ADF Bearing Drift Over the Sea
At night over the sea, 90 NM from an NDB, the ADF bearing pointer drifts slowly through about 15 degrees and the ident is weak but present. The station is an NDB co-located with a medium-power broadcast transmitter. Your heading is constant, the autopilot is holding altitude, and the wind is light. The magnetic compass and the standby attitude indicator agree with each other. Fuel is 50 minutes above final reserve, and there is no other navigation aid within 60 NM.
1. Explain the likely cause of the bearing drift and how to confirm it.
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The most likely cause is night-effect (skywave) interference. At night the ionospheric D layer weakens, so MF signals from distant transmitters, including broadcast stations, can propagate by skywave and arrive at the aircraft along with the groundwave. The ADF then indicates a bearing somewhere between the two arriving signals, causing a slow drift that is not matched by any real heading change.
Confirmation: note that the drift is slow, the ident is weak, and the aircraft is over water at night where groundwave from the station is weakening. Compare the indicated bearing with the bearing calculated from dead reckoning; a discrepancy that grows over time supports skywave corruption. A steady heading with a drifting bearing and no wind change is the classic signature.
2. Decide how much to rely on the ADF and what alternative navigation you would use.
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The ADF should be treated as unreliable for precise tracking. A 15-degree error at 90 NM is about 23 NM of cross-track uncertainty, which is far too large to use as a primary fix. Use the ADF only for a rough sense of orientation, such as confirming which quadrant the station lies in.
Alternative navigation: dead reckoning from the last good fix, using heading, TAS, time and the light wind; the magnetic compass and standby attitude indicator already agree, so basic instruments are trustworthy. If available, request radar vectors or a position report from ATC. Maintain a terrain-safe altitude and plan to divert to the alternate if the position uncertainty cannot be reduced before fuel becomes a factor.
3. What would make you abandon the ADF entirely and divert?
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Abandon the ADF if the ident disappears, the bearing becomes erratic rather than drifting smoothly, or the discrepancy with dead reckoning exceeds what the remaining fuel allows you to resolve. With 50 minutes above final reserve, set a decision point: if a reliable fix is not obtained before a defined time, divert.
The trigger should be based on position uncertainty, not on hope. If cross-track uncertainty grows beyond the width of the safe corridor to the destination, or if no independent fix is available when the fuel margin falls to about 30 minutes, divert. This keeps a reserve for the diversion and avoids pressing on with a single, corrupted aid.