What happened
While being radar vectored toward runway 35 at Essendon Airport, an aircraft descended below the radar minimum safe altitude of 2,100 ft. At the time of the event, the flight crew reported that the autopilot was active and the target altitude of 2,100 ft had been preselected. The crew did not realize the aircraft had breached the assigned altitude level until it had descended to 1,600 ft.
The investigation
Investigators examined the aircraft's automatic flight control system (AFCS) following the event. A subsequent engineering inspection revealed no mechanical or system faults within the AFCS. However, because no flight data could be retrieved from the aircraft, the ATSB could not verify the specific AFCS modes or settings in use, nor could they determine the exact technical reason why the aircraft descended below the preselected altitude.
Several environmental and operational factors were also reviewed. The flight had experienced significant delays, and the crew was aware of a tight schedule to complete their return flight before the 23:00 curfew. Additionally, the crew had no prior experience operating at night into Essendon Airport. The captain had also requested an ILS approach to runway 26, but air traffic control declined this due to traffic, necessitating the vectoring to runway 35.
Findings
- The pilot flying relied heavily on automation to capture the assigned altitude and diverted attention outside the cockpit to assist the pilot monitoring in locating the runway.
- The pilot flying's focus on external visual cues meant that neither crew member was actively monitoring the flight instruments or the descent path during the critical phase of flight.
- Difficulty in identifying runway 35 at night, combined with the pressure of the upcoming curfew and the flight delay, likely contributed to the lack of monitoring.
- While the crew had been awake for 15 hours, there was insufficient evidence to confirm that fatigue was operating at a level known to impair performance, though an elevated risk of fatigue was noted for their subsequent flights.