14 CFR § 121.229
Location of fuel tanks
July 13, 2020
(a) Fuel tanks must be located in accordance with §121.255.
(b) No part of the engine nacelle skin that lies immediately behind a major air outlet from the engine compartment may be used as the wall of an integral tank.
(c) Fuel tanks must be isolated from personnel compartments by means of fume- and fuel-proof enclosures.