Alternate selection and fuel-endurance shortfall on a 270 NM sector
You are planning a twin turboprop flight from Abuja (DNAA) to Lagos (DNMM), 270 NM, true airspeed 210 kt, fuel flow 240 kg/h in the cruise. Forecast wind at FL180 is 215/30, giving a 30 kt headwind component. Fuel at brakes is 610 kg; taxi fuel is 12 kg. The Lagos forecast for your ETA of 1740Z is: wind 220/10, visibility 6,000 m, scattered cloud at 2,000 ft, temporarily between 1700Z and 1900Z visibility 3,000 m in mist and broken cloud at 600 ft. Your filed alternate is Ibadan (DNIB), 60 NM from Lagos, where the forecast is 8 km and scattered at 3,000 ft; the wind gives a 20 kt tailwind component on that leg. Company and State planning rules require a 5 per cent contingency on trip fuel and a 45-minute final reserve. Maximum take-off and landing weights are not limiting on this sector.
1. Is a destination alternate aerodrome required for this flight? Justify your answer against the forecast.
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Answer: yes, an alternate is required.
The planning question is not what the weather will most probably be, but whether the destination will be at or above the prescribed criteria for the whole window around the ETA.
- The TEMPO band 1700Z to 1900Z brackets your ETA of 1740Z.
- Within that band the forecast is 3,000 m visibility and broken cloud at 600 ft, which is below the usual alternate-required criteria - commonly a ceiling of at least 2,000 ft and visibility of at least 5 km, or a defined margin above the approach minima. A 600 ft ceiling is far below any of these.
- A temporary condition that covers the ETA must be treated as a real possibility, not discounted because the prevailing forecast is good.
Ibadan therefore has to be carried, and it must itself be forecast at or above its minima for the relevant period. At 8 km and scattered at 3,000 ft it is comfortably suitable, and at 60 NM it satisfies the alternate distance/time limit in the rules.
Trap to avoid
Using the prevailing 6,000 m and 2,000 ft to justify departing without an alternate is a planning error; the forecast deliberately contains a deterioration in your arrival window.
2. Compute the required fuel for this flight, compare it with the 610 kg on board, and state your options.
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Current conditions
- Ground speed outbound: 210 - 30 = 180 kt, so 270 NM takes 1.5 h = 90 min.
- Trip fuel: 240 kg/h x 1.5 h = 360 kg.
- Contingency at 5 per cent: 18 kg.
- Alternate to Ibadan: 60 NM at 230 kt ground speed = 15.7 min, so about 64 kg.
- Final reserve, 45 min at 240 kg/h: 180 kg.
- Taxi: 12 kg.
Total required: 12 + 360 + 18 + 64 + 180 = 634 kg.
You have 610 kg at brakes, so you are 24 kg short. The 45-minute final reserve is not available for planning and cannot be counted as trip fuel; doing so would leave you with no legal reserve on arrival at Lagos.
Options
- Uplift fuel - at least 24 kg, and preferably 15 to 20 kg more for taxi variation and forecast error, if weights and performance permit.
- Offload 25 to 30 kg of payload or baggage.
- Shorten the diversion: if a suitable alternate closer than Ibadan is available, the alternate fuel falls.
- Re-brief on the forecast: if Lagos improves and the TEMPO is removed, no alternate is required and the requirement drops to 570 kg, leaving 40 kg (about 10 minutes) of discretion.
What you must not do is depart hoping the headwind will be less than forecast; the trip fuel must be computed on the wind you are given.
3. If no alternate were required and you departed with 610 kg, how much holding time could you accept at Lagos, and what would you say to a suggestion to simply plan on using part of the final reserve?
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Holding available without an alternate
With no alternate, the requirement is 12 + 360 + 18 + 180 = 570 kg.
Available for discretionary holding: 610 - 570 = 40 kg.
At 240 kg/h that is 0.167 h, i.e. about 10 minutes of holding. Any longer hold requires either less trip fuel than planned or a diversion.
On using the final reserve
The final reserve exists to protect you if the landing is delayed after the diversion; it is the buffer that covers an unexpected runway blockage, a go-around, or a change of destination caused by weather. Planning to eat into it converts a routine delay into a fuel emergency, and it is contrary to the planning rules and to the operator's fuel policy. If the flight needs more holding than 10 minutes, the answer is more fuel before departure or a different destination, not a smaller reserve.
Operational note
In the air the arithmetic changes with the actual wind: a stronger headwind eats the 40 kg quickly. The fuel figure to watch is not the total but the fuel over the destination when you start the descent.