Federal Aviation Administration, DOT § 25.971

(i) If no frequency of vibration result- (1) There must be pads, if necessary, ing from any r.p.m. within the normal to prevent chafing between the tank operating range of engine speeds is and its supports; critical, the test frequency of vibration (2) Padding must be nonabsorbent or must be 2,000 cycles per minute. treated to prevent the absorption of (ii) If only one frequency of vibration fluids; resulting from any r.p.m. within the (3) If a flexible tank liner is used, it normal operating range of engine must be supported so that it is not re- speeds is critical, that frequency of vi- quired to withstand fluid loads; and bration must be the test frequency. (4) Each interior surface of the tank compartment must be smooth and free (iii) If more than one frequency of vi- of projections that could cause wear of bration resulting from any r.p.m. with- the liner unless— in the normal operating range of en- (i) Provisions are made for protection gine speeds is critical, the most crit- of the liner at these points; or ical of these frequencies must be the (ii) The construction of the liner test frequency. itself provides that protection. (4) Under paragraphs (b)(3)(ii) and (b) Spaces adjacent to tank surfaces (iii) of this section, the time of test must be ventilated to avoid fume accu- must be adjusted to accomplish the mulation due to minor leakage. If the same number of vibration cycles that tank is in a sealed compartment, ven- would be accomplished in 25 hours at tilation may be limited to drain holes the frequency specified in paragraph large enough to prevent excessive pres- (b)(3)(i) of this section. sure resulting from altitude changes. (5) During the test, the tank assem- (c) The location of each tank must bly must be rocked at the rate of 16 to meet the requirements of § 25.1185(a). 20 complete cycles per minute, through (d) No engine nacelle skin imme- an angle of 15° on both sides of the hor- diately behind a major air outlet from izontal (30° total), about the most crit- the engine compartment may act as ical axis, for 25 hours. If motion about the wall of an integral tank. more than one axis is likely to be crit- (e) Each fuel tank must be isolated ical, the tank must be rocked about from personnel compartments by a fumeproof and fuelproof enclosure. each critical axis for 121⁄2 hours. (c) Except where satisfactory oper- § 25.969 Fuel tank expansion space. ating experience with a similar tank in Each fuel tank must have an expan- a similar installation is shown, non- sion space of not less than 2 percent of metallic tanks must withstand the test the tank capacity. It must be impos- specified in paragraph (b)(5) of this sec- sible to fill the expansion space inad- tion, with fuel at a temperature of 110 vertently with the airplane in the nor- ° F. During this test, a representative mal ground attitude. For pressure fuel- specimen of the tank must be installed ing systems, compliance with this sec- in a supporting structure simulating tion may be shown with the means pro- the installation in the airplane. vided to comply with § 25.979(b). (d) For pressurized fuel tanks, it must be shown by analysis or tests [Amdt. 25–11, 32 FR 6913, May 5, 1967] that the fuel tanks can withstand the § 25.971 Fuel tank sump. maximum pressure likely to occur on the ground or in flight. (a) Each fuel tank must have a sump with an effective capacity, in the nor- [Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as mal ground attitude, of not less than amended by Amdt. 25–11, 32 FR 6913, May 5, the greater of 0.10 percent of the tank 1967; Amdt. 25–40, 42 FR 15043, Mar. 17, 1977] capacity or one-sixteenth of a gallon unless operating limitations are estab- § 25.967 Fuel tank installations. lished to ensure that the accumulation (a) Each fuel tank must be supported of water in service will not exceed the so that tank loads (resulting from the sump capacity. weight of the fuel in the tanks) are not (b) Each fuel tank must allow drain- concentrated on unsupported tank sur- age of any hazardous quantity of water faces. In addition— from any part of the tank to its sump

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