320
14 CFR Ch. I (1–1–24 Edition)
§ 25.977
(4) Airspaces of tanks with inter-
connected outlets must be inter-
connected;
(5) There may be no point in any vent
line where moisture can accumulate
with the airplane in the ground atti-
tude or the level flight attitude, unless
drainage is provided;
(6) No vent or drainage provision may
end at any point—
(i) Where the discharge of fuel from
the vent outlet would constitute a fire
hazard; or
(ii) From which fumes could enter
personnel compartments; and
(7) Each fuel tank vent system must
prevent explosions, for a minimum of 2
minutes and 30 seconds, caused by
propagation of flames from outside the
tank through the fuel tank vents into
fuel tank vapor spaces when any fuel
tank vent is continuously exposed to
flame.
(b)
Carburetor vapor vents. Each car-
buretor with vapor elimination connec-
tions must have a vent line to lead va-
pors back to one of the fuel tanks. In
addition—
(1) Each vent system must have
means to avoid stoppage by ice; and
(2) If there is more than one fuel
tank, and it is necessary to use the
tanks in a definite sequence, each
vapor vent return line must lead back
to the fuel tank used for takeoff and
landing.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Docket No. FAA–2014–0500,
Amdt. No. 25–143, 81 FR 41207, June 24, 2016]
§ 25.977
Fuel tank outlet.
(a) There must be a fuel strainer for
the fuel tank outlet or for the booster
pump. This strainer must—
(1) For reciprocating engine powered
airplanes, have 8 to 16 meshes per inch;
and
(2) For turbine engine powered air-
planes, prevent the passage of any ob-
ject that could restrict fuel flow or
damage any fuel system component.
(b) [Reserved]
(c) The clear area of each fuel tank
outlet strainer must be at least five
times the area of the outlet line.
(d) The diameter of each strainer
must be at least that of the fuel tank
outlet.
(e) Each finger strainer must be ac-
cessible for inspection and cleaning.
[Amdt. 25–11, 32 FR 6913, May 5, 1967, as
amended by Amdt. 25–36, 39 FR 35460, Oct. 1,
1974]
§ 25.979
Pressure fueling system.
For pressure fueling systems, the fol-
lowing apply:
(a) Each pressure fueling system fuel
manifold connection must have means
to prevent the escape of hazardous
quantities of fuel from the system if
the fuel entry valve fails.
(b) An automatic shutoff means must
be provided to prevent the quantity of
fuel in each tank from exceeding the
maximum quantity approved for that
tank. This means must—
(1) Allow checking for proper shutoff
operation before each fueling of the
tank; and
(2) Provide indication at each fueling
station of failure of the shutoff means
to stop the fuel flow at the maximum
quantity approved for that tank.
(c) A means must be provided to pre-
vent damage to the fuel system in the
event of failure of the automatic shut-
off means prescribed in paragraph (b)
of this section.
(d) The airplane pressure fueling sys-
tem (not including fuel tanks and fuel
tank vents) must withstand an ulti-
mate load that is 2.0 times the load
arising from the maximum pressures,
including surge, that is likely to occur
during fueling. The maximum surge
pressure must be established with any
combination of tank valves being ei-
ther intentionally or inadvertently
closed.
(e) The airplane defueling system
(not including fuel tanks and fuel tank
vents) must withstand an ultimate
load that is 2.0 times the load arising
from the maximum permissible
defueling pressure (positive or nega-
tive) at the airplane fueling connec-
tion.
[Amdt. 25–11, 32 FR 6913, May 5, 1967, as
amended by Amdt. 25–38, 41 FR 55467, Dec. 20,
1976; Amdt. 25–72, 55 FR 29785, July 20, 1990]
§ 25.981
Fuel tank explosion preven-
tion.
(a) No ignition source may be present
at each point in the fuel tank or fuel
tank system where catastrophic failure
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Federal Aviation Administration, DOT
§ 25.981
could occur due to ignition of fuel or
vapors. This must be shown by:
(1) Determining the highest tempera-
ture allowing a safe margin below the
lowest expected autoignition tempera-
ture of the fuel in the fuel tanks.
(2) Demonstrating that no tempera-
ture at each place inside each fuel tank
where fuel ignition is possible will ex-
ceed the temperature determined under
paragraph (a)(1) of this section. This
must be verified under all probable op-
erating, failure, and malfunction con-
ditions of each component whose oper-
ation, failure, or malfunction could in-
crease the temperature inside the tank.
(3) Except for ignition sources due to
lightning addressed by § 25.954, dem-
onstrating that an ignition source
could not result from each single fail-
ure, from each single failure in com-
bination with each latent failure condi-
tion not shown to be extremely remote,
and from all combinations of failures
not shown to be extremely improbable,
taking into account the effects of man-
ufacturing variability, aging, wear,
corrosion, and likely damage.
(b) Except as provided in paragraphs
(b)(2) and (c) of this section, no fuel
tank Fleet Average Flammability Ex-
posure on an airplane may exceed three
percent of the Flammability Exposure
Evaluation Time (FEET) as defined in
Appendix N of this part, or that of a
fuel tank within the wing of the air-
plane model being evaluated, which-
ever is greater. If the wing is not a con-
ventional unheated aluminum wing,
the analysis must be based on an as-
sumed Equivalent Conventional
Unheated Aluminum Wing Tank.
(1) Fleet Average Flammability Ex-
posure is determined in accordance
with Appendix N of this part. The as-
sessment must be done in accordance
with the methods and procedures set
forth in the Fuel Tank Flammability
Assessment Method User’s Manual,
dated May 2008, document number
DOT/FAA/AR–05/8 (incorporated by ref-
erence, see § 25.5).
