[Federal Register Volume 87, Number 233 (Tuesday, December 6, 2022)]
[Rules and Regulations]
[Pages 74503-74505]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2022-26435]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2021-0891; Special Condition No. 25-825-SC]


Special Conditions: Airbus Model A321neoXLR Airplane; Passenger 
Protection From External Fire

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final special conditions.

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SUMMARY: These special conditions are issued for the Airbus Model 
A321neoXLR airplane. This airplane will have a novel or unusual design 
feature when compared to the technology envisaged by the airworthiness 
standards for transport category airplanes. This design feature is an 
integral rear center tank (RCT). 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 January 5, 2023.

FOR FURTHER INFORMATION CONTACT: Shannon Lennon, Human Machine 
Interface, AIR-626, Technical Innovation Policy Branch, Policy and 
Innovation Division, Aircraft Certification Service, Federal Aviation 
Administration, 2200 South 216th Street, Des Moines, Washington 98198; 
telephone and fax 206-231-3209; email [email protected].

SUPPLEMENTARY INFORMATION: 

Background

    On September 16, 2019, Airbus applied for an amendment to Type 
Certificate No. A28NM to include the new Model A321neoXLR airplane. The 
Model A321neoXLR airplane, which is a derivative of the Model 
A321neoACF airplane currently approved under Type

[[Page 74504]]

Certificate No. A28NM, is a twin-engine transport category aircraft 
that seats 244 passengers and has a maximum takeoff weight of 202,000 
lbs.

Type Certification Basis

    Under the provisions of title 14, Code of Federal Regulations (14 
CFR) 21.101, Airbus must show that the Model A321neoXLR airplane meets 
the applicable provisions of the regulations listed in Type Certificate 
No. A28NM, or the applicable regulations in effect on the date of 
application for the change, except for earlier amendments as agreed 
upon by the FAA.
    If the Administrator finds that the applicable airworthiness 
regulations (e.g., 14 CFR part 25) do not contain adequate or 
appropriate safety standards for the Airbus Model A321neoXLR airplane 
because of a novel or unusual design feature, special conditions are 
prescribed under the provisions of 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, or should any other model already included on 
the same type certificate be modified to incorporate the same novel or 
unusual design feature, these special conditions would also apply to 
the other model under Sec.  21.101.
    In addition to the applicable airworthiness regulations and special 
conditions, the Airbus Model A321neoXLR 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 issues special conditions, as defined in 14 CFR 11.19, in 
accordance with Sec.  11.38, and they become part of the type 
certification basis under Sec.  21.101.

Novel or Unusual Design Feature

    The Airbus Model A321neoXLR airplane will incorporate the following 
novel or unusual design feature:
    An integral RCT.

