276 

14 CFR Ch. I (1–1–14 Edition) 

§ 23.959 

conditions in § 23.959, except that full 
tanks must be used. 

(b) 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 any airplane component can 
occur because of overfilling of any 
tank. 

[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as 
amended by Amdt. 23–43, 58 FR 18972, Apr. 9, 
1993] 

§ 23.959

Unusable fuel supply. 

(a) The unusable fuel supply for each 

tank must be established as not less 
than that quantity at which the first 
evidence of malfunctioning occurs 
under the most adverse fuel feed condi-
tion occurring under each intended op-
eration and flight maneuver involving 
that tank. Fuel system component fail-
ures need not be considered. 

(b) The effect on the usable fuel 

quantity as a result of a failure of any 
pump shall be determined. 

[Amdt. 23–7, 34 FR 13093, Aug. 13, 1969, as 
amended by Amdt. 23–18, 42 FR 15041, Mar. 17, 
1977; Amdt. 23–51, 61 FR 5136, Feb. 9, 1996] 

§ 23.961

Fuel system hot weather oper-

ation. 

Each fuel system must be free from 

vapor lock when using fuel at its crit-
ical temperature, with respect to vapor 
formation, when operating the airplane 
in all critical operating and environ-
mental conditions for which approval 
is requested. For turbine fuel, the ini-
tial temperature must be 110 

°

F,  ¥0

°

, 

+5 

°

F or the maximum outside air tem-

perature for which approval is re-
quested, whichever is more critical. 

[Doc. No. 26344, 58 FR 18972, Apr. 9, 1993; 58 
FR 27060, May 6, 1993] 

§ 23.963

Fuel tanks: General. 

(a) Each fuel tank must be able to 

withstand, without failure, the vibra-
tion, inertia, fluid, and structural loads 
that it may be subjected to in oper-
ation. 

(b) Each flexible fuel tank liner must 

be shown to be suitable for the par-
ticular application. 

(c) Each integral fuel tank must have 

adequate facilities for interior inspec-
tion and repair. 

(d) The total usable capacity of the 

fuel tanks must be enough for at least 
one-half hour of operation at maximum 
continuous power. 

(e) Each fuel quantity indicator must 

be adjusted, as specified in § 23.1337(b), 
to account for the unusable fuel supply 
determined under § 23.959(a). 

[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964; 30 
FR 258, Jan. 9, 1965, as amended by Amdt 23– 
34, 52 FR 1832, Jan. 15, 1987; Amdt. 23–43, 58 
FR 18972, Apr. 9, 1993; Amdt. 23–51, 61 FR 5136, 
Feb. 9, 1996] 

§ 23.965

Fuel tank tests. 

(a) Each fuel tank must be able to 

withstand the following pressures with-
out failure or leakage: 

(1) For each conventional metal tank 

and nonmetallic tank with walls not 
supported by the airplane structure, a 
pressure of 3.5 p.s.i., or that pressure 
developed during maximum ultimate 
acceleration with a full tank, which-
ever is greater. 

(2) For each integral tank, the pres-

sure developed during the maximum 
limit acceleration of the airplane with 
a full tank, with simultaneous applica-
tion of the critical limit structural 
loads. 

(3) For each nonmetallic tank with 

walls supported by the airplane struc-
ture and constructed in an acceptable 
manner using acceptable basic tank 
material, and with actual or simulated 
support conditions, a pressure of 2 p.s.i. 
for the first tank of a specific design. 
The supporting structure must be de-
signed for the critical loads occurring 
in the flight or landing strength condi-
tions combined with the fuel pressure 
loads resulting from the corresponding 
accelerations. 

(b) Each fuel tank with large, unsup-

ported, or unstiffened flat sur-
faces,whose failure or deformation 
could cause fuel leakage, must be able 
to withstand the following test without 
leakage, failure, or excessive deforma-
tion of the tank walls: 

(1) Each complete tank assembly and 

its support must be vibration tested 
while mounted to simulate the actual 
installation. 

(2) Except as specified in paragraph 

(b)(4) of this section, the tank assembly 
must be vibrated for 25 hours at a total 
displacement of not less than 

1

⁄

32

of an 

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