Federal Aviation Administration, DOT

Section 25.961

(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 Section 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 compliance with this section is shown, and
the appropriate emergency pump must
be substituted for each main pump so
used.
(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 engine in not more than 20 seconds after
switching to any other fuel tank containing usable fuel when engine malfunctioning 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 addition 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]

spaschal on DSK3GDR082PROD with CFR

Section 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.
Section 25.959 Unusable fuel supply.
The unusable fuel quantity for each
fuel tank and its fuel system components 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 component 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]

Section 25.961 Fuel system hot weather operation.
(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 airport elected by the applicant to the
maximum altitude established as an
operating limitation under Section 25.1527. If a
climb test is elected, there may be no
evidence of vapor lock or other malfunctioning during the climb test conducted 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 critical altitude through the critical altitude. The time interval during which
takeoff power is used may not be less
than the takeoff time limitation.
(2) For turbine engine powered airplanes, the engines must operate at
takeoff power for the time interval selected 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, minimum crew, and the ballast necessary
to maintain the center of gravity within allowable limits.
(4) The climb airspeed may not exceed - 
(i) For reciprocating engine powered
airplanes, the maximum airspeed established for climbing from takeoff to
the maximum operating altitude with
the airplane in the following configuration:
(A) Landing gear retracted.
(B) Wing flaps in the most favorable
position.
(C) Cowl flaps (or other means of controlling the engine cooling supply) in

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