Off Track Diversion
Called Off Tack Diversion or Latest Point of Safe Diversion.
It's used for the latest point / time you can divert to an en route alternate (NOT a destination alternate) and arrive with reserves intact.
It's not a simple calculation, but it's not too difficult once you get your head around what you're calculating. Consider:
Melbourne to Perth. Perth weather is above minima but with 60 holding but you’re really not comfortable. You want to know the latest point in the flight that you can divert to Alice Springs and land with fixed reserve only. You don't have enough fuel to fly all the way to Perth then divert to Alice Springs(yes this is legal in Oz but that's another story....).
Data
Melbourne to Perth = 1461nm
Perth to Alice Springs = 1069nm
Using basic 2,000kg/hr fuel burn
GS 400kts MEL-PER and 475kts PER-ASP
FOB at departure = 10500kgs
Useable fuel = [10500-1000(Fixed Res)]/1.1 = 8636kg
Fuel required from MEL-PER-ASP =
(1461/400*2000)+(1069/475*2000)
7305+4501= 11806kgs
Approx distance to LPSD = [Useable Fuel/Fuel required for entire journey*distance destination to alternate}
8636/11806*1069
=781nm from MEL. Call this point L
Now check fuel required for MEL-L-ASP
MEL to L is 781nm @400 kts = 3905kgs
L to ASP is 760nm @ 450kts (crosswind now) = 3377kgs
Total is 3905+3377=7282kgs
You have 8636kg to play with.
Thus you have 1354kgs more for which you’d go another approx 40 mins (=2000kg/1354kg*60). Divide this by 2 and make L2 20 mins further on = =1/3 of an hour.
400*1/3 = 133nm
L2=781+133=914nm from MEL
L2 to ASP = 764nm
Fuel Req MEL-L2-ASP=
4570+3820=8390kgs
That’s within 5 mins of the Useable Fuel you had on departure. If it was more out than that, you’d refine again.
You can refine the average ground speeds for the diversion and reduced fuel burns further into the flight to get as accurate as you wish.