Fuel Planning with a Changing Forecast
You are planning a 2-hour 20-minute IFR flight in a twin-engine aeroplane. Trip fuel is 480 kg, taxi fuel 20 kg, and the forecast gives a 15 kt headwind at cruise, which you have already included. Your operator's fuel policy requires final reserve fuel of 30 minutes holding at 1,000 ft above destination at holding mass, plus contingency fuel of 5% of trip fuel, plus alternate fuel of 180 kg, plus 30 kg extra. On arrival at the destination, ATC advises holding for 20 minutes due to weather, and the alternate aerodrome forecast has deteriorated to below your minima, with a second alternate 25 minutes further away now the only viable option.
1. Calculate the required fuel at dispatch and explain how each component is justified.
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Components:
- Taxi: 20 kg.
- Trip: 480 kg.
- Contingency: 5% of trip = 24 kg.
- Alternate: 180 kg.
- Final reserve: 30 minutes holding. Holding fuel depends on holding mass and burn rate; if holding burn is, say, 120 kg/h, 30 minutes = 60 kg. (Use the AFM figure for the actual mass.)
- Extra: 30 kg.
Required fuel at dispatch = 20 + 480 + 24 + 180 + 60 + 30 = 794 kg (with the illustrative holding figure). The reasoning is that each block has a purpose: taxi covers ground movement, trip covers the planned route including the headwind, contingency covers forecast error and routing changes, alternate covers a diversion, final reserve covers an unexpected hold, and extra covers the commander's discretion. The final reserve is not available for normal planning; it is a last-resort margin.
2. On arrival, holding for 20 minutes and with the primary alternate unavailable, how do you assess the situation and what do you do?
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Assess fuel against time:
- Holding for 20 minutes consumes roughly 40 kg at a 120 kg/h burn, which eats into contingency and possibly the alternate allowance.
- The primary alternate is now below minima; the second alternate adds 25 minutes (about 50 kg) plus the diversion distance.
- Recompute fuel remaining at the point of decision: if remaining fuel is less than (fuel to second alternate + final reserve + a sensible margin), you must declare a fuel situation and act early.
Actions:
- Declare minimum fuel or Mayday fuel as appropriate to the actual state; do not wait until the final reserve is reached.
- Request priority and the shortest routing to the second alternate.
- Consider landing at the destination if the weather allows a safe approach within the hold time, rather than diverting further.
- Brief the passenger and crew and prepare for a possible missed approach at the alternate.
The exam tests the discipline of treating the alternate and final reserve as protected, and of making the diversion decision while options remain, not after fuel is critical.