Fuel Planning and In-Flight Fuel Management
The fuel stack
Commercial and private operations differ in detail, but the logic is the same: a stack of allowances, each with a purpose. Taxi fuel covers ground movement. Trip fuel covers start of take-off to landing at the destination. Reserve fuel protects against the unexpected and is normally split into contingency and final reserve. Alternate fuel covers a missed approach at destination and flight to the alternate. Additional fuel covers any extra you decide you need. Know which allowances are planned and which are protected in flight, and what the minimum at any point should be.
In flight
Fuel management is a continuous comparison of planned versus actual. Track fuel used against distance and time, and recalculate the fuel required at the destination as conditions change. If the actual burn exceeds plan, act early: a small excess discovered late leaves fewer options. Understand the decision points where you must divert, and how to compute the fuel needed to reach an alternate from your present position.
Traps
Questions often test whether contingency fuel is a percentage of trip fuel and whether final reserve is time-based rather than distance-based. Watch for the difference between fuel on board at take-off and fuel available for the trip: taxi fuel is not available. Another trap is computing endurance from total fuel while forgetting that reserves may not be usable for the intended flight.
Rule of thumb
For a quick diversion check, fuel required equals distance to alternate divided by groundspeed, multiplied by hourly burn, plus a fixed allowance for the approach and a margin. If that number exceeds your usable fuel minus final reserve, divert now rather than later.