Autopilot Mode Confusion on a Non-Precision Approach
A pilot is flying an RNAV (GNSS) approach to runway 03 at a regional airport. The autopilot is engaged in LNAV/VNAV. At 5 nm final, the autopilot annunciates 'ALT HOLD' instead of continuing the vertical path. The pilot selects a lower altitude on the mode control panel and the autopilot begins a descent, but the VNAV path is lost. The aircraft is at 1800 ft AGL, 3 nm from the runway, and the missed approach point is at 1 nm. The minimum descent altitude is 600 ft AGL. The pilot has the runway in sight. The autopilot is a two-axis system without autothrottle.
1. Explain what has happened and why the autopilot captured ALT HOLD instead of continuing VNAV.
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The autopilot captured ALT HOLD because the selected altitude on the mode control panel was set at or above the aircraft's current altitude, or because the VNAV path was interrupted by a mode change. Common causes:
- The MCP selected altitude was set to the current altitude rather than the next constraint.
- A vertical speed or level change mode was selected, overriding VNAV.
- The VNAV path became invalid (e.g. a discontinuity in the flight plan or a loss of GPS integrity), causing the autopilot to revert to ALT HOLD.
- The pilot selected a lower altitude, which caused the autopilot to descend in a non-path mode (e.g. VS or LVL CHG) rather than following the VNAV path.
Once VNAV is lost, the autopilot will not re-capture the path automatically unless the pilot re-engages VNAV with the correct constraints. The aircraft is now descending in a mode that does not guarantee terrain clearance, which is dangerous at 1800 ft AGL with rising terrain or obstacles.
2. State the pilot response to re-establish a stable approach or go around, and explain the limitations of the autopilot in this situation.
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Response:
- Disconnect the autopilot and hand-fly the approach using the flight director and raw data.
- Assess position and energy: 3 nm from the runway at 1800 ft AGL is high for a 600 ft MDA; a stable approach requires being on profile and on speed by 1000 ft AGL.
- Go around if the aircraft cannot be stabilised by 1000 ft AGL. Apply go-around power, pitch up, and follow the published missed approach.
- Re-engage the autopilot only after the aircraft is stabilised and the correct mode is confirmed.
Autopilot limitations:
- A two-axis autopilot without autothrottle cannot manage speed; the pilot must control power.
- VNAV is only as good as the navigation database and GPS integrity.
- Mode confusion is a classic threat; the pilot must monitor the FMA (flight mode annunciator) continuously.
- The autopilot does not guarantee terrain clearance; GPWS/EGPWS remains the backstop.