(2) Any fuel tank other than a main
fuel tank on an airplane must meet the
flammability exposure criteria of Ap-
pendix M to this part if any portion of
the tank is located within the fuselage
contour.
(3) As used in this paragraph,
(i)
Equivalent Conventional Unheated
Aluminum Wing Tank is an integral
tank in an unheated semi-monocoque
aluminum wing of a subsonic airplane
that is equivalent in aerodynamic per-
formance, structural capability, fuel
tank capacity and tank configuration
to the designed wing.
(ii)
Fleet Average Flammability Expo-
sure is defined in Appendix N to this
part and means the percentage of time
each fuel tank ullage is flammable for
a fleet of an airplane type operating
over the range of flight lengths.
(iii)
Main Fuel Tank means a fuel
tank that feeds fuel directly into one
or more engines and holds required fuel
reserves continually throughout each
flight.
(c) Paragraph (b) of this section does
not apply to a fuel tank if means are
provided to mitigate the effects of an
ignition of fuel vapors within that fuel
tank such that no damage caused by an
ignition will prevent continued safe
flight and landing.
(d) To protect design features that
prevent catastrophic ignition sources
within the fuel tank or fuel tank sys-
tem according to paragraph (a) of this
section, and to prevent increasing the
flammability exposure of the tanks
above that permitted in paragraph (b)
of this section, the type design must
include critical design configuration
control limitations (CDCCLs) identi-
fying those features and providing in-
structions on how to protect them. To
ensure the continued effectiveness of
those features, and prevent degrada-
tion of the performance and reliability
of any means provided according to
paragraphs (a), (b), or (c) of this sec-
tion, the type design must also include
necessary inspection and test proce-
dures, intervals between repetitive in-
spections and tests, and mandatory re-
placement times for those features.
The applicant must include informa-
tion required by this paragraph in the
Airworthiness Limitations section of
the Instructions for Continued Air-
worthiness required by § 25.1529. The
type design must also include visible
means of identifying critical features
of the design in areas of the airplane
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14 CFR Ch. I (1–1–24 Edition)
§ 25.991
where foreseeable maintenance ac-
tions, repairs, or alterations may com-
promise the CDCCLs.
[Doc. No. 1999–6411, 66 FR 23129, May 7, 2001,
as amended by Doc. No. FAA–2005–22997, 73
FR 42494, July 21, 2008; Doc. No. FAA– 2014–
1027, Amdt. No. 25–146, 83 FR 47556, Sept. 20,
2018]
F
UEL
S
YSTEM
C
OMPONENTS
§ 25.991
Fuel pumps.
(a)
Main pumps. Each fuel pump re-
quired for proper engine operation, or
required to meet the fuel system re-
quirements of this subpart (other than
those in paragraph (b) of this section,
is a main pump. For each main pump,
provision must be made to allow the
bypass of each positive displacement
fuel pump other than a fuel injection
pump (a pump that supplies the proper
flow and pressure for fuel injection
when the injection is not accomplished
in a carburetor) approved as part of the
engine.
(b)
Emergency pumps. There must be
emergency pumps or another main
pump to feed each engine immediately
after failure of any main pump (other
than a fuel injection pump approved as
part of the engine).
§ 25.993
Fuel system lines and fittings.
(a) Each fuel line must be installed
and supported to prevent excessive vi-
bration and to withstand loads due to
fuel pressure and accelerated flight
conditions.
(b) Each fuel line connected to com-
ponents of the airplane between which
relative motion could exist must have
provisions for flexibility.
(c) Each flexible connection in fuel
lines that may be under pressure and
subjected to axial loading must use
flexible hose assemblies.
(d) Flexible hose must be approved or
must be shown to be suitable for the
particular application.
(e) No flexible hose that might be ad-
versely affected by exposure to high
temperatures may be used where exces-
sive temperatures will exist during op-
eration or after engine shut-down.
(f) Each fuel line within the fuselage
must be designed and installed to allow
a reasonable degree of deformation and
stretching without leakage.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–15, 32 FR 13266, Sept.
20, 1967]
§ 25.994
Fuel system components.
Fuel system components in an engine
nacelle or in the fuselage must be pro-
tected from damage that could result
in spillage of enough fuel to constitute
a fire hazard as a result of a wheels-up
landing on a paved runway under each
of the conditions prescribed in
§ 25.721(b).
[Amdt. 25–139, 79 FR 59430, Oct. 2, 2014]
§ 25.995
Fuel valves.
In addition to the requirements of
§ 25.1189 for shutoff means, each fuel
valve must—
(a) [Reserved]
(b) Be supported so that no loads re-
sulting from their operation or from
accelerated flight conditions are trans-
mitted to the lines attached to the
valve.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–40, 42 FR 15043, Mar. 17,
1977]
§ 25.997
Fuel strainer or filter.
There must be a fuel strainer or filter
between the fuel tank outlet and the
inlet of either the fuel metering device
or an engine driven positive displace-
ment pump, whichever is nearer the
fuel tank outlet. This fuel strainer or
filter must—
(a) Be accessible for draining and
cleaning and must incorporate a screen
or element which is easily removable;
(b) Have a sediment trap and drain
except that it need not have a drain if
the strainer or filter is easily remov-
able for drain purposes;
(c) Be mounted so that its weight is
not supported by the connecting lines
or by the inlet or outlet connections of
the strainer or filter itself, unless ade-
quate strength margins under all load-
ing conditions are provided in the lines
and connections; and
(d) Have the capacity (with respect to
operating limitations established for
the engine) to ensure that engine fuel
system functioning is not impaired,
with the fuel contaminated to a degree
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