Offchocks, not quite I'm afraid, presuming we're taking EU OPS here. In case of re-planning in flight same terms and conditions apply.
An operator shall ensure that in-flight re-planning procedures for calculating
usable fuel required when a flight has to proceed along a route or to a
destination aerodrome other than originally planned includes:
1. trip fuel for the remainder of the flight; and
2. reserve fuel consisting of:
(i) contingency fuel; and
(ii) alternate fuel, if a destination alternate aerodrome is required (this
does not preclude selection of the departure aerodrome as the
destination alternate aerodrome); and
(iii) final reserve fuel; and
(iv) additional fuel, if required by the type of operation (e.g. ETOPS);
and
3. extra fuel if required by the commander.
requirements for an alternate are as follows:
An operator must select at least one destination alternate for each IFR flight
unless:
1. both:
(i) the duration of the planned flight from take-off to landing or, in the
event of in-flight re-planning in accordance with EU- OPS
1.255(d), the remaining flying time to destination does not exceed
six hours, and
(ii) two separate runways (see EU - OPS 1.192) are available and
usable at the destination aerodrome and the appropriate weather
reports or forecasts for the destination aerodrome, or any
combination thereof, indicate that for the period from one hour
before until one hour after the expected time of arrival at the
destination aerodrome, the ceiling will be at least 2 000 ft or circling
height + 500 ft, whichever is greater, and the visibility will be at least
5 km;
Funny enough if you call it re-planning in flight you risk to fall short unlike if you simply divert in which case Final reserve is all you need.
