455 

Federal Aviation Administration, DOT 

Pt. 25, App. N 

M25.1 of this appendix with any airplane or 
engine configuration affecting the perform-
ance of the FRM for which approval is 
sought. 

M25.3

Reliability indications and mainte-

nance access. 

(a) Reliability indications must be pro-

vided to identify failures of the FRM that 
would otherwise be latent and whose identi-
fication is necessary to ensure the fuel tank 
with an FRM meets the fleet average flam-
mability exposure requirements listed in 
paragraph M25.1 of this appendix, including 
when the FRM is inoperative. 

(b) Sufficient accessibility to FRM reli-

ability 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 sys-
tem), and to any other confined spaces or en-
closed areas that could contain hazardous at-
mosphere under normal conditions or failure 
conditions, must be permanently stenciled, 
marked, or placarded to warn maintenance 
personnel of the possible presence of a poten-
tially hazardous atmosphere. 

M25.4

Airworthiness limitations and proce-

dures. 

(a) If FRM is used to comply with para-

graph M25.1 of this appendix, Airworthiness 
Limitations must be identified for all main-
tenance or inspection tasks required to iden-
tify failures of components within the FRM 
that are needed to meet paragraph M25.1 of 
this appendix. 

(b) Maintenance procedures must be devel-

oped to identify any hazards to be considered 
during maintenance of the FRM. These pro-
cedures 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 reli-
ability, such as component failures. 

(b) Unless alternative reporting procedures 

are approved by the responsible Aircraft Cer-
tification 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 reli-
ability of the FRM meets, and will continue 
to meet, the exposure requirements of para-
graph M25.1 of this appendix. 

(c) Develop service instructions or revise 

the applicable airplane manual, according to 
a schedule approved by the responsible Air-
craft 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 Av-
erage Flammability Exposure to more than 
that required by paragraph M25.1 of this ap-
pendix. 

[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] 

A

PPENDIX

N TO P

ART

25—F

UEL

T

ANK

 

F

LAMMABILITY

E

XPOSURE AND

R

ELI

-

ABILITY

A

NALYSIS

 

N25.1

General. 

(a) This appendix specifies the require-

ments for conducting fuel tank fleet average 
flammability exposure analyses required to 
meet § 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 conven-
tional unheated wing tank. 

(b) This appendix defines parameters af-

fecting fuel tank flammability that must be 
used in performing the analysis. These in-
clude parameters that affect all airplanes 
within the fleet, such as a statistical dis-
tribution of ambient temperature, fuel flash 
point, flight lengths, and airplane descent 
rate. Demonstration of compliance also re-
quires 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 in-
stalled. 

(c) The following definitions, input vari-

ables, and data tables must be used in the 
program to determine fleet average flamma-
bility 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 compart-
ments. 

(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 dis-
embarked. 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, Defi-
nitions). A non-flammable ullage is one 
where the fuel-air vapor is too lean or too 

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