Sometimes the destination airport is temporarily closed due to meteorology, incidents or acidents, etc.
How do you decide for how long you wait until you divert? Obviously, that depends on the available holding fuel. You might want to "brush up" on those fuel calculations!
Fuel consumption= Fuel Flow (FF) (kg/h)
Fuel used (FU) versus distance (FU Vs Dist)
FU Vs Dist = FF/GS (kg/nm)
Fuel used to destination (Fuel2Dest)
Fuel2Dest = Actual FU + Distance2Go x FU Vs Dist (kg)
Remaining Fuel at Destination (RF)
RF = Fuel on Board - Fuel2Dest (kg)
Holding Fuel (HF)
HF = RF – (Alternate + Final reserve) (kg)
Estimated Maximum Holding Time (MHT)
MHT = HF/ FF x 60 (minutes)
The mobile application allows you to practice these calculations. Have fun!