Magnetic Heading and Zone-Time Decision on a Westbound Leg
You are planning a flight from an airfield at 52°N 004°E to a destination at 52°N 030°E. The planned true course is 274°T, TAS 140 kt, and the forecast wind is 220°/25 kt. The chart shows variation 6°W at the departure end and 4°W near the destination. The aircraft compass card gives a deviation of 2°E for headings near 270° and 1°W for headings near 090°. The leg distance is 420 NM, and departure is at 09:00 UTC. The destination operates on UTC+2 and requires arrival before 15:00 local time. The aircraft has 150 L of usable fuel, burning 30 L/h, with a 45-minute reserve.
1. Compute the compass heading to steer for the first part of the leg, showing the order of corrections.
Read the worked answer
Wind triangle
- Angle between TC 274° and wind source 220° = 54°.
- Crosswind = 25 sin 54° = 20.2 kt from the left; headwind = 25 cos 54° = 14.7 kt.
- WCA = arcsin(20.2/140) ≈ 8.3° right → TH = 274 + 8.3 = 282.3°T.
- Variation 6°W: MH = TH + 6 = 288.3°M.
- Deviation 2°E: CH = MH − 2 = 286.3°C, i.e. steer about 286°C.
The departure variation is the correct one for the initial heading; using the destination 4°W would give 284°C and put the aircraft progressively left of track. Re-check the deviation card at the actual heading rather than assuming the 270° value applies.
2. Determine the ETA in UTC and destination local time, and decide whether the fuel plan is acceptable.
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GS = 140 cos 8.3° − 25 cos 54° ≈ 138.5 − 14.7 = 123.8 kt. Time = 420 / 123.8 = 3.39 h = 3 h 24 min. ETA = 12:24 UTC = 14:24 UTC+2, so the 15:00 local restriction is met by 36 min. Fuel: trip = 30 × 3.39 = 101.8 L; reserve = 30 × 0.75 = 22.5 L; total = 124.3 L. With 150 L available, margin = 25.7 L ≈ 51 min. The flight is feasible, but the margin depends on the wind holding; a 10 kt stronger headwind would add about 17 min and 8.5 L, still inside the margin.
3. Explain how you would revise the heading if the actual deviation at 286°C is 4°E rather than 2°E, and what operational risk this creates.
Read the worked answer
Desired MH is 288.3°M. With actual deviation 4°E, the required compass heading is 288.3 − 4 = 284.3°C. If you keep 286.3°C, the actual magnetic heading becomes 286.3 + 4 = 290.3°M, i.e. 2° right of the desired 288.3°M. Over 420 NM that is a lateral displacement of about 420 tan 2° ≈ 14.7 NM, enough to miss a destination airfield or enter controlled airspace. Deviation is heading-dependent and can change with electrical load, so use the card value for the actual heading and cross-check with a ground-based navaid or GNSS; if the discrepancy is large, carry out a compass swing.