[Federal Register Volume 89, Number 89 (Tuesday, May 7, 2024)]
[Proposed Rules]
[Pages 38004-38007]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-09660]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2023-2412; Notice No. 25-23-06-SC]


Special Conditions: Airbus Model A321neo Extra-Long Range (XLR) 
Airplane; Cabin Evacuation--Protection From Fuel Tank Explosion Due to 
External Fuel-Fed Ground Fire

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 
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 
fire-safety performance of fuel-tank skin or structure in a post-crash 
external fuel-fed ground fire. 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 comments on or before June 21, 2024.

ADDRESSES: Send comments identified by Docket No. FAA-2023-2412 using 
any of the following methods:
    Federal eRegulations Portal: Go to www.regulations.gov and follow 
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.
    Docket: Background documents or comments received may be read at 
www.regulations.gov at any time. Follow the online instructions for 
accessing the docket or go 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.

FOR FURTHER INFORMATION CONTACT: Douglas Bryant, Engine and Propulsion 
Section, AIR-625, Technical Policy Branch, Policy and Standards 
Division, Aircraft Certification Service, Federal Aviation 
Administration, 2200 South 216th Street, Des Moines, Washington 98198; 
telephone and fax 206-231-3166; email [email protected].

SUPPLEMENTARY INFORMATION:

Comments Invited

    The FAA invites 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 proposed special conditions, 
explain the reason for any recommended change, and include supporting 
data.
    The FAA will consider all comments received by the closing date for 
comments, and will consider comments filed late if it is possible to do 
so without incurring delay. The FAA may change these special conditions 
based on the comments received.

Privacy

    Except for Confidential Business Information (CBI) as described in 
the following paragraph, and other information as described in title 
14, Code of Federal Regulations (14 CFR) 11.35, the FAA will post all 
comments received without change to www.regulations.gov, including any 
personal information you provide. The FAA will also post a report

[[Page 38005]]

summarizing each substantive verbal contact received about these 
special conditions.

Confidential Business Information

    Confidential Business Information (CBI) is commercial or financial 
information that is both customarily and actually treated as private by 
its owner. Under the Freedom of Information Act (FOIA) (5 U.S.C. 552), 
CBI is exempt from public disclosure. If your comments responsive to 
these special conditions contain commercial or financial information 
that is customarily treated as private, that you actually treat as 
private, and that is relevant or responsive to these special 
conditions, it is important that you clearly designate the submitted 
comments as CBI. Please mark each page of your submission containing 
CBI as ``PROPIN.'' The FAA will treat such marked submissions as 
confidential under the FOIA, and the indicated comments will not be 
placed in the public docket of these special conditions. Send 
submissions containing CBI to the individual listed in the For Further 
Information Contact section. Comments the FAA receives, which are not 
specifically designated as CBI, will be placed in the public docket for 
these special conditions.

Background

    On September 16, 2019, Airbus applied for an amendment to Type 
Certificate No. A28NM to include the new Model A321neo XLR series 
airplane. The Airbus Model A321neo XLR series airplane, which is a 
derivative of the Model A321neo Airbus Cabin Flex (ACF) currently 
approved under Type Certificate No. A28NM, is a twin-engine transport 
category aircraft that seats up to 244 passengers and has a maximum 
takeoff weight of 222,667 lbs.

Type Certification Basis

    Under the provisions of title 14, Code of Federal Regulations (14 
CFR) 21.101, Airbus must show that the Model A321neo XLR series 
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 A321neo XLR series 
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 A321neo XLR series airplane must comply 
with the fuel venting 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 Features

    The Airbus Model A321neo XLR series airplane will incorporate the 
following novel or unusual design feature:
    An integral RCT.

