14 CFR §25.971

1. (a)Each fuel tank must have a sump with an effective capacity, in the normal ground attitude, of not less than the greater of 0.10 percent of the tank capacity or one-sixteenth of a gallon unless operating limitations are established to ensure that the accumulation of water in service will not exceed the sump capacity.
2. (b)Each fuel tank must allow drainage of any hazardous quantity of water from any part of the tank to its sump with the airplane in the ground attitude.
3. (c)Each fuel tank sump must have an accessible drain that—
   1. (1)Allows complete drainage of the sump on the ground;
   2. (2)Discharges clear of each part of the airplane; and
   3. (3)Has manual or automatic means for positive locking in the closed position.