Drift Correction After a Mid-Leg Fix
You are pilot flying a twin turboprop on a scheduled sector. The planned route is a straight leg of 300 NM on track 090°T. Planned TAS is 240 kt and the flight plan wind is 360°T/30 kt, giving a planned heading of 083°T. You have been maintaining 080°T because the drift experienced after departure looked stronger than forecast. Thirty minutes after the last positive fix, a new fix is obtained: the aircraft is 10 NM right of the intended track and 110 NM along track from the starting point. Fuel is sufficient, but the destination has a narrow slot and ATC asks for your revised ETA at the 300 NM point. The aircraft has a basic GNSS receiver and a standby compass; no flight director. You must decide whether to regain track now or continue and correct later.
1. Determine the actual wind and groundspeed from the fix, showing the vector reasoning.
Read the worked answer
Use the fix as a velocity triangle over 0.5 h.
- Along-track component = 110 / 0.5 = 220 kt.
- Cross-track component = 10 / 0.5 = 20 kt right (south).
- Ground vector = (220, −20).
- Air vector on 080°T at 240 kt = (240 sin 80, 240 cos 80) = (236.4, 41.7).
- Wind vector = ground − air = (220 − 236.4, −20 − 41.7) = (−16.4, −61.7).
This is a wind toward about 195°T at 63.8 kt, i.e. wind from 015°T/64 kt. Check: GS = √(220² + 20²) = 220.9 kt; track made good = 090 + atan(20/220) = 095.2°T; drift = 15.2° right, consistent with a strong left crosswind. The forecast 360/30 is clearly wrong; use the new wind.
2. Calculate the heading required to maintain 090°T and the revised ETA at the 300 NM point. Compare with the planned ETA.
Read the worked answer
On track 090°T, the wind from 015°T/64 kt gives a crosswind component of 64 sin 75° = 61.8 kt from the left.
- Drift = arcsin(61.8 / 240) = 14.9° right.
- Heading to hold 090°T = 090 − 14.9 = 075°T.
- Groundspeed = 240 cos 14.9° − 64 cos 75° = 231.9 − 16.6 = 215 kt.
Remaining distance to the 300 NM point = 300 − 110 = 190 NM. Time = 190 / 215 = 0.884 h = 53 min. ETA = 30 + 53 = 83 min after start.
Planned GS with 360/30 and heading 083°T is about 240 cos 7.2° = 238 kt; planned time = 300 / 238 = 75.6 min. The aircraft is therefore about 7 min late, and the ETA must be passed to ATC.
3. Would you regain track immediately or continue and correct later? Give a heading plan and the operational reasoning.
Read the worked answer
To parallel the intended track, steer 075°T. To be back on track at the 300 NM point, add a closing angle of atan(10/190) = 3.0° left, so steer about 072°T until within roughly 2 NM of track, then return to 075°T. A 15–18° left correction is modest and usually acceptable if airspace and terrain allow. If airspace prevents it, continue and correct later, but update the ETA and fuel. Avoid large late corrections near destination; they increase workload and make the final approach less stable.