[Federal Register Volume 89, Number 138 (Thursday, July 18, 2024)]
[Rules and Regulations]
[Pages 58253-58257]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-15853]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2023-2412; Special Conditions No. 25-868-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: Final special conditions.

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SUMMARY: These special conditions are issued for the Airbus Model 
A321neo XLR 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 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 July 18, 2024.

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:

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 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 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

[[Page 58254]]

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, 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. The 
special conditions herein address a different flammability aspect of 
the A321neo XLR's integral RCT.
    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. 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.
    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 proposed 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 
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 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 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

[[Page 58255]]

this 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 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.
    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.
    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 studies mentioned above 
showing prevention of fuselage burn-through for approximately 5 minutes 
enhances occupant survivability. The requirements of the 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.
    Airbus may consider a flammability reduction system or ignition 
mitigation means that complies with Sec.  25.981 when showing 
compliance with these special conditions, provided the system's 
performance is demonstrated to meet the 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 these special conditions.
    The 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. 25-23-06-
SC for the Airbus Model A321neo XLR airplane, which was published in 
the Federal Register on May 7, 2024 (89 FR 38004). The FAA received 
several comments from an individual regarding the proposed special 
conditions.
    The commenter requested the FAA consider how passengers will be 
made aware of what the commenter described as the ``unique'' 
configuration of a fuel tank directly under passenger seats in what is 
traditionally a location for baggage and cargo. The commenter suggested 
that the FAA make passengers aware of their proximity to the airplane 
fuel.
    In the Notice of Proposed Special Conditions No. 25-23-06-SC, which 
was published in the Federal Register, the FAA informed the public of 
the proposed configuration. As stated in that Notice, while the subject 
integral RCT is a novel or unusual design feature, the configuration is 
not unique. Many transport airplanes incorporate fuel tank 
configurations that result in fuel in close proximity to some 
passengers. These special conditions address standards for post-crash 
fire-safety performance of fuel-tank skin or structure. No changes were 
made to these special conditions as a result of this comment.
    The commenter requested the FAA clarify how it addressed the 
crashworthiness requirements of a fuel tank integral to the fuselage 
applied by the FAA to the Model A321neo XLR series airplane. The 
commenter recognized the request is beyond the proposed special 
conditions.
    The FAA disagrees that additional clarification of crashworthiness 
requirements for the RCT is necessary for these special conditions. The 
FAA discussed the type certification basis of the Model A321neo XLR 
series airplane in the Notice of Proposed Special Conditions No. 25-23-
06-SC. The crashworthiness requirements applicable to the Model A321neo 
XLR series airplane are addressed by the type certification basis, and 
as acknowledged by the commenter, are outside the scope of these 
special conditions. Therefore, no changes were made to these special 
conditions as a result of this comment.
    The commenter requested the FAA explain what considerations the FAA 
is making relative to an otherwise survivable accident when the RCT is 
ruptured and there is an external fuel-fed ground fire already present.
    The FAA infers that the commenter requests the FAA further clarify 
the requirements the FAA applied to the Model A321neo XLR series 
airplane related to a ruptured RCT in addition to an external fuel-fed 
ground fire. The FAA considers the commenter's request to be beyond the 
scope of these special conditions, which addresses standards 
specifically for the post-crash fire safety performance of fuel-tank 
skin or structure by establishing a requirement to prevent the ignition 
of fuel vapor during an external fuel-fed ground fire.
    The FAA stated in Notice of Proposed Special Conditions No. 25-23-
06-SC, and restated in the discussion above, that several part 25 
requirements applicable to the Model A321neo XLR series airplane 
address fire-safety performance of the fuel tanks and fuselage in the 
Model A321neo XLR certification basis. These standards address failure 
conditions or minimize the hazard to the occupants in the event 
ignition of flammable fluids or vapors occurs. The potential for a 
ruptured RCT is thus already addressed in the Model A321neo XLR 
certification basis. Therefore, no changes were made to these special 
conditions as a result of this comment.
    The commenter stated that ``the applicant should show not that the 
design prevents but that it eliminates the possibility that ignition 
will occur.'' The FAA interprets this statement as a request that the 
FAA require the applicant to fully eliminate any possibility of fuel 
ignition in the RCT, rather than to minimize the probability of 
ignition to an acceptable level through ignition-preventative design 
measures. The FAA does not agree that it is practical to eliminate the 
possibility that ignition will occur from a design in the case of a 
fuel tank exposed to a post-crash fuel-fed ground fire. Service 
experience has shown that existing designs would not meet this standard 
since aircraft fuel tanks exposed to an external fuel-fed ground fire 
would eventually experience conditions that would support fuel tank 
ignition (for example, refer to the fuel tank explosions discussed in 
the China Airlines Boeing 737 accident report \1\). These special 
conditions are necessary

