Holding fuel vs. alternate at destination
You are planning a twin-turboprop IFR flight, departure mass 5,600 kg, MTOM 5,700 kg. Trip fuel to destination is 620 kg, taxi-out 15 kg, taxi-in 12 kg. The destination forecast gives TEMPO TSRA for the arrival window; the only suitable alternate is 95 NM away, requiring 210 kg diversion fuel plus 40 kg approach/missed-approach allowance. Company policy requires 30 minutes holding at 1,500 ft above the alternate at a fuel flow of 480 kg/h, plus 10% of trip fuel as contingency. You have no tankering requirement and the runway at destination is forecast wet.
1. Compute the minimum take-off fuel and determine whether you can depart at the planned mass.
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Contingency = 10% × 620 = 62 kg. Holding = 0.5 h × 480 = 240 kg. Alternate fuel = 210 + 40 = 250 kg. Reserve (contingency + alternate + holding + final reserve) - final reserve for a twin-turboprop is 30 min at 1,500 ft at holding flow = 240 kg. Minimum take-off fuel = taxi-out 15 + trip 620 + contingency 62 + alternate 250 + holding 240 + final reserve 240 = 1,427 kg. Trip + contingency + reserves = 1,412 kg (excluding taxi). Fuel is not limiting on mass here; the question is whether the wet runway and TSRA change anything. The TEMPO TSRA at destination means you must plan to the alternate, which you have. Departure mass is already fixed at 5,600 kg, so you depart as planned provided performance on the wet runway is satisfied.
2. At 40 NM before destination, ATC advises the destination is below minima and the alternate is now forecast to go below minima in 50 minutes. What is the correct decision?
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You must reassess before committing. Remaining fuel at 40 NM out is roughly trip fuel used so far plus reserves; you have approximately contingency + alternate + holding + final reserve available - about 792 kg. If you divert now, the 210 kg diversion plus 40 kg approach is comfortably within that. If you continue to destination first and then divert, add the missed-approach and climb fuel plus the extra distance; you may erode the final reserve. Because the alternate is now also threatened, you should not simply continue. The safe action is to divert immediately to the alternate (or an en-route suitable aerodrome) while fuel permits, and inform ATC. Continuing to a destination known to be below minima with a deteriorating alternate risks landing with less than final reserve.