Fuel Margin Call
A two-engine trainer burns 11 gallons per hour per engine. The planned leg is 345 nautical miles, and in still air the aircraft's groundspeed is 115 knots. Each wing tank holds 50 gallons, of which 4 gallons per tank are unusable. Company rules require a 45-minute reserve at cruise burn, and the pilot treats that reserve as untouchable. At the last minute the forecaster calls a steady headwind that will drop groundspeed to 100 knots for the entire leg. No fuel stop is available short of the destination. The pilot must decide whether to go, and must be able to state the groundspeed at which the flight would just barely be legal.
1. With no wind, what fuel is required and what margin remains?
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Burn rate: 2 engines x 11 gal/hr = 22 gal/hr.
Trip fuel: 345 nm at 115 kt = 345 / 115 = 3.0 hr. 3.0 x 22 = 66 gal.
Reserve: 45 min = 0.75 hr, x 22 = 16.5 gal.
Required total: 66 + 16.5 = 82.5 gal.
Usable fuel: (50 - 4) x 2 = 92 gal.
Margin: 92 - 82.5 = 9.5 gal, which is 9.5 / 22 = 0.43 hr, about 26 minutes.
So the no-wind plan is legal but thin: roughly a quarter hour of slop beyond the reserve, in an aircraft whose fuel gauges are not certified to a quarter hour.
2. Recompute with the forecast 100-knot groundspeed. Does the flight go?
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Time: 345 / 100 = 3.45 hr.
Trip fuel: 3.45 x 22 = 75.9 gal.
With reserve: 75.9 + 16.5 = 92.4 gal required against 92 gal usable.
Shortfall: 0.4 gal, about 1.1 minutes of fuel.
The arithmetic says no. But the judgment says no more firmly than the arithmetic does:
- A 0.4-gallon shortfall is smaller than the uncertainty in the forecast itself. A 5-knot wind error changes trip time by about 10 minutes, or ~3.7 gallons.
- Completing the flight would put the aircraft on the ground 44.5 minutes after the last checkpoint - under the legal reserve, and with zero protection against a missed approach, a reroute, or a go-around.
- The reserve is a floor, not a budget line. Dipping into it does not convert a marginal flight into a safe one; it converts a known risk into an unmeasured one.
Decision: no-go. The correct fix is a later departure, a shorter leg, or a lighter aircraft - not a smaller reserve.
3. What groundspeed makes the flight exactly legal, and how much should that number influence the decision?
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Set fuel available for the trip equal to usable fuel minus reserve:
75.5 gal for the trip / 22 gal/hr = 3.4318 hr.
Groundspeed = 345 / 3.4318 = about 100.5 kt.
So 100 knots fails by about half a knot of groundspeed.
That tiny gap is exactly why the number should not carry the decision. A break-even of 100.5 kt against a forecast of 100 kt is not a margin; it is a coincidence. Forecast winds, fuel burn at a different power setting, and taxi fuel all move the true break-even by several knots in either direction.
A useful operating habit: if the break-even groundspeed is within about 10 knots of the forecast, treat the flight as not legal, because you cannot measure your way to the right side of a half-knot line in the air. Here the honest answer is that this load only works in still air.