Originally Posted by
disininer
So your answer would be you don't need any other considerations when proceeding to your alternate. Being above minima would be enough although the decision is been made long before TOD. What if the decision to proceed to the alternate was other but not the impossibility to reach your original destination?
Once you're airborne, the overriding requirement is to be able to land somewhere with fixed reserves in the tanks. If you've planned to fly to B, the passengers want to get to B, the company want you to go to B, the next flight for you and the aircraft commences from B, it's not inflight replanning if you decide to divert to C without first proceeding to B. It would be inflight replanning if you planned to B but the passengers want to get to C, you want to get to C, the company wants you to get to C, the next leg commences from C, and on your way to B you replan to C. In that latter case you would need to have additional reserves at C over and above the basic fixed. As
FullWings says, this is done in cases where you can't plan A to C because you can't take enough fuel, so you plan A to B then when you get to your inflight decision point you recalculate the fuel to continue to C holding D as an alternate (if required). The idea being that you won't have burnt your contingency fuel, you may have saved some fuel with direct tracking and maybe you have some left over taxi fuel, also from that point you don't need as much contingency fuel.