CAR-OPS 1.375 In-Flight Fuel Management
(a) ....
(b) A commander shall ensure that the amount of usable fuel remaining in flight is not less than the fuel required to proceed to an aerodrome where a safe landing can be made, with final reserve fuel remaining.
(c) The commander shall declare an emergency when the actual usable fuel on board is less than final reserve fuel.
The general principle is that:
If you think you might land with fuel below 'final reserve' you will declare a PAN.
If you know you will land with fuel below 'final reserve' you will declare a MAYDAY.
"The crew decided to divert to Ottawa declaring emergency indicating they might need to cut into their final fuel reserve of 30 minutes in case of any delays. The aircraft climbed to FL230 enroute to Ottawa,on approach to Ottawa the crew reported 6.3 tons of fuel/14000 lbs of fuel on board and cancelled their emergency under the condition that they were vectored directly for a RNAV approach to runway 14. The aircraft landed safely on runway 14 about 50 minutes after aborting the approach in Toronto with more fuel than final reserve remaining..."
clearly our rules stipulate to declare pan (fuel advisory in the USA) if you 'think' eating into final-res, and mayday if you actually 'will'.
So declaring mayday ahead of the legal state, just to clear your way into the alternate, then subsequently canceling it, is this wise precaution or not quite sop?
Imagine a Saab 340 doing that and have some big one go-around for it (as reported) and wait for the flak for the poor guy!
Quod licet jovi non licet bovi I guess