Fuel, Wind and Ten Kilograms of Margin
A light twin is planned for a 520 NM leg. Usable tank capacity is 260 kg. Fixed fuel allowances are: taxi 8 kg, contingency/reserve 25 kg, and alternate plus holding 37 kg. Cruise consumption is 45 kg/h at a still-air TAS of 150 kt. The forecast wind for the leg is a 20 kt headwind. Basic empty mass is 620 kg, crew 85 kg, baggage 15 kg, and maximum take-off mass is 1050 kg. The operator's rule is that the reserve and alternate figures may be planned but never consumed. Dispatch must be decided within the hour; the forecast is the only wind data available and its stated accuracy is plus or minus 10 kt.
1. Compute the planned trip fuel, the total fuel required, the spare fuel on board, and the spare expressed as endurance in minutes.
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Ground speed: 150 − 20 = 130 kt
Trip time: 520 ÷ 130 = 4.00 h
Trip fuel: 4.00 × 45 = 180 kg
Fixed allowances: 8 + 25 + 37 = 70 kg
Total required: 180 + 70 = 250 kg
Spare: 260 (full tanks) − 250 = 10 kg
Endurance equivalent: 10 ÷ 45 = 0.222 h = 13.3 min
Check the mass: 620 + 85 + 15 + 260 = 980 kg against a 1050 kg limit. Mass is not the constraint - the tanks are. Because the tanks can only hold 260 kg, no extra fuel can be loaded to widen the margin; 10 kg is the entire buffer.
2. Using the same allowances, find the strongest headwind at which the flight can still be dispatched legally. Compare it with the forecast and state what the comparison means.
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Fuel available for the trip: 260 − 70 = 190 kg
Maximum trip time: 190 ÷ 45 = 4.222 h
Minimum ground speed: 520 ÷ 4.222 = 123.2 kt
Maximum headwind: 150 − 123.2 = 26.8 kt, i.e. about 26 kt
The forecast is 20 kt, so the legal margin is only about 6 kt of wind - and the forecast's own stated accuracy is ±10 kt. The flight is dispatchable on paper but sits inside the uncertainty band of the only data available.
Check the other direction: at a 30 kt headwind, GS = 120 kt, time = 520 ÷ 120 = 4.333 h, trip fuel = 195 kg, total = 265 kg - 5 kg more than the aircraft can carry. A 10 kt forecast error turns a legal plan into an illegal one, with the shortfall landing inside the reserve that may never be used.
3. The captain argues that because the plan is legal on the forecast, the flight should go. Give your recommendation and the reasoning that supports it, including at least one alternative that preserves the sortie.
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Recommendation: do not dispatch on the forecast alone.
The plan is legal only because the tanks happen to be full; the entire buffer is 10 kg (13 minutes), and a routine 10 kt wind error exceeds it by 5 kg. A plan whose legality depends on the forecast being exactly right is not a plan, it is a bet - and the loss is taken out of the reserve the operator has already declared unusable.
Options that keep the sortie alive:
- Re-plan against 30 kt. If the numbers do not close, the leg must change, not the reserve.
- Insert a fuel stop and split 520 NM into two legs; this restores margin at the cost of time.
- Delay until an actual wind or an updated forecast inside 26 kt is available, and record the figure used.
- Re-route to a shorter track if terrain and airspace allow.
What must not happen: counting the 25 kg contingency or the 37 kg alternate/holding as trip fuel, or dispatching and "seeing how it goes". The correct answer is a re-plan, not a re-interpretation of the reserve.