Discussion

    The Airbus Model A321neoXLR incorporates an integral RCT. This tank 
is a ``center'' fuel tank, in that it is located in the airplane 
fuselage rather than in its wings. The tank is a ``rear'' tank, in that 
it is located aft of the wheel bay; it will be in an area of the lower 
fuselage that partially replaces the aft cargo compartment of the 
airplane from which this model is derived. The top of the tank will be 
directly below the floor of the passenger cabin. The fuel tank will be 
``integral'' to the airplane, in that its walls will be part of the 
airplane structure. The exterior skin of the airplane fuselage will 
constitute part of the walls of the fuel tank, and these areas will 
lack the thermal/acoustic insulation that usually lines the exterior 
skin of an airplane fuselage.
    This design was not envisaged by the FAA's regulatory requirements 
for insulation installations on transport category airplanes. 14 CFR 
25.856(b) requires all thermal/acoustic insulation in the lower half of 
the airplane fuselage and their installation to comply with the flame 
penetration resistance test of appendix F, part VII. The FAA adopted 
Sec.  25.856(b) to raise the level of post-crash fire safety on 
transport category airplanes. Part VII of appendix F to part 25 
requires a stringent test method for all thermal/acoustic insulation 
proposed for installation in the lower half of the fuselage. The FAA's 
intent in imposing this requirement was to ensure that this insulation 
provides an additional barrier between the occupants and an external 
post-crash fire, especially a fire resulting from a pool of spilled 
aviation fuel.\1\ This barrier extends the time available for 
evacuation.
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    \1\ See pg. 2 of FAA Advisory Circular 25.856-2A, Installation 
of Thermal/Acoustic Insulation for Burnthrough Protection.
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    While the rule applies to the thermal/acoustic insulation that an 
applicant proposes as part of their design, it does not require 
applicants to install such insulation. Since the fuselage skins of the 
lower half of transport category airplanes are generally insulated, and 
were at the time these standards were developed, the FAA considered 
this approach to be sufficient to ensure safety. The rule also noted, 
however, that if applicants began to propose designs that omitted this 
thermal/acoustic insulation, the FAA would revisit the need for a 
specific fuselage burnthrough standard.\2\
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    \2\ Improved Flammability Standards for Thermal/Acoustic 
Insulation Materials Used In Transport Category Airplanes, 68 FR 
45046, 45049 (Jul. 31, 2003).
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    Thus, since this design will lack thermal/acoustic insulation under 
the fuselage skin in the area of the fuel tank, current FAA regulations 
do not ensure that it will provide a continuous flame penetration 
(burnthrough) resistant barrier between the passengers and an external 
fire, nor that it will provide enough protection, against an external 
post-crash fire, to allow time for passengers to evacuate.
    According to Airbus, its design does not allow for compliant 
thermal/acoustic insulation to be placed beneath the cabin floor. This 
large volume of unheated liquid (fuel), directly below the floor of the 
passenger cabin, would, without mitigation, create a 'cold feet' effect 
for the passengers above it. Therefore, Airbus will install insulation 
panels between the fuel tank and the cabin floor, for comfort reasons. 
These insulation panels would normally be required to meet Sec.  
25.856(b). However, Airbus states that it is technically not feasible 
to install thermal/acoustic insulation that complies with Sec.  
25.856(b), due to the lack of space in this area and the need to keep 
nearby decompression panels free of blockages and ensure adequate 
ventilation.
    Special conditions are needed to address the assumption in the 
FAA's current flammability standards that proposed airplane designs 
would include thermal/acoustic insulation in the lower fuselage, and to 
ensure that this proposed design does not reduce the time available for 
passenger evacuation in the case of a post-crash external fire. 
Specifically, the FAA will require that the lower half of the airplane 
fuselage, spanning the longitudinal area of the tank, be resistant to 
fire penetration. ``Resistant to fire penetration'' will, for this 
special condition, mean that this area provides fire penetration 
resistance equivalent to the resistance which would be provided if the 
fuselage were lined with thermal/acoustic insulation that meets the 
flame penetration resistance test requirements of part VII of appendix 
F of part 25. The applicant's method of compliance may, but is not 
required to, be based upon any inherent flame penetration resistance 
capability provided by the construction of the fuel tank and/or other 
surrounding features.
    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

    The FAA issued Notice of Proposed Special Conditions No. 25-21-04-
SC for the Model A321neoXLR airplane, which was published in the 
Federal Register on April 6, 2022 (87 FR 19811). The FAA received four 
comments from the Boeing Company (Boeing).
    Comment Summary: Boeing requested that the discussion section of 
these special conditions describe the RCT as an ``auxiliary,'' rather 
than ``center,'' fuel tank because the airplane also has a ``center'' 
wing (main) tank, and because, as described by Advisory Circular (AC) 
25-8, Auxiliary Fuel

[[Page 74505]]

