Wellington fog arrival: is the planned take-off fuel legal and sufficient?
An A320-200 is planned Auckland (NZAA) to Wellington (NZWLG). Planned data: taxi 180 kg; trip 3,120 kg; contingency 5% of trip; destination forecast requires 20 min holding at 1,500 ft; alternate Palmerston North (NZPM) fuel 1,050 kg; final reserve 30 min at 2,400 kg/h; additional fuel 15 min holding. The dispatcher plans a take-off fuel of 5,900 kg. Weather at Wellington is marginal: fog is forecast to form after arrival, and the latest TAF gives a 30% probability of below-minima conditions for 2 hours after ETA. The aircraft has a fuel capacity allowing 6,500 kg take-off fuel; performance is not limiting. After 45 minutes in the cruise, 1,400 kg has been used and 4,900 kg remains. ATC then advises that Wellington holding is now 45 minutes due to weather.
1. Calculate the minimum take-off fuel required under the operator's Part 121-style fuel policy and assess the planned 5,900 kg.
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Build the requirement from the stated components:
- Taxi fuel: 180 kg
- Trip fuel: 3,120 kg
- Contingency: 5% × 3,120 = 156 kg
- Alternate fuel: 1,050 kg
- Final reserve: 30 min at 2,400 kg/h = 1,200 kg
- Additional holding: 15 min at 2,400 kg/h = 600 kg
Minimum take-off fuel = 180 + 3,120 + 156 + 1,050 + 1,200 + 600 = 6,306 kg.
The planned 5,900 kg is 406 kg below the minimum. The flight cannot depart as planned. Uplift to at least 6,306 kg, and consider more because the TAF gives a 30% probability of below-minima conditions after ETA; a 20-minute holding allowance may be optimistic if the fog arrives early. A take-off fuel closer to 6,500 kg would preserve the alternate and final reserve while giving some operational margin.
2. Using the in-flight figures, recalculate the fuel needed to continue to Wellington with the revised 45-minute hold and decide what to do.
Read the worked answer
At the decision point, 4,900 kg remains. Fuel needed from here:
- Remaining trip: 3,120 − 1,400 = 1,720 kg
- Contingency (planned): 156 kg
- Alternate Palmerston North: 1,050 kg
- Final reserve: 1,200 kg
- Revised holding: 45 min at 2,400 kg/h = 1,800 kg
Required = 1,720 + 156 + 1,050 + 1,200 + 1,800 = 5,926 kg.
Available 4,900 kg is 1,026 kg short. Continuing to Wellington expecting a 45-minute hold is not acceptable. A diversion to Palmerston North from the present position might take, say, 650 kg plus 1,200 kg final reserve and 200 kg approach allowance = 2,050 kg, which is comfortably available. The safe decision is to divert to Palmerston North or to negotiate a much shorter hold only if the latest weather confirms improvement; the aircraft must not become committed to Wellington with insufficient alternate and reserve fuel.