DGCA CPL Tech Specific: Wind, fuel and a deteriorating forecast en route

Wind, fuel and a deteriorating forecast en route

You are planning a light single-engine IFR flight of 280 NM on true track 090. TAS at the planned power is 135 kt, cruise fuel flow 34 L/h, usable fuel 120 L, taxi allowance 8 L. Forecast wind at the cruising level is 340/30. The alternate is 50 NM from the destination on roughly the same track. Company procedure requires a 45-minute final reserve at cruise flow plus the fuel to fly the alternate. One hour after departure the GPS shows a groundspeed of 128 kt, and you still have 135 NM to run. The destination weather has just gone to 400 m in fog with vertical visibility 100 ft; the alternate is still forecast above minima. Nothing has been added or dumped, the fuel flow has matched the plan, and no ATC delay has occurred.

1. Compute the planned heading, groundspeed and ETA.

Read the worked answer

Wind triangle, track 090, wind 340/30, TAS 135 kt.

Angle between track and wind direction = 110° (wind from 340 is 110° to the left of the nose, so it is a rear-left wind).

Crosswind component = 30 × sin 110° = 30 × 0.940 = 28.2 kt
Head/tailwind component = 30 × cos 110° = 30 × (−0.342) = −10.3 kt, i.e. a 10.3 kt tailwind

WCA = arcsin[(28.2) / 135] = arcsin(0.209) ≈ 12° The wind pushes you right of track, so the heading must be offset left: heading = 090 − 12 = 078° true.

Groundspeed = 135 + 10.3 ≈ 145 kt

Time en route = 280 / 145 = 1.93 h = 1 h 56 min

Fuel check on the plan:

Trip fuel = 34 × 1.93 = 65.6 L
Alternate 50 NM at 145 kt = 20.7 min → 11.7 L
Final reserve 45 min = 25.5 L
Taxi 8 L
Total required = 8 + 65.6 + 11.7 + 25.5 = 110.8 L, against 120 L usable → 9.2 L spare.

Legal on paper, but only about 16 minutes of extra fuel - that margin is small and it is about to be eroded.

2. Recompute at the one-hour point using the actual groundspeed, and quantify the margin over the required reserves.

Read the worked answer

Fuel state after one hour:

Fuel used = 34 L (flow matched the plan) + 8 L taxi = 42 L
Remaining = 120 − 42 = 78 L

Time to destination at the actual GS:

135 NM / 128 kt = 1.055 h = 63 min
Fuel to destination = 34 × 1.055 = 35.9 L

Reserve requirement:

Final reserve 45 min = 25.5 L
Alternate 50 NM at 128 kt = 23.4 min → 34 × 0.39 = 13.3 L
Total reserve and alternate = 38.8 L

Margin:

78 − 35.9 − 38.8 = 3.3 L, about 6 minutes of cruise fuel.

That margin is not a margin. Three factors make it worse, not better:

The GS loss is a trend, not a one-off: the wind is stronger than forecast and may increase.
The destination is now at 400 m and VV100 - an approach, a missed approach and a diversion are all realistic outcomes, and a missed approach costs 10–15 minutes of fuel that has not been budgeted.
The final reserve is a legal minimum you must land with, not fuel available for planning.

3. What is your decision, and how do you justify it to yourself and to ATC?

Read the worked answer

Decision: stop and divert to an airport along the route with fuel to spare - do not press on to the destination.

Reasoning:

With 3.3 L (6 minutes) above trip + alternate + 45-minute reserve, a single missed approach at the destination would consume the reserve and leave the alternate in doubt. That is an unacceptable position to enter deliberately.
The destination weather (400 m, VV100) makes a missed approach more likely than not, and the alternate is 50 NM further from that point.
The correct answer is not to shave the reserve. The 45-minute final reserve is the number below which you must not plan to descend.

Actions:

Inform ATC of a decision to divert, with fuel state and intentions; request priority and the shortest routing.
Select an airport that is behind or abeam the present position, with weather above minima and a runway suitable for the aircraft, so the diversion does not add another 50 NM.
Reduce power to an economy setting if it increases range, and recalculate with the new TAS/flow.
If no suitable airport is within range at the required reserves, declare MAYDAY FUEL (or PAN, depending on the urgency) and obtain direct routing and priority.
Land with the reserve intact. The forecast is not a contract; the fuel gauge and the clock are.