Originally Posted by
usmc-sgt
Most FMSs will calculate it for you by changing the EFC time. If not...
Add:
- fuel required to get from the hold to destination
-fuel required to get from destination to alternate
-at least :45 minutes after the alternate.
-10% for error
Subtract that from FOB.
So-
You’re holding and have 10000 lbs on board. It’ll take you 2000lbs to get to your destination and another 2000 to get to the alternate. Min fuel is 4500lbs.
2000+2000+4500= 8500lbs + 10% = 9350lbs.
You have 650lbs you can hold with which in my jet is about 6 minutes, or not very long.
Pen and calculator answer: this.
FMS answer, without knowing your particular one and its special tricks: keep putting in fake EFC's until your arrival fuel at the alternate is what you want.
Other thoughs: yeah the 45 minutes is not required once you've launched, but it's a good starting point and personally I wouldn't go below it without a seriously good set of circumstances lining up, like a few good alternates in the same direction, etc. Conversely you may decide to bump it up if there's only 1 alternate, its weather is getting worse, other diversions are already saturating its arrival rate, etc. Our you wanna give yourself fuel for a missed and second attempt at landing at the destination.