RNAV Approach with a FMS Position Disagreement at the Final Approach Fix
You are flying a single-pilot IFR helicopter on an RNAV (GNSS) approach to a regional airfield. The approach uses LNAV minima; there is no vertical guidance. At 8 NM from the threshold, on the inbound track, you notice the FMS map shows you 0.4 NM left of the magenta line, but the CDI is centred and the GPS receiver reports 'RAIM available' with 8 satellites. The airfield has a 400 ft obstacle 1.2 NM before the threshold, and the LNAV MDA is 620 ft. The weather is 1,500 m visibility, BKN008. The autopilot is coupled in LNAV. You are 6 minutes from the FAF. ATC has cleared you for the approach.
1. What is the significance of the 0.4 NM map/CDI disagreement, and what do you do before the FAF?
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Significance. The CDI and the FMS map are derived from the same GNSS position, so a 0.4 NM disagreement means one of the following: (a) the map is displaying a different source (e.g. dead-reckoning or a stale position after a brief loss of RAIM), (b) a display/scaling anomaly, or (c) the aircraft is genuinely off track and the CDI is not responding correctly. Any of these invalidates the assumption that the CDI is telling the truth.
Why it matters here. The approach has a 400 ft obstacle 1.2 NM before the threshold, and the MDA is 620 ft. A 0.4 NM lateral error at that range, combined with an obstacle close to the final approach track, is not a small error - it could put you outside the obstacle clearance area.
Actions before the FAF (6 minutes is enough):
- Cross-check position against a second source - DME, a VOR radial, or the map's own bearing/distance to a known waypoint. Do not rely on a single display.
- Check the annunciation - confirm the active source is GNSS and that the CDI is in the correct sensitivity (terminal vs approach).
- If the disagreement cannot be resolved, do not continue the approach. Request vectors or a hold, and if necessary a different approach.
- Do not 'fly the CDI' blindly just because it is centred - a centred CDI with a conflicting map is a red flag, not reassurance.
2. If you decide to continue and the CDI later shows a full-scale deflection at 3 NM, what is the correct response and why?
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At 3 NM, inside the FAF, a full-scale CDI deflection is a go-around trigger. The approach is not stable and the lateral containment is lost.
Response:
- Go around - apply missed-approach procedure immediately; do not attempt to re-intercept the final track at low level near the 400 ft obstacle.
- Climb on the published missed-approach track to the published altitude, then advise ATC.
- Do not descend below MDA during the go-around; the obstacle is 1.2 NM before the threshold, and a low, mis-positioned go-around is exactly the scenario the missed-approach procedure is designed to protect against.
Why not 'correct back'? At 3 NM with a full-scale deflection you are well outside the lateral tolerance. A correction turn at low height, in IMC (BKN008, 1,500 m), near an obstacle, is a classic CFIT setup. The disciplined answer is: go around, re-position, and if the navigation anomaly persists, divert. Single-pilot IFR discipline means treating a navigation anomaly as an approach-integrity failure, not a minor tracking error.