Discussion

    The proposed Airbus Model A321neo XLR series airplane incorporates 
an integral RCT. This tank is a ``center'' fuel tank, that would, if 
approved, be located in the airplane fuselage rather than in its wings. 
The tank is a ``rear'' tank, that would be located aft of the center 
wing fuel tank and behind the wheel bay; it would be in an area of the 
lower section of the fuselage, partially replacing the aft cargo 
compartment of the airplane from which this proposed model is derived. 
The top of the tank would be directly below the floor of the passenger 
cabin. The fuel tank would be ``integral'' to the airplane, in that its 
walls would be part of the airplane structure. The exterior skin of the 
airplane fuselage would constitute part of the walls of the fuel tank, 
and these areas are usually separate boundaries (not integral) on other 
fuselage fuel tanks. An integral fuel tank may be referred to as a 
conformal fuselage structural fuel tank since boundaries of the fuel 
tank ``conform'' with the airplane exterior. The integral RCT is 
installed in a location that may be exposed to the direct effects of 
post-crash ground, or pool, fuel-fed fires. An external fuel-fed ground 
fire or external fuel-fed pool fire is also referred to as `external 
ground fire'.
    The airworthiness standards applicable to the Model A321neo XLR do 
not contain specific standards for post-crash fire-safety performance 
of fuel-tank skin or structure. In addition, the integral RCT on the 
A321neo XLR was not envisaged by the FAA when promulgating requirements 
related to occupant protection when fuel tanks are exposed to external 
fuel-fed fires. The FAA considered fuel tank designs in widespread use 
on transport airplanes, including main fuel tanks and auxiliary fuel 
tanks when promulgating requirements related to occupant protection. 
Auxiliary fuel tanks are normally located in the center wing and within 
cargo holds, and in such cases are sometimes referred to as an 
auxiliary center tank (ACT).
    Airplane manufacturers commonly incorporate a center wing fuel tank 
as an auxiliary fuel tank to make fuel available for increasing the 
flight range of the airplane. Continued expansion of range performance 
requirements has resulted in airplane designs using other areas of the 
airplane to carry fuel, such as incorporating fuel tanks in the 
empennage and fuselage. The Airbus model A321neo XLR airplane includes 
a center wing fuel tank, an integral RCT and the option for additional 
ACTs within the fuselage. Unlike an integral RCT, a center wing fuel 
tank and optional ACTs are not expected by the FAA or manufacturers to 
be exposed to the direct effects of post-crash ground fire because the 
fuel tank walls are not exterior airplane skin on the center fuel tank 
or ACT designs.
    Due to its unusual configuration, the A321neo XLR's integral RCT 
will also not incorporate the insulation that usually lines the 
fuselage skin of a modern transport category airplane. Therefore the 
FAA has issued, after notice and comment, a set of special conditions 
that address that novel or unusual aspect of the A321neo XLR's integral 
RCT with regard to certain of the FAA's regulatory requirements for 
thermal/acoustic insulation installations, specifically 14 CFR 
25.856(b). Those special conditions, No. 25-825-SC, require that the 
lower half of the fuselage spanning the longitudinal location of the 
RCT resist penetration from an external fuel-fed fire, in order to 
ensure that the design provides the same level of passenger protection 
from such fires as do the FAA's existing regulations for such 
insulation.\1\ The special conditions

[[Page 38006]]

