Fuel Planning with a Deteriorating Forecast
A single-engine piston aircraft is planned for a 240 nautical mile leg from an inland aerodrome to a coastal destination. True airspeed is 115 knots, forecast wind is 290°/25 knots, and the track is 300° true. The aircraft burns 34 litres per hour. Usable fuel at start-up is 160 litres. Taxi and run-up consume 4 litres. The destination forecast is temporarily marginal, with a 30% probability of conditions below the alternate minima at the estimated arrival time, and the only suitable alternate is 35 nautical miles beyond the destination on the same track. Reserve policy requires 45 minutes of holding fuel plus a 10% trip fuel contingency. The pilot has 1,050 total hours and is current.
1. Calculate the wind component, ground speed, trip time and fuel required, including reserves.
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The wind is 290° and the track is 300°, so the wind is 10° off the nose.
Headwind component = 25 × cos 10° = 25 × 0.985 = 24.6 kt. Ground speed = 115 − 24.6 = 90.4 kt. Trip time = 240 / 90.4 = 2.66 h = 2 h 39 min. Trip fuel = 2.66 × 34 = 90.3 L.
Contingency: 10% of trip fuel = 9.0 L. Holding reserve: 45 min = 0.75 × 34 = 25.5 L. Alternate fuel: 35 nm at approximately the same ground speed (same track) = 35 / 90.4 = 0.39 h = 13.2 L. Taxi and run-up: 4 L.
Total required = 90.3 + 9.0 + 25.5 + 13.2 + 4 = 142.0 L. Fuel available = 160 L, so the surplus is 18 L, or about 32 minutes at cruise burn. That is a thin margin for a destination with a 30% chance of being below alternate minima.
2. Explain how the 30% probability of marginal weather should change the fuel and diversion decision.
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The 30% figure is a probability, not a forecast of failure, but it means roughly one arrival in three would require the alternate. The plan already budgets for the alternate, which is correct, but the surplus of only 18 L (about 32 minutes) is the real issue. If the aircraft has to hold at the destination while waiting for a clearance or an improvement, the 45-minute holding reserve is consumed and the alternate fuel is still needed. The pilot should therefore either: (a) add fuel before departure, since 18 L is less than the 45-minute hold and a single go-around plus diversion could exhaust the margin; (b) nominate a closer alternate if one exists with suitable minima; or (c) delay departure until the forecast improves. The key reasoning is that reserves are not spare fuel to be spent at the destination - they are protected for the diversion. A 30% risk of needing the alternate means the alternate fuel is 'live', so the discretionary surplus is effectively zero.
3. The pilot decides to depart. State the in-flight decision points and the criteria that would trigger a diversion.
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Before top of descent, the pilot should obtain the latest destination and alternate weather and compute the fuel remaining against the alternate requirement. A sensible trigger is: if the destination is not forecast to be at or above the required minima at the estimated arrival time, or if the actual weather at the destination is below minima when within 20–30 minutes of arrival, divert to the alternate. A second trigger is fuel: if remaining fuel falls to the alternate fuel plus the final reserve, the diversion must be executed immediately rather than holding. The pilot should also set a 'bingo' figure before descent - for example, the fuel at which the alternate plus 45 minutes is the only protected quantity - and treat it as a hard limit. Because the alternate is on the same track, the diversion adds only 35 nm, so the decision can be made late, but the fuel trigger must be monitored continuously. The pilot should record the decision and inform air traffic services of any diversion.