NDB/ADF Tracking with Wind: Interpreting Relative Bearing
You are inbound to an NDB in a remote part of New Zealand, maintaining heading 090°M at TAS 200 kt. The ADF relative bearing is 030° and the station is 60 nm away. Forecast wind is 180°M/40 kt. You intend to track directly to the NDB. The aircraft has no GNSS and the VOR is out of service, so the ADF is your primary aid. The minimum safe altitude in the area is 6,000 ft and you are at 8,000 ft. You must decide what heading to fly and estimate the revised time to the station. The ADF needle is steady, so there is no bearing corruption. The wind is from the south. You need to interpret the relative bearing correctly and allow for drift. Show your vector reasoning and explain why the ADF relative bearing is not the same as the turn required.
1. Determine the current track made good and groundspeed.
Read the worked answer
Bearing to the station = heading + relative bearing = 090° + 030° = 120°M.
Air vector at heading 090°M, TAS 200 kt: North = 0 kt; East = 200 kt.
Wind from 180°M at 40 kt blows toward 360°M: North = +40 kt; East = 0 kt.
Ground vector: North = 40 kt; East = 200 kt.
Track made good = atan2(200, 40) = 78.7°M, approximately 079°M. Groundspeed = sqrt(40² + 200²) = 204 kt.
So the aircraft is tracking 079°M, not 090°M; the wind has pushed it left of heading.
2. Calculate the heading needed to track directly to the NDB and the revised groundspeed and ETA.
Read the worked answer
To track to the station, desired track is 120°M. Wind from 180°M gives an angle of 60° between track and wind direction.
Crosswind = 40 sin 60° = 34.6 kt. The wind is from the left of track 120°M, so drift is right. Drift = asin(34.6 / 200) = 10.0° right.
Heading = 120° − 10° = 110°M.
Along-track wind: wind toward 360°M is 120° from track 120°M, giving 40 cos 120° = −20 kt, i.e. a 20 kt headwind.
Groundspeed = 200 cos 10° − 20 = 197 − 20 = 177 kt.
Time from 60 nm = 60 / 177 = 0.339 h = 20.3 min. At the current 204 kt the time would be 17.6 min, so the correct tracking heading adds about 2.7 min.
3. Explain why a 30° right turn is not the correct response to the ADF relative bearing.
Read the worked answer
The ADF relative bearing of 030° means the station bearing is 120°M, i.e. 30° right of the aircraft nose at heading 090°M. However, the aircraft is not tracking 090°M; it is tracking 079°M because of the wind. Relative to the current track, the station is about 41° right.
The correct turn is from heading 090°M to 110°M, a 20° right turn. At 110°M the wind produces 10° right drift, so the resulting track is 120°M directly to the station. If you turned 30° right to 120°M, the wind would still push you right of the desired track. The ADF gives bearing to the station, not the wind-corrected heading.