spaschal on DSK3GDR082PROD with CFR

Federal Aviation Administration, DOT

Pt. 25, App. N

M25.1 of this appendix with any airplane or
engine configuration affecting the performance of the FRM for which approval is
sought.
M25.3 Reliability indications and maintenance access.
(a) Reliability indications must be provided to identify failures of the FRM that
would otherwise be latent and whose identification is necessary to ensure the fuel tank
with an FRM meets the fleet average flammability exposure requirements listed in
paragraph M25.1 of this appendix, including
when the FRM is inoperative.
(b) Sufficient accessibility to FRM reliability indications must be provided for
maintenance personnel or the flightcrew.
(c) The access doors and panels to the fuel
tanks with FRMs (including any tanks that
communicate with a tank via a vent system), and to any other confined spaces or enclosed areas that could contain hazardous atmosphere under normal conditions or failure
conditions, must be permanently stenciled,
marked, or placarded to warn maintenance
personnel of the possible presence of a potentially hazardous atmosphere.
M25.4 Airworthiness limitations and procedures.
(a) If FRM is used to comply with paragraph M25.1 of this appendix, Airworthiness
Limitations must be identified for all maintenance or inspection tasks required to identify failures of components within the FRM
that are needed to meet paragraph M25.1 of
this appendix.
(b) Maintenance procedures must be developed to identify any hazards to be considered
during maintenance of the FRM. These procedures must be included in the instructions
for continued airworthiness (ICA).
M25.5 Reliability reporting.
The effects of airplane component failures
on FRM reliability must be assessed on an
on-going basis. The applicant/holder must do
the following:
(a) Demonstrate effective means to ensure
collection of FRM reliability data. The
means must provide data affecting FRM reliability, such as component failures.
(b) Unless alternative reporting procedures
are approved by the responsible Aircraft Certification Service office, as defined in part 26
of this subchapter, provide a report to the
FAA every six months for the first five years
after service introduction. After that period,
continued reporting every six months may
be replaced with other reliability tracking
methods found acceptable to the FAA or
eliminated if it is established that the reliability of the FRM meets, and will continue
to meet, the exposure requirements of paragraph M25.1 of this appendix.
(c) Develop service instructions or revise
the applicable airplane manual, according to
a schedule approved by the responsible Aircraft Certification Service office, as defined

in part 26 of this subchapter, to correct any
failures of the FRM that occur in service
that could increase any fuel tank-s Fleet Average Flammability Exposure to more than
that required by paragraph M25.1 of this appendix.
[Doc. No. FAA-2005-22997, 73 FR 42494, July
21, 2008, as amended by Doc. No. FAA-2018-
0119, Amdt. 25-145, 83 FR 9169, Mar. 5, 2018]

APPENDIX N TO PART 25 - FUEL TANK
FLAMMABILITY EXPOSURE AND RELIABILITY ANALYSIS
N25.1 General.
(a) This appendix specifies the requirements for conducting fuel tank fleet average
flammability exposure analyses required to
meet Section 25.981(b) and Appendix M of this part.
For fuel tanks installed in aluminum wings,
a qualitative assessment is sufficient if it
substantiates that the tank is a conventional unheated wing tank.
(b) This appendix defines parameters affecting fuel tank flammability that must be
used in performing the analysis. These include parameters that affect all airplanes
within the fleet, such as a statistical distribution of ambient temperature, fuel flash
point, flight lengths, and airplane descent
rate. Demonstration of compliance also requires application of factors specific to the
airplane model being evaluated. Factors that
need to be included are maximum range,
cruise mach number, typical altitude where
the airplane begins initial cruise phase of
flight, fuel temperature during both ground
and flight times, and the performance of a
flammability reduction means (FRM) if installed.
(c) The following definitions, input variables, and data tables must be used in the
program to determine fleet average flammability exposure for a specific airplane model.
N25.2 Definitions.
(a) Bulk Average Fuel Temperature means
the average fuel temperature within the fuel
tank or different sections of the tank if the
tank is subdivided by baffles or compartments.
(b) Flammability Exposure Evaluation Time
(FEET). The time from the start of preparing
the airplane for flight, through the flight
and landing, until all payload is unloaded,
and all passengers and crew have disembarked. In the Monte Carlo program, the
flight time is randomly selected from the
Flight Length Distribution (Table 2), the
pre-flight times are provided as a function of
the flight time, and the post-flight time is a
constant 30 minutes.
(c) Flammable. With respect to a fluid or
gas, flammable means susceptible to igniting
readily or to exploding (14 CFR Part 1, Definitions). A non-flammable ullage is one
where the fuel-air vapor is too lean or too

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