[[Page 58256]]

to establish a level of safety equivalent to that established by the 
existing airworthiness standards. The commenter's proposal would set a 
requirement beyond existing airworthiness standards and place an 
unnecessary burden on applicants. Therefore, no changes were made to 
these special conditions as a result of this comment.
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    \1\ Japan Transport Safety Board, Aircraft Accident 
Investigation Report, AA2009-7, China Airlines B18616, August 28, 
2009. www.mlit.go.jp/jtsb/eng-air_report/B18616.pdf.
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    The commenter requested the applicant show compliance by testing 
the capability of the design. The FAA acknowledges that some testing 
may be necessary to show compliance with these special conditions but 
does not agree that only testing must be used. To obtain a type 
certificate the applicant must follow the requirements of Sec.  
21.33(b)(1). No specific aspect of the proposed integral RCT, nor 
requirement of these special conditions, necessitates or requires that 
the applicant must demonstrate by test to show compliance. These 
special conditions do not include specific means of compliance since 
more than one means of compliance may be acceptable. No changes were 
made to these special conditions as a result of this comment.
    The commenter requested the FAA define the flame size and intensity 
the applicant must use for representing an external fuel-fed ground 
fire when showing compliance with these special conditions. Such 
definition is unnecessary. A post-crash external fuel-fed ground fire 
depends on many factors, including the specific airplane design and 
fuel types approved for use. Well-established industry standard fire 
test methods currently exist for powerplant installation fire 
protection, as well as cabin safety fire protection, that include 
standardized fire test conditions that are intended to represent a 
large pool fire. Applicants may consider these standards and any other 
available fire testing method, if shown to be applicable to these 
special conditions, when developing test methods for these special 
conditions. The FAA does not consider it is necessary to identify any 
specific test conditions as requirements for these special conditions. 
Therefore, no changes were made to these special conditions as a result 
of this comment.
    The commenter requested the FAA clarify what it means by sufficient 
time to evacuate to include occupants to move safely away from the 
aircraft due to the potential impact from a fuel tank explosion to the 
surrounding area. The commenter stated the 90-second evacuation test 
time would be insufficient and the 5-minute time referenced in the 
Notice of Proposed Special Conditions No. 25-23-06-SC may be acceptable 
if justified by the applicant. The commenter also stated the applicant 
should include an assessment of other aircraft accidents and time to 
move survivors clear of the aircraft in the justification.
    The FAA does not agree to specify a requirement in these special 
conditions for additional time for airplane occupants to move away from 
the airplane once safely evacuated. These special conditions are 
necessary to establish a level of safety equivalent to that established 
by the existing airworthiness standards. The considerations of moving 
occupants away from the airplane as proposed by the commenter apply 
generally to all airplane designs and are not specifically associated 
with or affected by the novel or unusual design feature of the RCT. 
Since these special conditions are intended to establish the same level 
of safety as the relevant regulations in this model's certification 
basis, by providing sufficient time for a safe evacuation of all 
occupants after the initiation of an external fuel-fed ground fire, it 
is unnecessary to include an additional assessment to account for 
moving occupants away from the airplane. Therefore, no changes were 
made to these special conditions as a result of this comment.
    The commenter requested that the FAA clarify how it considered 
maintainability of the design features needed to ensure the original 
design intent for each airplane as it ages. The FAA infers the 
commenter requests the FAA to include requirements for the airplane 
manufacturer to require airplane operators to maintain the critical 
features of the type design associated with these special conditions 
for the life of the airplane.
    The FAA agrees that critical features that need to be identified by 
the applicant and maintained in service should be appropriately 
managed; however, the FAA does not agree these special conditions 
should include a dedicated requirement to address this need. The FAA 
considers that the Model A321neo XLR certification basis already 
includes airworthiness standards that account for ensuring critical 
design features are maintained in service. Specifically, Sec. Sec.  
25.901(c) and 25.1309(b) include requirements for system safety 
analysis of propulsion and airplane systems. FAA Advisory Circular 25-
19A, Certification Maintenance Requirements, provides guidance on the 
selection, documentation, and control of Certification Maintenance 
Requirements (CMR). A CMR is a required scheduled maintenance task 
established during the design certification of the airplane systems as 
an operating limitation of the type certificate. The FAA considers it 
unnecessary to include additional requirements in these special 
conditions to maintain the type design of critical features since the 
Model A321new XLR certification basis includes airworthiness 
requirements that address this issue. Therefore, no changes were made 
to these special conditions as a result of this comment.
    In conclusion, no changes were made to the special conditions as a 
result of these comments, and the special conditions are adopted as 
proposed.

Applicability

    As discussed above, these 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.
    Under standard practice, the effective date of final special 
conditions would be 30 days after the date of publication in the 
Federal Register. However, as the certification date for the Airbus 
Model A321neo XLR series airplane is imminent, the FAA finds that good 
cause exists to make these special conditions effective upon 
publication.

Conclusion

    This action affects only a certain novel or unusual design feature 
on one model series 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 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

[[Page 58257]]

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 July 12, 2024.
Patrick R. Mullen,
Manager, Technical Policy Branch, Policy and Standards Division, 
Aircraft Certification Service.
[FR Doc. 2024-15853 Filed 7-17-24; 8:45 am]
BILLING CODE 4910-13-P