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.
[Doc. No. 6258, 29 FR 19222, Dec. 31, 1964, by Amdt. 121–251, 60 FR 65935, Dec. 20, 1995]
121.645 Fuel supply: Turbine-engine powered airplanes, other than turbo propeller: Flag and supplemental operations.
(a) Any flag operation within the 48 contiguous United States and the District of Columbia may use the fuel requirements of §121.639.
(b) For any certificate holder conducting flag or supplemental operations outside the 48 contiguous United States and the District of Columbia, unless authorized by the Administrator in the operations specifications, no person may release for flight or takeoff a turbine-engine powered airplane (other than a turbo-propeller powered airplane) unless, considering wind and other weather conditions expected, it has enough fuel—
(1) To fly to and land at the airport to which it is released;
(2) After that, to fly for a period of 10 percent of the total time required to fly from the airport of departure to, and land at, the airport to which it was released;
(3) After that, to fly to and land at the most distant alternate airport specified in the flight release, if an alternate is required; and
(4) After that, to fly for 30 minutes at holding speed at 1,500 feet above the alternate airport (or the destination airport if no alternate is required) under standard temperature conditions.
(c) No person may release a turbine-engine powered airplane (other than a turbo-propeller airplane) to an airport for which an alternate is not specified under §121.621(a)(2) or §121.623(b) unless it has enough fuel, considering wind and other weather conditions expected, to fly to that airport and thereafter to fly for at least two hours at normal cruising fuel consumption.
(d) The Administrator may amend the operations specifications of a certificate holder conducting flag or supplemental operations to require more fuel than any of the minimums stated in paragraph (a) or (b) of this section if he finds that additional fuel is necessary on a particular route in the interest of safety.
(e) For a supplemental operation within the 48 contiguous States and the District of Columbia with a turbine engine powered airplane the fuel requirements of §121.643 apply.
[Doc. No. 6258, 29 FR 19222, Dec. 31, 1964, as amended by Amdt. 121–10, 30 FR 10025, Aug. 12, 1965; Amdt. 121–144, 43 FR 22649, May 25, 1978; Amdt. 121–253, 61 FR 2615, Jan. 26, 1996]
121.647 Factors for computing fuel required.
Each person computing fuel required for the purposes of this subpart shall consider the following:
(a) Wind and other weather conditions forecast.
(b) Anticipated traffic delays.
(c) One instrument approach and possible missed approach at destination.
(d) Any other conditions that may delay landing of the aircraft.
For the purposes of this section, required fuel is in addition to unusable fuel
I don't have my OPS manual in front of me, I know shame on me! But, the 10% rule can be wittled down to only haveing to have 10% of the fuel during the time which the airplanes location cannot be reliably (sp?) fixed for an hour or more, ie North Atlantic Track.
For simplicity, if you are going from JFK to Stansted, say your cruise fuel is 75000, alternate is 5000, 30 minute hold speed at 1500 is 8000 you need 88000 so far. Now the 10% would "normally" be 7500 (.10 * 75000) but you use the exemption and now only have to have 10% of about 3 hours worth of fuel (a rough estimate of normal crossing time). So that is 10% of about 12000 or 1200.
So I go from a total "required" fuel load of 95500 to 89200. 6300 more pounds of cargo or pax I can legally take, or have a farther alternate if needed.