Contingency Fuel Decision on a Long Sector
You are planning a 4 h 20 min flight from a departure aerodrome to a destination where the only usable alternate is 55 min away at the planned mass. The route has one en-route alternate 1 h 40 min along track. Forecast winds give a 12 % headwind component on the first half and a 20 % tailwind on the second half. Company policy permits contingency fuel of either 5 % of trip fuel or 5 min holding at 1500 ft, whichever is greater, provided a suitable en-route alternate exists and the operator's risk assessment is satisfied. Trip fuel is computed as 9800 kg. Tankering is not permitted. The dispatcher proposes reducing contingency to the 5 min holding figure to raise payload by 400 kg.
1. Compute both candidate contingency fuel values and decide which is legally usable, stating the conditions that must hold.
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5 % of trip fuel: 0.05 x 9800 = 490 kg.
5 min holding at 1500 ft: holding flow is roughly half of cruise flow at low level. If cruise burn is about 2200 kg/h, holding burn is approximately 1100 kg/h, so 5 min gives about 92 kg.
The greater value is 490 kg, so under the percentage rule contingency is 490 kg. The reduced 5 min option is only available when the operator's risk assessment supports it and a suitable en-route alternate is available. Here the en-route alternate is 1 h 40 min along track, so it exists, but the decision must be justified by the assessment, not by payload convenience. Reducing contingency to 92 kg saves 398 kg, close to the 400 kg payload gain claimed, but the dispatcher's proposal must be rejected unless the documented risk assessment specifically allows it for this route and the en-route alternate is confirmed serviceable and weather-compliant at the ETA.
2. The en-route alternate's weather later falls below planning minima. What is the effect on the contingency fuel that may be carried?
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With the en-route alternate no longer usable, the condition for the reduced contingency fails. Contingency must revert to the greater of 5 % trip fuel (490 kg) or 5 min holding (92 kg), i.e. 490 kg. The payload increase of 400 kg must be offloaded or the flight re-planned. The key judgement is that contingency fuel is a function of the fuel plan's validity, not a fixed company number; when the supporting alternate disappears, the reduced figure loses its basis. Note also that the destination alternate remains 55 min away, which is unaffected by the en-route alternate's status, but the en-route case is what authorised the reduction.