title-14-part-25-sec25-955

315

Federal Aviation Administration, DOT

§ 25.955

the most critical condition for icing
likely to be encountered in operation.

(d) Each fuel system for a turbine en-

gine powered airplane must meet the
applicable fuel venting requirements of
part 34 of this chapter.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–23, 35 FR 5677, Apr. 8,
1970; Amdt. 25–36, 39 FR 35460, Oct. 1, 1974;
Amdt. 25–38, 41 FR 55467, Dec. 20, 1976; Amdt.
25–73, 55 FR 32861, Aug. 10, 1990]

§ 25.952

Fuel system analysis and test.

(a) Proper fuel system functioning

under all probable operating conditions
must be shown by analysis and those
tests found necessary by the Adminis-
trator. Tests, if required, must be made
using the airplane fuel system or a test
article that reproduces the operating
characteristics of the portion of the
fuel system to be tested.

(b) The likely failure of any heat ex-

changer using fuel as one of its fluids
may not result in a hazardous condi-
tion.

[Amdt. 25–40, 42 FR 15043, Mar. 17, 1977]

§ 25.953

Fuel system independence.

Each fuel system must meet the re-

quirements of § 25.903(b) by—

(a) Allowing the supply of fuel to

each engine through a system inde-
pendent of each part of the system sup-
plying fuel to any other engine; or

(b) Any other acceptable method.

§ 25.954

Fuel system lightning protec-

tion.

(a) For purposes of this section—
(1) A critical lightning strike is a

lightning strike that attaches to the
airplane in a location that, when com-
bined with the failure of any design
feature or structure, could create an
ignition source.

(2) A fuel system includes any compo-

nent within either the fuel tank struc-
ture or the fuel tank systems, and any
airplane structure or system compo-
nents that penetrate, connect to, or are
located within a fuel tank.

(b) The design and installation of a

fuel system must prevent catastrophic
fuel vapor ignition due to lightning and
its effects, including:

(1) Direct lightning strikes to areas

having a high probability of stroke at-
tachment;

(2) Swept lightning strokes to areas

where swept strokes are highly prob-
able; and

(3) Lightning-induced or conducted

electrical transients.

this section, catastrophic fuel vapor ig-
nition must be extremely improbable,
taking into account flammability, crit-
ical lightning strikes, and failures
within the fuel system.

(d) To protect design features that

prevent catastrophic fuel vapor igni-
tion caused by lightning, the type de-
sign must include critical design con-
figuration control limitations
(CDCCLs) identifying those features
and providing information to protect
them. To ensure the continued effec-
tiveness of those design features, the
type design must also include inspec-
tion and test procedures, intervals be-
tween repetitive inspections and tests,
and mandatory replacement times for
those design features used in dem-
onstrating compliance to paragraph (b)
of this section. The applicant must in-
clude the information required by this
paragraph in the Airworthiness Limi-
tations section of the Instructions for
Continued Airworthiness required by
§ 25.1529.

[Doc. No. FAA–2014–1027, Amdt. 25–146, 83 FR
47556, Sept. 20, 2018]

§ 25.955

Fuel flow.

(a) Each fuel system must provide at

least 100 percent of the fuel flow re-
quired under each intended operating
condition and maneuver. Compliance
must be shown as follows:

(1) Fuel must be delivered to each en-

gine at a pressure within the limits
specified in the engine type certificate.

(2) The quantity of fuel in the tank

may not exceed the amount established
as the unusable fuel supply for that
tank under the requirements of § 25.959
plus that necessary to show compliance
with this section.

(3) Each main pump must be used

that is necessary for each operating
condition and attitude for which com-
pliance with this section is shown, and
the appropriate emergency pump must
be substituted for each main pump so
used.

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14 CFR Ch. I (1–1–24 Edition)

§ 25.957

(4) If there is a fuel flowmeter, it

must be blocked and the fuel must flow
through the meter or its bypass.

(b) If an engine can be supplied with

fuel from more than one tank, the fuel
system must—

(1) For each reciprocating engine,

supply the full fuel pressure to that en-
gine in not more than 20 seconds after
switching to any other fuel tank con-
taining usable fuel when engine mal-
functioning becomes apparent due to
the depletion of the fuel supply in any
tank from which the engine can be fed;
and

(2) For each turbine engine, in addi-

tion to having appropriate manual
switching capability, be designed to
prevent interruption of fuel flow to
that engine, without attention by the
flight crew, when any tank supplying
fuel to that engine is depleted of usable
fuel during normal operation, and any
other tank, that normally supplies fuel
to that engine alone, contains usable
fuel.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–11, 32 FR 6912, May 5,
1967]

§ 25.957

Flow between interconnected

tanks.

If fuel can be pumped from one tank

to another in flight, the fuel tank
vents and the fuel transfer system
must be designed so that no structural
damage to the tanks can occur because
of overfilling.

§ 25.959

Unusable fuel supply.

The unusable fuel quantity for each

fuel tank and its fuel system compo-
nents must be established at not less
than the quantity at which the first
evidence of engine malfunction occurs
under the most adverse fuel feed condi-
tion for all intended operations and
flight maneuvers involving fuel feeding
from that tank. Fuel system compo-
nent failures need not be considered.

[Amdt. 25–23, 35 FR 5677, Apr. 8, 1970, as
amended by Amdt. 25–40, 42 FR 15043, Mar. 17,
1977]

§ 25.961

Fuel system hot weather oper-

ation.

(a) The fuel system must perform

satisfactorily in hot weather operation.
This must be shown by showing that

the fuel system from the tank outlets
to each engine is pressurized, under all
intended operations, so as to prevent
vapor formation, or must be shown by
climbing from the altitude of the air-
port elected by the applicant to the
maximum altitude established as an
operating limitation under § 25.1527. If a
climb test is elected, there may be no
evidence of vapor lock or other mal-
functioning during the climb test con-
ducted under the following conditions:

(1) For reciprocating engine powered

airplanes, the engines must operate at
maximum continuous power, except
that takeoff power must be used for the
altitudes from 1,000 feet below the crit-
ical altitude through the critical alti-
tude. The time interval during which
takeoff power is used may not be less
than the takeoff time limitation.

(2) For turbine engine powered air-

planes, the engines must operate at
takeoff power for the time interval se-
lected for showing the takeoff flight
path, and at maximum continuous
power for the rest of the climb.

(3) The weight of the airplane must

be the weight with full fuel tanks, min-
imum crew, and the ballast necessary
to maintain the center of gravity with-
in allowable limits.

(4) The climb airspeed may not ex-

ceed—

(i) For reciprocating engine powered

airplanes, the maximum airspeed es-
tablished for climbing from takeoff to
the maximum operating altitude with
the airplane in the following configura-
tion:

(A) Landing gear retracted.
(B) Wing flaps in the most favorable

position.

trolling the engine cooling supply) in
the position that provides adequate
cooling in the hot-day condition.

(D) Engine operating within the max-

imum continuous power limitations.

(E) Maximum takeoff weight; and
(ii) For turbine engine powered air-

planes, the maximum airspeed estab-
lished for climbing from takeoff to the
maximum operating altitude.

(5) The fuel temperature must be at

least 110

(b) The test prescribed in paragraph

(a) of this section may be performed in
flight or on the ground under closely