14 C.F.R. § 25.967 – Fuel tank installations.

(a) Each fuel tank must be supported so that tank loads (resulting from the weight of the fuel in the tanks) are not concentrated on unsupported tank surfaces. In addition—
1. (1) There must be pads, if necessary, to prevent chafing between the tank and its supports;
2. (2) Padding must be nonabsorbent or treated to prevent the absorption of fluids;
3. (3) If a flexible tank liner is used, it must be supported so that it is not required to withstand fluid loads; and
4. (4) Each interior surface of the tank compartment must be smooth and free of projections that could cause wear of the liner unless—
  1. (i) Provisions are made for protection of the liner at these points; or
  2. (ii) The construction of the liner itself provides that protection.
(b) Spaces adjacent to tank surfaces must be ventilated to avoid fume accumulation due to minor leakage. If the tank is in a sealed compartment, ventilation may be limited to drain holes large enough to prevent excessive pressure resulting from altitude changes.
(c) The location of each tank must meet the requirements of § 25.1185(a).
(d) No engine nacelle skin immediately behind a major air outlet from the engine compartment may act as the wall of an integral tank.
(e) Each fuel tank must be isolated from personnel compartments by a fumeproof and fuelproof enclosure.