277 

Federal Aviation Administration, DOT 

§ 23.967 

inch (unless another displacement is 
substantiated) while 

2

⁄

3

filled with 

water or other suitable test fluid. 

(3) The test frequency of vibration 

must be as follows: 

(i) If no frequency of vibration result-

ing from any rpm within the normal 
operating range of engine or propeller 
speeds is critical, the test frequency of 
vibration is: 

(A) The number of cycles per minute 

obtained by multiplying the maximum 
continuous propeller speed in rpm by 
0.9 for propeller-driven airplanes, and 

(B) For non-propeller driven air-

planes the test frequency of vibration 
is 2,000 cycles per minute. 

(ii) If only one frequency of vibration 

resulting from any rpm within the nor-
mal operating range of engine or pro-
peller speeds is critical, that frequency 
of vibration must be the test fre-
quency. 

(iii) If more than one frequency of vi-

bration resulting from any rpm within 
the normal operating range of engine 
or propeller speeds is critical, the most 
critical of these frequencies must be 
the test frequency. 

(4) Under paragraph (b)(3) (ii) and (iii) 

of this section, the time of test must be 
adjusted to accomplish the same num-
ber of vibration cycles that would be 
accomplished in 25 hours at the fre-
quency specified in paragraph (b)(3)(i) 
of this section. 

(5) During the test, the tank assem-

bly must be rocked at a rate of 16 to 20 
complete cycles per minute, through 
an angle of 15

° 

on either side of the hor-

izontal (30

° 

total), about an axis par-

allel to the axis of the fuselage, for 25 
hours. 

(c) Each integral tank using methods 

of construction and sealing not pre-
viously proven to be adequate by test 
data or service experience must be able 
to withstand the vibration test speci-
fied in paragraphs (b)(1) through (4) of 
this section. 

(d) Each tank with a nonmetallic 

liner must be subjected to the sloshing 
test outlined in paragraph (b)(5) of this 
section, with the fuel at room tempera-
ture. In addition, a specimen liner of 
the same basic construction as that to 
be used in the airplane must, when in-
stalled in a suitable test tank, with-

stand the sloshing test with fuel at a 
temperature of 110 

°

F. 

[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as 
amended by Amdt. 23–43, 58 FR 18972, Apr. 9, 
1993; Amdt. 23–43, 61 FR 253, Jan. 4, 1996; 
Amdt. 23–51, 61 FR 5136, Feb. 9, 1996] 

§ 23.967

Fuel tank installation. 

(a) Each fuel tank must be supported 

so that tank loads are not con-
centrated. In addition— 

(1) There must be pads, if necessary, 

to prevent chafing between each tank 
and its supports; 

(2) Padding must be nonabsorbent or 

treated to prevent the absorption of 
fuel; 

(3) If a flexible tank liner is used, it 

must be supported so that it is not re-
quired to withstand fluid loads; 

(4) Interior surfaces adjacent to the 

liner must be smooth and free from 
projections that could cause wear, un-
less— 

(i) Provisions are made for protection 

of the liner at those points; or 

(ii) The construction of the liner 

itself provides such protection; and 

(5) A positive pressure must be main-

tained within the vapor space of each 
bladder cell under any condition of op-
eration, except for a particular condi-
tion for which it is shown that a zero 
or negative pressure will not cause the 
bladder cell to collapse; and 

(6) Syphoning of fuel (other than 

minor spillage) or collapse of bladder 
fuel cells may not result from improper 
securing or loss of the fuel filler cap. 

(b) Each tank compartment must be 

ventilated and drained to prevent the 
accumulation of flammable fluids or 
vapors. Each compartment adjacent to 
a tank that is an integral part of the 
airplane structure must also be venti-
lated and drained. 

(c) No fuel tank may be on the engine 

side of the firewall. There must be at 
least one-half inch of clearance be-
tween the fuel tank and the firewall. 
No part of the engine nacelle skin that 
lies immediately behind a major air 
opening from the engine compartment 
may act as the wall of an integral 
tank. 

(d) Each fuel tank must be isolated 

from personnel compartments by a 
fume-proof and fuel-proof enclosure 

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