Fuel, Endurance and the Alternate
You plan a VFR cross-country in a single-engine piston aeroplane. Trip fuel to destination is 68 L, taxi and start 4 L, and the planned cruise burn is 32 L/h. You want a 45-minute final reserve and a 20-minute contingency. The aerodrome forecast at destination shows a 30% probability of temporary ceiling below VFR minima during your arrival window, and there is one suitable alternate 22 min away. Fuel on board at start is 130 L usable. Take-off mass is at the maximum, and the aeroplane's fuel gauge has a known tendency to over-read by about 5 L at full tanks.
1. Compute the required fuel and decide whether the flight can depart as planned. Show your arithmetic.
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Burn rate 32 L/h = 0.533 L/min. Contingency 20 min = 10.7 L; final reserve 45 min = 24 L; alternate 22 min = 11.7 L; trip 68 L; taxi 4 L. Sum = 68 + 4 + 10.7 + 11.7 + 24 = 118.4 L. Fuel on board 130 L, so surplus is about 11.6 L. But the gauge over-reads by ~5 L at full tanks, so treat usable as ~125 L and surplus as ~6.6 L. That is legal on paper but thin: the alternate is only justified if weather actually requires it, and any headwind increase or ATC delay erodes the margin. Departure is defensible only with a firm go/no-go at destination and a planned diversion trigger.
2. Weather at destination deteriorates en route and the alternate is now 35 min away due to wind. What is your decision framework?
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Recompute: alternate leg 35 min = 18.7 L, raising requirement to ~125.4 L, which exceeds the ~125 L usable estimate. The flight is no longer safely plannable to that alternate. Options, in order of preference: divert to a closer aerodrome with known-good weather; hold only if holding fuel is genuinely available and the forecast supports improvement; or return. Do not dip into the 45-minute final reserve to make the alternate work - the reserve is not flight fuel. The trigger for diversion should be set before the point of no return, based on the last reliable weather and fuel state, not on hope of improvement.
3. What does the 5 L gauge over-read teach about fuel planning generally?
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Never plan to the last litre. Use dipstick or totaliser figures where available, cross-check gauges against calculated burn at each waypoint, and treat indicated fuel as an upper bound when the gauge is known to over-read. Build a personal minimum margin above the legal requirement, and note that contingency fuel exists precisely for the unexpected - using it as trip fuel converts a legal plan into an illegal one.