Night NDB approach with static and suspect bearing
You are flying a night NDB approach to a coastal aerodrome. The only approved aid is the WAI NDB, 4 NM inland from the coast. The ADF needle is fluctuating between 005° and 020° relative to the aircraft nose; ident is readable but distorted by static from thunderstorms 60 NM south. You are on the final approach track of 360°M, heading 010°M, TAS 120 kt, and the forecast wind is 200/25. You have GNSS on board but it is not approved for the approach, and the moving map shows a 1.5 NM offset from the ADF bearing. The weather is 600 ft AGL and 3 km visibility; the NDB MDA is 640 ft AGL. ATC has cleared you for the approach and there is no traffic.
1. Assess the reliability of the ADF bearing and the GNSS offset.
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The ADF is not reliable enough for final descent. Needle swings of 15°, distorted ident, and thunderstorm static all indicate bearing errors; a coastal station only 4 NM inland also raises the risk of coastal refraction at night. A 1.5 NM offset at 4 NM is roughly 20°, which is larger than the needle fluctuation, so either the GNSS map or the ADF is significantly wrong - and the GNSS is not approved for the approach. Do not chase the needle. Average readings only for situational awareness, not for descent.
2. What action do you take at or before the FAF?
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Discontinue the approach. Fly the missed approach, advise ATC, and either hold for conditions to improve or divert. Do not descend below the NDB MDA of 640 ft AGL unless the required visual reference is unmistakable; with weather at 600 ft AGL, that is unlikely. If diverting, use GNSS for enroute navigation only if it is approved for that purpose, otherwise use VOR/DR. Do not re-attempt the NDB approach until a reliable bearing can be established.
3. What independent cross-check would make continuing the NDB approach acceptable?
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A positive fix from a serviceable VOR/DME, or an approved GNSS overlay at the FAF, with the ADF bearing stable within a few degrees and ident continuous, would support continuing. The cross-check must be independent and timely; a single fluctuating ADF bearing is not enough, and a non-approved moving map does not meet the approach requirement. Even with a cross-check, the visual segment at 600 ft AGL must be assessed at the MDA, and the missed approach must remain available.