49 CFR § 238.423
Fuel tanks
November 18, 2020
(a) External fuel tanks. Each type of external fuel tank must be approved by FRA's Associate Administrator for Safety upon a showing that the fuel tank provides a level of safety at least equivalent to a fuel tank that complies with the external fuel tank requirements in §238.223(a).
(b) Internal fuel tanks. Internal fuel tanks shall comply with the requirements specified in §238.223(b).