spaschal on DSK3GDR082PROD with CFR

Federal Aviation Administration, DOT

Pt. 25, App. N

(5) Fuel tank thermal characteristics. If
fuel temperature affects fuel tank flammability, 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 compartments, 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 temperature 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 evaluated, the applicant must provide substantiation that the number of flights per day
and the number of hours per flight for that
airplane model is consistent with the existing 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 flammability requirements of Section 25.981 and Appendix M of this part. The program must determine the time periods during each flight
phase when the fuel tank or compartment
with the FRM would be flammable. The following factors must be considered in establishing these time periods:
(1) Any time periods throughout the flammability exposure evaluation time and under
the full range of expected operating conditions, 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 assumed in the reliability analysis (60 flight
hours must be used for a 10-day MMEL dispatch 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 inoperative.
(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 oxygen 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 evolution from the fuel in the tank or compartment 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 Section 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 defined in Table 4, during a 12-hour overnight
period.
(e) The applicant must submit to the responsible Aircraft Certification Service
officefor approval the fuel tank flammability
analysis, including the airplane-specific parameters 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 exposure, substantiating data, and any airworthiness limitations and other conditions
assumed in the analysis.
N25.4 Variables and data tables.
The following data must be used when conducting a flammability exposure analysis to
determine the fleet average flammability exposure. Variables used to calculate fleet
flammability exposure must include atmospheric 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 exposure during a given flight, the variation of
ground ambient temperatures, cruise ambient 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 Section1-standard deviation value.
(2) Ambient Temperature: Under the program, the ground and cruise ambient temperatures 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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