What happened
While established on an ILS approach approximately 8 NM from Alice Springs, the pilot was operating in instrument meteorological conditions (IMC). During the manual flight phase, the aircraft began to deviate from the published approach track, experiencing bank angles of up to 65° and directional changes of up to 90°. During these maneuvers, the aircraft continued to descend, eventually dropping below the minimum sector altitude (MSA). The aircraft remained below this altitude for over one minute before a sustained climb was initiated. During this period of deviation, the aircraft came within 810 ft of terrain.
The investigation
The investigation focused on the pilot's perception of instrument failure versus the actual aircraft state. The pilot believed that vacuum instruments were providing erroneous indications of a right-hand turn, leading them to manually correct with a left turn. However, flight data analysis confirmed that the instrument indications actually correlated with the aircraft's true bank and track. While a post-incident inspection of the artificial horizon revealed a gradual drift in pitch and roll, there was no evidence of the sudden, absolute failure the pilot had reported, and vacuum system inspections showed no faults in the pumps or check valves.
As a result, the investigation concluded that the pilot likely experienced spatial disorientation, interpreting real instrument readings as false because they conflicted with the pilot's perceived orientation. Furthermore, although the pilot notified air traffic control of the loss of glidepath guidance and instrument issues, they did not initiate a missed approach once approach tolerances were exceeded.
Findings
- The pilot likely succumbed to spatial disorientation due to the lack of visual cues in IMC.
- The pilot failed to execute a missed approach after deviating from the ILS track and descending below the MSA.
- The pilot did not broadcast a PAN PAN call, which could have prompted air traffic control to issue a safety alert.
- Air traffic control did not issue a safety alert despite the pilot's indications of stress and loss of guidance.
- The pilot did not utilize the autopilot, which could have mitigated workload and the risk of disorientation.
- A post-occurrence inspection identified a gradual drift in the artificial horizon, though not severe enough to explain the reported failure.