-17578]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 79, Number 143 (Friday, July 25, 2014)]
[Rules and Regulations]
[Pages 43236-43237]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2014-17578]


-----------------------------------------------------------------------

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2013-0911; Special Conditions No. 25-539-SC]


Special Conditions: Airbus Model A350-900 Airplanes; Lateral-Trim 
Function Through Differential Flap Setting

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final special conditions.

-----------------------------------------------------------------------

SUMMARY: These special conditions are issued for Airbus Model A350-900 
airplanes. These airplanes will have a novel or unusual design feature 
associated with a lateral-trim function that deploys flaps 
asymmetrically for airplane lateral-trim control. This function 
replaces the traditional method of providing airplane lateral trim over 
a small range through flap and aileron mechanical rigging. 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: Robert C. Jones, FAA, Propulsion and 
Mechanical Systems, ANM-112, Transport Airplane Directorate, Aircraft 
Certification Service, 1601 Lind Avenue SW., Renton, Washington 98057-
3356; telephone (425) 227-1234; facsimile (425) 227-1320.

SUPPLEMENTARY INFORMATION:

Background

    On August 25, 2008, Airbus applied for a type certificate for their 
new Model A350-900 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 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.
    On conventional airplanes, small, lateral, airplane asymmetries 
typically have been addressed through flap and aileron rigging (e.g., 
using shims). On Model A350-900 airplanes, an order for asymmetric flap 
deployment will be computed by the primary flight-control system as a 
function of the aileron position. The current airworthiness standards 
do not contain adequate safety standards for asymmetric use of the 
flaps for Airbus Model A350-900 airplanes. Special conditions are 
needed to account for the aspects of a function used to command an 
intended flap asymmetry. The lateral-trim function is intended to be 
performed once during climb and once during cruise to compensate for 
airplane small lateral asymmetries.
    The lateral-trim function is not a trim-control system in the 
conventional sense as it has no pilot interface and is not governed by 
Title 14, Code of Federal Regulations (14 CFR) 25.677. Some fly-by-wire 
airplanes have no pilot-operated lateral trim at all. The lateral-trim 
function is simply an additional fly-by-wire flight-control function 
that nulls small roll asymmetries in certain flight phases with small, 
asymmetric flap deployments. Although the function operates under 
normal conditions within the small range of the traditional rigging, 
failure cases may result in a significant out-of-range asymmetric flap 
condition. An asymmetry threshold protects the system against excessive 
flap asymmetry.

Type Certification Basis

    Under 14 CFR 21.17, Airbus must show that the Model A350-900 
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 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 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 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 airplane incorporates the following novel 
or unusual design features: The asymmetric use of flaps to address 
lateral trim, which is not adequately addressed by Sec.  25.701.

[[Page 43237]]

Discussion

    Section 25.701(a) requires that, unless the airplane has safe-
flight characteristics with the flaps or slats retracted on one side 
and extended on the other, flap and slat surfaces must be synchronized 
by either a mechanical interconnection or any equivalent means that has 
the same integrity. Synchronization is interpreted to mean that flap 
movement is symmetrical throughout the full range of flap motion. 
Because the lateral-trim function intentionally creates asymmetric flap 
motions, the flap-system installation of the Model A350-900 airplane 
does not meet the requirement of Sec.  25.701(a) and (d).
    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. 25-13-22-SC for Airbus 
Model A350-900 airplanes was published in the Federal Register on 
January 8, 2014 (79 FR 1339). 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 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 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 airplanes:

Lateral-Trim Function Through Differential Flap Setting

    Current airworthiness standards, specifically Sec.  25.701, do not 
contain adequate safety standards for this airplane design. In lieu of 
the requirements of Sec.  25.701(a) and (d) for the lateral-trim 
function, the following special condition are issued:
    1. Airbus must demonstrate that an unsafe condition is not created 
by using the flaps asymmetrically.
    2. The degree of acceptable asymmetry must be defined and justified 
for all flight phases with respect to:
    a. Section 25.701(b) and (c), with the worst-case asymmetric flap 
configurations, and
    b. Providing equivalent protection against excess asymmetry in the 
same manner as Sec.  25.701 provides to systems that are synchronized, 
or use another equivalent means to prevent asymmetry.
    3. This lateral-trim function is a flight-control system and 
therefore must be shown to comply with both general system requirements 
as well as general flight-control requirements. Therefore, the function 
must be demonstrated not to have significant latent failures, where 
practicable.

    Issued in Renton, Washington, on July 9, 2014.
Jeffrey E. Duven,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 2014-17578 Filed 7-24-14; 8:45 am]
BILLING CODE 4910-13-P