FAR121.639
Sec. 121.639
Fuel supply: All domestic operations.
No person may dispatch or take off an airplane unless it has enough fuel--
(a) To fly to the airport to which it is dispatched;
(b) Thereafter, to fly to and land at the most distant alternate airport (where required) for the airport to which dispatched; and
(c) Thereafter, to fly for 45 minutes at normal cruising fuel consumption or, for certificate holders who are authorized to conduct day VFR operations in their operations specifications and who are operating nontransport category airplanes type certificated after December 31, 1964, to fly for 30 minutes at normal cruising fuel consumption for day VFR operations.
Amdt. 121-251, Eff. 1/19/96
Nothing in the books about after takeoff other than how to report minimum fuel or emergency fuel.