In-flight DR revision and diversion fuel judgement
You are on an IFR flight from Alpha to Bravo, a planned 150 NM leg. TAS is 105 kt, the planned wind is 240/25, and the flight-plan groundspeed is 95 kt. Fuel at top of climb is 150 L and burn is 34 L/h. After 30 minutes you fix your position 42 NM from Alpha. Bravo now reports weather below your alternate minima, so you must carry a 45-minute reserve and a 40 NM alternate from Bravo. The alternate is 40 NM from Bravo. The front is moving faster than forecast, and the wind at your level is stronger than planned. ATC offers a diversion now to the alternate, which is 95 NM from your present position, or you can continue to Bravo. There is no icing and no other traffic.
1. Calculate actual groundspeed, revised time and fuel to Bravo.
Read the worked answer
Actual groundspeed = 42 NM / 0.5 h = 84 kt. Remaining distance to Bravo = 150 − 42 = 108 NM. Time = 108 / 84 = 1.286 h = 77.1 min. Fuel burned since top of climb = 34 × 0.5 = 17 L, so fuel now = 150 − 17 = 133 L. Fuel to Bravo = 34 × 1.286 = 43.7 L. Fuel at Bravo = 133 − 43.7 = 89.3 L. The revised ETA is about 77 minutes from the fix, not the flight-plan time based on 95 kt.
2. Compare continuing to Bravo with diverting now to the alternate, including reserve. What do you recommend?
Read the worked answer
Divert now: 95 NM at 84 kt = 1.131 h = 67.9 min; fuel = 34 × 1.131 = 38.5 L; fuel at alternate = 133 − 38.5 = 94.5 L. Reserve = 45 min = 25.5 L; margin = 69 L. Continue to Bravo then alternate: fuel at Bravo = 89.3 L; 40 NM at 84 kt = 0.476 h; fuel = 16.2 L; fuel at alternate = 73.1 L; margin above reserve = 47.6 L. Both are legal. But Bravo is already below alternate minima, the wind trend is unfavourable, and continuing gives the smaller margin. Divert now: it reduces exposure and still leaves about 69 L above reserve.
3. What would you tell ATC if the alternate is now also reported below minima?
Read the worked answer
Advise ATC that you no longer have a legal destination and request the nearest suitable aerodrome with an approach, or vectors/holding as appropriate. If fuel is becoming critical, declare minimum fuel or an emergency rather than pressing toward an aerodrome below minima. Do not use the 45-minute reserve as traffic fuel; it is there for the unexpected. The decision must be made early, while the divert fuel is still available.