proposed herein address a different flammability aspect of the A321neo 
XLR's integral RCT.
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    \1\ Special Conditions: Airbus Model A321neoXLR Airplane; 
Passenger Protection from External Fire. 87 FR 74503 (Dec. 6, 2022).
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    Pertinent to the fuel tank structure, post-crash-fire occupant 
survivability is dependent on the time available for occupant 
evacuation prior to fuel-tank breach or structural failure. Structural 
failure can be a result of degradation in load-carrying capability 
caused by a fuel-fed ground fire. Structural failure can also be a 
result of over-pressurization caused by ignition of fuel vapors inside 
the fuel tank.
    Past experience indicates that occupant survivability following a 
post-crash fire is greatly influenced by the size and intensity of any 
fire that occurs. The ability of main fuel tanks, when they have 
aluminum wing surfaces wetted by fuel on their interior surface, to 
withstand post-crash-fire conditions, has been demonstrated by tests 
conducted at the FAA William J. Hughes Technical Center.\2\ Results of 
these tests have verified adequate dissipation of heat across wetted 
aluminum fuel-tank surfaces so that localized hot spots do not occur, 
thus minimizing the threat of explosion. This inherent capability of 
aluminum to dissipate heat also allows the aircraft's lower surface, 
which is also the fuel tank boundary, to retain its load-carrying 
characteristics during a fuel-fed ground fire, and significantly delays 
structural collapse or burn-through for a time interval that usually 
exceeds evacuation times. In addition, as an aluminum fuel tank with 
significant quantities of fuel inside is heated, fuel vapor accumulates 
in the ullage space, exceeding the upper flammability limit relatively 
quickly and thus reducing the threat of a fuel-tank explosion prior to 
fuel-tank burn-through.
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    \2\ Hill, R., and Johnson, G.R., ``Investigation of Aircraft 
Fuel Tank Explosions and Nitrogen Inerting Requirements During 
Ground Fires,'' FAA Report DOT/FAA/RD-75-119, October 1975. 
Available via the FAA Technical Center website for Fire Safety at 
http://www.fire.tc.faa.gov/.
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    The center wing tank and optional ACTs are surrounded by fuselage 
structure and would not be directly exposed to a post-crash ground 
fire. This inherent separation is also expected to significantly delay 
structural collapse or burn-through and reduce the threat of explosion 
for a time interval that usually exceeds evacuation times. Service 
history of conventional aluminum airplanes has shown that fuel-tank 
explosions caused by ground fires have been rare on airplanes 
configured with flame arrestors in the fuel-tank vent lines. The Model 
A321neo XLR integral RCT may or may not have equivalent capability of 
past designs approved with existing regulations, due to the RCT design 
and location being integral with the fuselage.
    There are several Part 25 requirements that address fire-safety 
performance of the fuel tanks and fuselage in the Model A321neo XLR 
certification basis. However, these requirements do not directly or 
adequately address standards for post-crash fire-safety performance of 
fuel-tank skin or structure. These standards address failure conditions 
or minimize the hazard to the occupants in the event ignition of 
flammable fluids or vapors occurs. For example, Sec.  25.863 requires 
applicants to minimize the probability of ignition and resultant 
hazards if ignition occurs for flammable fluid systems on the airplane. 
Another example is Sec.  25.981(a) which requires applicants to 
demonstrate no ignition source may be present at each point in the fuel 
tank or fuel tank system where catastrophic failure could occur due to 
ignition of fuel or vapors. Specifically, Sec.  25.981(a)(1) requires 
``determining the highest temperature allowing a safe margin below the 
lowest expected autoignition temperature of the fuel in the fuel 
tanks.'' Then Sec.  25.981(a)(2) requires ``demonstrating that no 
temperature at each place inside each fuel tank where fuel ignition is 
possible will exceed the temperature determined under paragraph (a)(1) 
of this section. This must be verified under all probable operating, 
failure, and malfunction conditions of each component whose operation, 
failure, or malfunction could increase the temperature inside the 
tank.'' In addition, Sec.  25.981(a)(3) requires ``except for ignition 
sources due to lightning addressed by Sec.  25.954, demonstrating that 
an ignition source could not result from each single failure, from each 
single failure in combination with each latent failure condition not 
shown to be extremely remote, and from all combinations of failures not 
shown to be extremely improbable, taking into account the effects of 
manufacturing variability, aging, wear, corrosion, and likely damage.'' 
These airworthiness requirements address ignition sources and are part 
of the FAA's regulatory framework for preventing fires and explosions; 
however, taken together, they do not adequately address the potential 
for a post-crash external ground fire to affect the safety of airplane 
occupants.
    The FAA therefore determined that the airworthiness standards 
applicable to the Model A321neo XLR airplane do not contain adequate 
standards for post-crash fire-safety performance of fuel-tank skin or 
structure. The FAA therefore proposes that special conditions are 
needed for the Model A321neo XLR airplane, because the integral RCT 
design, including location in the lower fuselage, is considered an 
unusual or novel design feature that could expose the RCT to an 
external ground fire. Factors influencing occupant survival time when a 
fuel tank is exposed to a ground-fed fire are the structural integrity 
of the tank; burn-through resistance; flammability of the tank; and the 
presence of auto-ignition threats during exposure to a fire. As 
previously discussed, the FAA issued Special Conditions No. 25-825-SC 
were issued to address the novel or unusual aspect of the A321neo XLR's 
integral RCT with regard to requirements for thermal/acoustic 
insulation installations. The FAA considers the occupant survival time 
related to the burn-through resistance of the integral RCT to be 
adequately accounted for in those special conditions.
    These proposed special conditions address standards for post-crash 
fire-safety performance of fuel-tank skin or structure by proposing a 
requirement to prevent the ignition of fuel vapor during an external 
fuel-fed ground fire. These proposed special conditions include 
accounting for the potential for hot surface ignition created by the 
external fuel-fed fire. As described in FAA Advisory Circular 25.981-
1D, ``Fuel Tank Ignition Source Prevention Guidelines,'' hot surfaces 
that can exceed the autoignition temperature of the flammable vapor 
under consideration are considered to be ignition sources. The FAA 
intends this proposed requirement to adequately protect the airplane 
occupants from the consequences of an integral RCT exposed to an 
external fuel-fed ground, or pool fire.
    The intention of the proposed requirement for the design to prevent 
ignition is for the applicant to show that ignition sources do not 
occur, such as from a hot surface, due to the external heat applied to 
the integral RCT from an external fuel-fed ground fire. Where 
previously discussed, Sec.  25.981(a) requires applicants to 
demonstrate that no ignition source may be present but does not 
specifically address ignition due to an external fuel-fed ground fire.
    To provide the same level of safety as provided by the relevant 
regulations in this model's certification basis, Airbus must 
demonstrate that the Model A321neo XLR series airplane has sufficient 
post-crash fire-safety performance of fuel-tank skin or structure to 
enable occupants to safely evacuate in the event that the integral RCT 
is exposed to an external fuel-fed ground fire.

