14 CFR 29.971 - Fuel tank sump.
(a) Each fuel tank must have a sump with a capacity of not less than the greater of -
(1) 0.10 per cent of the tank capacity; or
(b) The capacity prescribed in paragraph (a) of this section must be effective with the rotorcraft in any normal attitude, and must be located so that the sump contents cannot escape through the tank outlet opening.
(c) Each fuel tank must allow drainage of hazardous quantities of water from each part of the tank to the sump with the rotorcraft in any ground attitude to be expected in service.
(d) Each fuel tank sump must have a drain that allows complete drainage of the sump on the ground.