MD11,
Would really depend on your operations manual in theory if your flight was planned with an alternate and everything went to plan you would land with final reserve, alternate and contingency.
If things started to go wrong inflight first you would use your contingency (that's why it's there)
We then have a provision in our operations manual to commit to the destination airport subject to various conditions.. Then in these circumstances we can use the alternate fuel.
The commander shall ensure that the flight is conducted so that the expected useable fuel remaining on arrival at the destination aerodrome is not less than:
• The required Alternate Fuel plus Final Reserve Fuel, or
• The Final Reserve Fuel if no alternate aerodrome is required
If as the result of an in-flight fuel check, the expected useable fuel remaining on arrival at the destination aerodrome is less than:
(a) The required Alternate Fuel plus Final Reserve Fuel, the Commander must take into account the traffic and the operational conditions prevailing at the destina-tion aerodrome, at the destination alternate aerodrome and at any other adequate aerodrome, in deciding whether to proceed to the destination aerodrome or to divert so as to perform a safe landing with not less than Final Reserve Fuel, or
(b) The Final Reserve Fuel if no alternate aerodrome is required, the Commander must take appropriate action and proceed to an adequate aerodrome so as to perform a safe landing with not less than Final Reserve Fuel.
In all cases the following fuel conservation measures should be considered in the event of a fuel shortfall:
(a) Decrease aircraft speed (down to Max Range Speed / Cost Index minimum)
(b) Obtain a more direct route
(c) Fly closer to the optimum FL (taking the wind into account)
(d) Select a closer alternate aerodrome
(e) Land and refuel
NOTE: A landing is “assured” if, in the judgement of the Flight Crew, it could be completed in the event of any forecast deterioration in the weather and plausible single failures of ground and/or airborne facilities e.g. CAT II/III to CAT I.
LOW FUEL STATE If at any time, it becomes apparent that the aircraft may land with less than Final Reserve Fuel; an emergency “PAN” call to ATC must be made, reporting fuel remaining in minutes. ATC must thereafter be kept fully informed of the situation. If at any time, it is evident that the aircraft will land with less than Final Reserve Fuel remaining; an emergency “MAYDAY” call must be made, reporting fuel remaining in minutes.
ADDITIONAL CONDITIONS FOR ALTERNATIVE FUEL PROCEDURES
RCF PROCEDURE On a flight using the RCF procedure, in order to proceed to the Destination 1 aerodrome, the Commander must ensure that the useable fuel remaining at the decision point is at least the total of:
(a) Trip Fuel from the decision point to the destination 1 aerodrome; and
(b) Contingency Fuel equal to 5% of the trip fuel from the decision point to destination 1 aerodrome; and
(c) Destination 1 aerodrome Alternate Fuel, if a destination 1 alternate aerodrome is required; and
(d) Final Reserve Fuel