Track Error After an Unforecast Wind Change
You are pilot-in-command on a VFR cross-country from Ilorin (ILR) to Abuja (ABV), 150 nm. You plan a true track of 060°T at 100 kt TAS. The forecast wind is 120°/15 kt, magnetic variation is 3°W, and your planned heading is 053°T (056°M). That gives an expected groundspeed of about 92 kt and a planned flight time of 97 minutes. Thirty minutes after departure you identify a town 40 nm from Ilorin and estimate you are 5 nm left of track. The heading indicator agrees with the magnetic compass, and fuel burn is 9 US gal/h. The aircraft has 34 US gal usable and 28 gal remain. There is no convective weather, but the horizon is hazy and the next confirmed feature is 30 nm ahead. ATC is not providing a radar service. You must decide whether to correct now or wait for another fix, and you still have to plan for a 30-minute alternate at Abuja.
1. Calculate the track error and the heading to steer to regain the intended track at Abuja. Give the result in °T and °M.
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The fix gives 40 nm made good and 5 nm left of track. Using the 1-in-60 rule, track error = (5 × 60) / 40 = 7.5° left. The remaining distance to Abuja is 150 − 40 = 110 nm. Closing angle = (5 × 60) / 110 = 2.7° right. Total correction = 7.5° right to parallel the track plus 2.7° right to converge = 10.2° right. New true heading = 053°T + 10° = 063°T. With 3°W variation, magnetic heading = 063° + 3° = 066°M. This is a deliberate correction, not a random turn: the aircraft has been drifting left because the forecast crosswind did not materialise as planned. If the next fix shows the offset increasing again, repeat the calculation rather than adding more correction blindly.
2. Revise the ETA and assess the fuel situation, including a 30-minute alternate.
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The fix shows 40 nm in 30 minutes, so actual groundspeed is 80 kt, not the planned 92 kt. Remaining time = 110 nm / 80 kt = 1.38 h, or about 82 minutes. Revised total time = 30 + 82 = 112 minutes, which is 15 minutes later than planned. Fuel used so far = 0.5 h × 9 = 4.5 gal, leaving 23.5 gal. The remaining leg at 9 gal/h will use about 12.3 gal, leaving about 11.2 gal at Abuja. A 30-minute alternate at 9 gal/h needs 4.5 gal, leaving roughly 6.7 gal, or about 45 minutes, before any final reserve. That is acceptable only if the alternate is genuinely available and the destination weather remains VFR. If the ETA slips further or a headwind increases, the margin disappears quickly and a diversion or reduced alternate distance should be considered.
3. State what you would tell ATC and why.
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I would report the revised ETA and position in plain language: for example, “Five miles left of track, 40 nm from Ilorin, estimating Abuja at 112 minutes, request traffic information.” The reason is not just courtesy. ATC may have traffic crossing the intended track, and the controller can warn other aircraft that I am correcting right. I would not declare an emergency at this stage because fuel and weather remain within limits. I would also state that I am operating VFR and not requesting a radar service unless the controller offers one. If the next fix shows the aircraft still left of track, I would update the estimate and say so, rather than let the controller work from a stale ETA.