Fuel Monitoring When the Gauge Is an Upper Limit
A piston single is planned for a VFR leg of 2 h 20 min at a cruise burn of 32 L/h. Fuel on board at engine start is 140 L. The operator's flight-planning policy requires taxi fuel 8 L, contingency 5% of trip fuel, alternate fuel for 25 min at the same burn, and a final reserve of 30 min. The destination forecast is acceptable, but the alternate is 25 min away. After 1 h 15 min the pilot reads 78 L remaining; the planned burn to that point was 40 L. The fuel gauge is known to overread by up to 5 L when the tanks are warm. A totalizer is fitted but the pilot has not reset it after refuelling. The aircraft is 35 NM from destination; the alternate is 45 NM away; current groundspeed is 120 kt.
1. Compute the required fuel and decide whether departure with 140 L meets policy.
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Trip fuel = 2 h 20 min = 2.333 h × 32 L/h = 74.7 L. Contingency = 5% × 74.7 = 3.7 L. Alternate = 25 min = 0.417 h × 32 = 13.3 L. Final reserve = 30 min = 0.5 h × 32 = 16.0 L. Taxi = 8 L.
Required total = 74.7 + 3.7 + 13.3 + 16.0 + 8 = 115.7 L. Fuel on board is 140 L, so the margin is 24.3 L. Departure meets the operator's policy on the planned figures. The margin is not large, so the pilot should monitor fuel early and treat any unexplained loss as significant.
2. Recompute the actual burn rate at 1 h 15 min and the fuel needed from that point to destination, alternate and reserve. Is 78 L enough?
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Fuel used since engine start = 140 − 78 = 62 L. Subtract taxi fuel: 62 − 8 = 54 L airborne. Time airborne = 1.25 h, so actual burn = 54 ÷ 1.25 = 43.2 L/h.
Remaining trip time = 2 h 20 min − 1 h 15 min = 1 h 05 min = 1.083 h. Remaining trip fuel = 1.083 × 43.2 = 46.8 L. Contingency = 5% × 46.8 = 2.3 L. Alternate = 25 min at 43.2 L/h = 18.0 L. Final reserve = 30 min at 43.2 L/h = 21.6 L. Required from present position = 46.8 + 2.3 + 18.0 + 21.6 = 88.7 L.
Indicated remaining is 78 L, already 10.7 L short; if the gauge overreads by 5 L, the true remaining could be 73 L and the shortfall 15.7 L. The aircraft does not have enough fuel to continue to destination, alternate and reserve at the observed burn. Divert to the nearest suitable airport and land with fuel remaining.
3. What could cause the discrepancy, and what should the pilot do if a fuel leak is suspected?
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Possible causes include a higher-than-planned power setting or richer mixture, unrecorded fuel before start, a gauge error, or a leak. The totalizer not reset after refuelling makes it unreliable for this sector. The pilot should compare gauge, totalizer and known start fuel, and look for visible stains, fuel smell or a rapid unexplained decrease.
If a leak is suspected, land immediately at the nearest suitable airport; do not continue to the destination or alternate. A fuel leak can worsen rapidly and may become a fire risk. If no leak is found, treat the gauge reading as an upper limit, divert to the nearest airport with fuel, and advise ATC if minimum fuel or urgency applies. Do not rely on the planned burn rate once the actual burn is 35% higher than planned.