617
Federal Aviation Administration, DOT
Pt. 25, App. N
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 FAA
Oversight Office for approval the fuel tank
flammability analysis, including the air-
plane-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 af-
fect 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 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
curve, given by the 50 percent value and a
±
1-
standard deviation, as shown in Table 1 of
this appendix.
(ii) The flammability envelope of the fuel
that must be used for the flammability expo-
sure analysis is a function of the flash point
of the fuel selected by the Monte Carlo for a
given flight. The flammability envelope for
the fuel is defined by the upper flammability
limit (UFL) and lower flammability limit
(LFL) as follows:
(A) LFL at sea level = flash point tempera-
ture of the fuel at sea level minus 10
°
F. LFL
decreases from sea level value with increas-
ing altitude at a rate of 1
°
F per 808 feet.
(B) UFL at sea level = flash point tempera-
ture of the fuel at sea level plus 63.5
°
F. UFL
decreases from the sea level value with in-
creasing altitude at a rate of 1
°
F per 512
feet.
(4) For each flight analyzed, a separate
random number must be generated for each
of the three parameters (ground ambient
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