14 CFR § 29.967

Each fuel tank must be supported so that tank loads are not concentrated on unsupported tank surfaces. In addition—

There must be pads, if necessary, to prevent chafing between each tank and its supports;

The padding must be nonabsorbent or treated to prevent the absorption of fuel;

If flexible tank liners are used, they must be supported so that they are not required to withstand fluid loads; and

Each interior surface of tank compartments must be smooth and free of projections that could cause wear of the liner, unless—

There are means for protection of the liner at those points; or

The construction of the liner itself provides such protection.

Any spaces adjacent to tank surfaces must be adequately ventilated to avoid accumulation of fuel or fumes in those spaces due to minor leakage. If the tank is in a sealed compartment, ventilation may be limited to drain holes that prevent clogging and that prevent excessive pressure resulting from altitude changes. If flexible tank liners are installed, the venting arrangement for the spaces between the liner and its container must maintain the proper relationship to tank vent pressures for any expected flight condition.

The location of each tank must meet the requirements of § 29.1185(b) and (c).

No rotorcraft skin immediately adjacent to a major air outlet from the engine compartment may act as the wall of an integral tank.