Systems Installations, the RCT would be connected to the main tank with 
a fuel feed line. Boeing also requested that the discussion section 
describe the tank as an ``aft'' fuel tank rather than a ``rear'' tank, 
because it will be aft of the wheel bay.
    FAA Response: No change to the terms used to describe the RCT in 
these special conditions is necessary. The existing terms are accurate, 
consistent with the applicant's nomenclature, and adequate for their 
purpose.
    Comment Summary: Boeing requested that the discussion section of 
these special conditions acknowledge that AC 25.856-2A \3\ provides 
guidance for center wing tank designs. Boeing further requested that 
the discussion, according to guidance provided in that AC for the wing 
box area, also indicate that insulation panels installed above a fuel 
tank are not required to meet Sec.  25.856(b).
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    \3\ See pg. 2 of FAA Advisory Circular 25.856-2A, Installation 
of Thermal/Acoustic Insulation for Burnthrough Protection.
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    FAA Response: The discussion for these special conditions 
acknowledges that Sec.  25.856(b) does not adequately address designs 
like the RCT of the A321neoXLR. This aircraft presents a novel fuselage 
design that does not incorporate thermal/acoustic insulation in areas 
where the RCT is integral to the fuselage, nor does it include thermal/
acoustic insulation above the RCT that will meet Sec.  25.856(b). This 
design presents a fire penetration resistance (burnthrough) 
vulnerability that is addressed by these special conditions. The same 
vulnerability does not exist with transport airplane wing box 
construction due to that structure's significant mass, and large 
surface area that dissipates heat. Therefore, adding insulation over 
the wingbox, would not contribute to its fire penetration resistance. 
14 CFR 25.856(b) excepts the installation of insulation in locations 
where it would not contribute to fire penetration resistance. However, 
the wing box example in AC 25.856-2A only addresses the FAA's 
assessment of the wing box area in consideration of thermal/acoustic 
insulation installations that would not contribute to fire penetration 
resistance. It does not suggest that all center fuel tanks do not 
necessitate the installation of thermal/acoustic insulation that meets 
Sec.  25.856(b). For this reason, the FAA declines to change the 
discussion section of these special conditions.
    Comment Summary: Boeing requested that the special conditions 
require the RCT fire penetration resistance capability to either be 
equivalent to the capability provided by the wing box area or meet the 
requirements of 14 CFR 25.963(e)(2). Boeing's rationale was that the 
FAA's proposed standard of fire penetration resistance equivalent to 
that of a fuselage lined with thermal/acoustic insulation that meets 
the flame penetration resistance test requirements of part VII of 
appendix F, does not address hazards associated with fuel tanks and is 
not applicable to the wing box area.
    FAA Response: These special conditions are intended to ensure that 
the existing RCT area fuselage design establishes the same level of 
safety as would 14 CFR 25.856(b). When thermal/acoustic insulation is 
installed, either along the fuselage skin or under the passenger cabin 
floor, it should be fire penetration resistant and delay the onset of 
fire into the passenger cabin. These special conditions are not 
intended to ensure the RCT is constructed to provide a fire penetration 
resistance capability that is similar to that of the wing box area. It 
is also unnecessary to require that the RCT meet rules such as 14 CFR 
25.963(e)(2), which provides standards for fuel tank access covers.
    The special conditions are adopted as proposed.

Applicability

    As discussed above, these special conditions are applicable to the 
Airbus Model A321neoXLR airplane. Should Airbus apply at a later date 
for a change to the type certificate to include another model 
incorporating the same novel or unusual design feature, these special 
conditions would apply to that model as well.

Conclusion

    This action affects only a certain novel or unusual design feature 
on one model of airplane. It is not a rule of general applicability.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

Authority Citation

    The authority citation for these special conditions is as follows:

    Authority: 49 U.S.C. 106(f), 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 A321neoXLR airplanes.

Passenger Protection From External Fire

    The lower half of the fuselage, spanning the longitudinal location 
of the rear center fuel tank, must be resistant to fire penetration.

    Issued in Kansas City, Missouri, on November 30, 2022.
Patrick R. Mullen,
Manager, Technical Innovation Policy Branch, Policy and Innovation 
Division, Aircraft Certification Service.
[FR Doc. 2022-26435 Filed 12-5-22; 8:45 am]
BILLING CODE 4910-13-P