In-flight replanning when the destination deteriorates
You are 40 minutes from destination at FL 240 with 1 500 kg of fuel. The destination forecast was 3 000 m visibility and a 1 000 ft ceiling, but the latest observation is 800 m visibility with a 200 ft ceiling and vertical visibility 200 ft. The approach minima are 400 ft and 1 200 m. Your alternate is 25 minutes away and its weather is 1 500 m and a 600 ft ceiling, above its 500 ft and 1 500 m minima. You have 1 500 kg of fuel; the alternate fuel planned was 300 kg, final reserve 150 kg, and the missed approach allowance is 100 kg.
1. Decide whether to continue, hold or divert, showing the fuel arithmetic.
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The destination observation is below the approach minima (800 m versus 1 200 m; 200 ft versus 400 ft), so an approach cannot be commenced unless conditions improve. Fuel to the alternate = 25/60 x cruise burn. At a representative 1 200 kg/h that is 500 kg, more than the 300 kg planned. Fuel on arrival at the alternate = 1 500 - 500 = 1 000 kg, leaving 850 kg above final reserve, which is adequate.
Holding is not attractive because the weather is deteriorating and the alternate is marginal but usable. The correct decision is to divert now, before fuel is eroded. If a hold is attempted, set a hard limit: divert when fuel reaches the alternate fuel plus final reserve plus a 10 percent margin, i.e. 500 + 150 + 65 = 715 kg, which at 1 200 kg/h is about 39 minutes of holding from the present state. That limit must be monitored continuously.
2. What ATS messages and records are required for the diversion?
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Advise ATC of the diversion and request a revised clearance and, if needed, a change of level. File or amend the flight plan so the new destination and ETA are recorded; if the flight was under IFR, the amended route and destination must be passed to ATS. Record the time of the decision, fuel at the decision point, the new ETA and the reason for the diversion. On arrival, note the actual fuel and compare it with the final reserve; if the reserve was approached, file a report. The commander remains responsible for the decision and for ensuring the alternate weather is checked again before committing.