[[Page 38007]]

    The FAA assessed post-crash-survival time during the adoption of 
Sec.  25.856 and revisions to appendix F to part 25 at Amendment 25-111 
for fuselage burn-through protection. Studies conducted by and on 
behalf of the FAA indicated that following a survivable accident, 
prevention of fuselage burn-through for approximately 5 minutes can 
significantly enhance survivability.\3\
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    \3\ Cherry, R. and Warren, K. ``Fuselage Burnthrough Protection 
for Increased Postcrash Occupant Survivability: Safety Benefit 
Analysis Based on Past Accidents, ``FAA Report DOT/FAA/AR-99/57, 
September 1999 and R G W Cherry & Associates Limited, ``A Benefit 
Analysis for Cabin Water Spray Systems and Enhanced Fuselage 
Burnthrough Protection,'' FAA Report DOT/FAA/AR-02/49, April 7, 
2003.
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    The FAA would consider Airbus showing the design prevents ignition 
of fuel tank vapors in the integral RCT during at least 5 minutes of 
exposure to an external fuel-fed ground fire as a sufficient time 
duration for the purposes of these special conditions. The time 
duration of 5 minutes is consistent with the aforementioned studies 
showing prevention of fuselage burn-through for approximately 5 minutes 
enhances occupant survivability. The requirements of the proposed 
special conditions and the time duration are consistent with the 
European Union Aviation Safety Agency Special Conditions No. SC-
D25.863-01, Cabin Evacuation--Protection from Fuel Tank Explosion due 
to External Fuel Fed Ground Fire applicable to integral RCTs.\4\
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    \4\ SC-D25.863-01, Issue 2, dated 24 October 2023 https://www.easa.europa.eu/en/document-library/product-certification-consultations/final-special-condition-ref-sc-d25863-01-cabin.
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    Airbus may consider a flammability reduction system or ignition 
mitigation means that complies with Sec.  25.981 when showing 
compliance with the proposed special conditions, provided the system's 
performance is demonstrated to meet the proposed special conditions. As 
discussed previously, showing compliance with only Sec.  25.981(b) is 
insufficient to show post-crash fire-safety performance of fuel-tank 
skin or structure. Airbus must also meet the proposed special 
conditions.
    The 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.

Applicability

    As discussed above, these proposed special conditions are 
applicable to the Airbus Model A321neo XLR series airplane 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 apply to the 
other model as well.

Conclusion

    This action affects only certain novel or unusual design feature on 
A321neo XLR 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.

Authority Citation

    The authority citation for these special conditions is as follows:

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

The Proposed Special Conditions

    [squf] Accordingly, the Federal Aviation Administration (FAA) 
proposes the following special conditions as part of the type 
certification basis for Airbus Model A321neo XLR series airplanes.
    Cabin Evacuation--Protection from Fuel Tank Explosion Due to 
External Fuel-Fed Ground Fire.
    The applicant must show the design prevents ignition of fuel tank 
vapors (due to hot surface) from occurring in the integral rear center 
tank during the time required for evacuation. The applicant's showing 
must also demonstrate that the design provides sufficient time for a 
safe evacuation of all occupants after the initiation of an external 
fuel-fed ground fire.

     Issued in Kansas City, Missouri, on April 29, 2024.
Patrick R. Mullen,
Manager, Technical Policy Branch, Policy and Standards Division, 
Aircraft Certification Service.
[FR Doc. 2024-09660 Filed 5-6-24; 8:45 am]
BILLING CODE 4910-13-P