Incident Overview
A pilot was manually flying an ILS approach in instrument meteorological conditions (IMC) when the aircraft deviated from the published approach path. The pilot reported being concerned about the aircraft’s location relative to terrain but believed the aircraft was under control. The unusual attitudes indicated on the artificial horizon were attributed by the pilot to instrument error rather than the actual aircraft attitude.
Pilot's Actions
The pilot reported that false indications from the vacuum instruments incorrectly showed a turn to the right. The pilot’s initial response was a turn to the left, believing the vacuum instruments still indicated a level of bank when the aircraft was level. However, analysis of recorded data confirmed that the instrument indications correlated with high levels of bank and the aircraft’s track. The pilot descended below the minimum sector altitude (MSA) before the deviations and did not execute a missed approach as quickly as practicable to achieve a safe altitude after deviating from the approach. The aircraft subsequently flew below MSA for greater than one minute, tracking perpendicular to the approach path without intent, with left and right bank angles up to 65° and directional changes of up to 90°.
Instrument Indications
Post-incident inspection of the vacuum system found no identified problems with the vacuum pumps or check valves. The pilot recalled observing a low vacuum pressure indication during the flight, but the source states it is almost certain there was no instrument malfunction to the extent believed by the pilot. Post-occurrence maintenance fault-finding of the artificial horizon identified a gradual drift in pitch (up to 7°) and roll (up to 4°) over 20 minutes. This drift was not consistent with the sudden and absolute failure reported by the pilot. The pilot likely became spatially disoriented due to the lack of visual cues in IMC, interpreting real instrument indications as false because they mismatched the pilot's sensed orientation.
ATC Communication
The pilot notified ATC that they had incorrect artificial horizon information and had lost glidepath guidance. The pilot used their callsign twice and displayed other verbal cues indicating stress. The controller did not issue a safety alert, although being below MSA and off the ILS with indications of stress was an opportunity to do so. The source indicates that a PAN PAN call would have been appropriate and would have almost certainly triggered a safety alert to climb immediately.
Risk of Terrain Collision
From leaving the approach profile to commencing a sustained climb to a safe altitude, the aircraft remained below MSA for greater than one minute. The time elapsed from notifying ATC (while below MSA) to the start of a sustained climb was greater than 30 seconds. During this time in IMC, the aircraft came within 810 ft of terrain.