Originally Posted by
Harry Grout
Some people think too much. If you believe you will land with less than final reserve then declare a mayday. If the situation should subsequently change in your favour, cancel it. Fill out ASR. Job done.
Spot on. If, based on the information
currently in your possession and it not changing, you believe you
will land with less than your FFR at the nearest suitable, you
must, declare MAYDAY FUEL. If, having committed to land at a particular airport (destination or otherwise) and you
may have less than your FFR
if your current clearance changes, then you must declare MINIMUM FUEL.
If the situation improves after the declaration (Cleared direct, or planned holding cancelled for eg) then great, but that may not, and in fact, probably
won't happen, unless you declare minimum or mayday fuel.
Originally Posted by The AIP
Emergency Fuel
The pilot in command must declare a situation of emergency fuel by broadcasting MAYDAY MAYDAY MAYDAY FUEL, when the calculated usable fuel predicted to be available upon landing at the nearest aerodrome where a safe landing can be made is less than the final reserve fuel.