In-Flight Diversion With Weather at Minima
After 2 h 10 min in cruise, a light twin has 150 kg of usable fuel remaining. The destination is 70 NM ahead, but its weather is now at the alternate minima with a 300 ft cloud base and visibility 3 km; one approach and missed approach would consume 12 kg. The planned alternate is 45 NM beyond the destination, but a closer alternate is 35 NM off the current track. Current ground speed is 110 kt and fuel flow is 60 kg/h. The operator's final reserve is 30 min. The crew has already used the planned contingency; no additional fuel is available. ATC can offer either direct to the destination or a diversion to the closer alternate. The aircraft is not equipped for icing and the closer alternate is reporting an icing risk in cloud. The destination runway is available but the missed approach is not guaranteed to be clear of terrain in the current ceiling.
1. Calculate the fuel required from the present position to the destination, one approach and missed approach, and the planned alternate, including final reserve. Is the destination plan still legal?
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Fuel flow is 60 kg/h = 1 kg/min.
To destination: 70 ÷ 110 = 0.636 h = 38.2 min → 38.2 kg. Approach and missed approach: 12 kg. Destination to planned alternate: 45 ÷ 110 = 0.409 h = 24.5 min → 24.5 kg. Final reserve: 30 min → 30 kg.
Total required = 38.2 + 12 + 24.5 + 30 = 104.7 kg. With 150 kg remaining, the margin is 45.3 kg, so the destination plan is legal on fuel. Fuel legality does not, however, resolve the terrain and weather risk.
2. Calculate the fuel required to divert now to the closer alternate and compare the margins.
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Distance to closer alternate = 35 NM. Time = 35 ÷ 110 = 0.318 h = 19.1 min → 19.1 kg.
Required fuel = 19.1 + 30 reserve = 49.1 kg. With 150 kg remaining, the margin is 100.9 kg, about 56 kg more than the destination plan. The closer alternate is therefore much stronger on fuel, but its icing risk must be considered separately because the aircraft is not icing-equipped.
3. Recommend a course of action. Address the icing risk and the destination missed-approach terrain issue.
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Do not attempt the destination approach merely because the fuel calculation is legal. The destination is at minima and the missed approach is not guaranteed clear of terrain; a go-around would place the aircraft in a high-workload, low-margin situation.
Divert now to the closer alternate only if it can be reached and flown without entering icing conditions, for example by remaining clear of cloud or in above-freezing air. If the closer alternate cannot be used without icing exposure, proceed to the planned alternate; the fuel to destination plus planned alternate leaves about 45 kg margin, and a direct diversion would leave more. Holding is not attractive: at 1 kg/min, 30 min of holding would consume the entire destination-plan margin. The crew should declare the diversion promptly and monitor fuel against the new plan.