315 

Federal Aviation Administration, DOT 

§ 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 § 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. 

(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. 

(C) Cowl flaps (or other means of con-

trolling the engine cooling supply) in 

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