457 

Federal Aviation Administration, DOT 

Pt. 25, App. N 

(5) Fuel tank thermal characteristics. If 

fuel temperature affects fuel tank flamma-
bility, inputs to the Monte Carlo analysis 
must be provided that represent the actual 
bulk average fuel temperature within the 
fuel tank at each point in time throughout 
each of the flights being evaluated. For fuel 
tanks that are subdivided by baffles or com-
partments, bulk average fuel temperature 
inputs must be provided for each section of 
the tank. Input values for these data must be 
obtained from ground and flight test data or 
a thermal model of the tank that has been 
validated by ground and flight test data. 

(6) Maximum airplane operating tempera-

ture limit, as defined by any limitations in 
the airplane flight manual. 

(7) Airplane Utilization. The applicant 

must provide data supporting the number of 
flights per day and the number of hours per 
flight for the specific airplane model under 
evaluation. If there is no existing airplane 
fleet data to support the airplane being eval-
uated, the applicant must provide substan-
tiation that the number of flights per day 
and the number of hours per flight for that 
airplane model is consistent with the exist-
ing fleet data they propose to use. 

(d) 

Fuel Tank FRM Model. 

If FRM is used, 

an FAA approved Monte Carlo program must 
be used to show compliance with the flam-
mability requirements of § 25.981 and Appen-
dix M of this part. The program must deter-
mine the time periods during each flight 
phase when the fuel tank or compartment 
with the FRM would be flammable. The fol-
lowing factors must be considered in estab-
lishing these time periods: 

(1) Any time periods throughout the flam-

mability exposure evaluation time and under 
the full range of expected operating condi-
tions, when the FRM is operating properly 
but fails to maintain a non-flammable fuel 
tank because of the effects of the fuel tank 
vent system or other causes, 

(2) If dispatch with the system inoperative 

under the Master Minimum Equipment List 
(MMEL) is requested, the time period as-
sumed in the reliability analysis (60 flight 
hours must be used for a 10-day MMEL dis-
patch limit unless an alternative period has 
been approved by the Administrator), 

(3) Frequency and duration of time periods 

of FRM inoperability, substantiated by test 
or analysis acceptable to the FAA, caused by 
latent or known failures, including airplane 
system shut-downs and failures that could 
cause the FRM to shut down or become inop-
erative. 

(4) Effects of failures of the FRM that 

could increase the flammability exposure of 
the fuel tank. 

(5) If an FRM is used that is affected by ox-

ygen concentrations in the fuel tank, the 
time periods when oxygen evolution from the 
fuel results in the fuel tank or compartment 
exceeding the inert level. The applicant 

must include any times when oxygen evo-
lution from the fuel in the tank or compart-
ment under evaluation would result in a 
flammable fuel tank. The oxygen evolution 
rate that must be used is defined in the Fuel 
Tank Flammability Assessment Method 
User’s Manual, dated May 2008, document 
number DOT/FAA/AR–05/8 (incorporated by 
reference in § 25.5). 

(6) If an inerting system FRM is used, the 

effects of any air that may enter the fuel 
tank following the last flight of the day due 
to changes in ambient temperature, as de-
fined in Table 4, during a 12-hour overnight 
period. 

(e) The applicant must submit to the re-

sponsible Aircraft Certification Service 
officefor approval the fuel tank flammability 
analysis, including the airplane-specific pa-
rameters identified under paragraph N25.3(c) 
of this appendix and any deviations from the 
parameters identified in paragraph N25.3(b) 
of this appendix that affect flammability ex-
posure, substantiating data, and any air-
worthiness limitations and other conditions 
assumed in the analysis. 

N25.4

Variables and data tables. 

The following data must be used when con-

ducting a flammability exposure analysis to 
determine the fleet average flammability ex-
posure. Variables used to calculate fleet 
flammability exposure must include atmos-
pheric ambient temperatures, flight length, 
flammability exposure evaluation time, fuel 
flash point, thermal characteristics of the 
fuel tank, overnight temperature drop, and 
oxygen evolution from the fuel into the 
ullage. 

(a) Atmospheric Ambient Temperatures 

and Fuel Properties. 

(1) In order to predict flammability expo-

sure during a given flight, the variation of 
ground ambient temperatures, cruise ambi-
ent temperatures, and a method to compute 
the transition from ground to cruise and 
back again must be used. The variation of 
the ground and cruise ambient temperatures 
and the flash point of the fuel is defined by 
a Gaussian curve, given by the 50 percent 
value and a 

±

1-standard deviation value. 

(2) Ambient Temperature: Under the pro-

gram, the ground and cruise ambient tem-
peratures are linked by a set of assumptions 
on the atmosphere. The temperature varies 
with altitude following the International 
Standard Atmosphere (ISA) rate of change 
from the ground ambient temperature until 
the cruise temperature for the flight is 
reached. Above this altitude, the ambient 
temperature is fixed at the cruise ambient 
temperature. This results in a variation in 
the upper atmospheric temperature. For cold 
days, an inversion is applied up to 10,000 feet, 
and then the ISA rate of change is used. 

(3) Fuel properties: 
(i) For Jet A fuel, the variation of flash 

point of the fuel is defined by a Gaussian 

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