What happened
While en route to Canberra, the flight crew was cleared to track direct from waypoint AVBEG to Canberra Airport, with instructions to descend below 8,500 ft AMSL to exit controlled airspace. Instead of following the direct track, the crew transitioned to the AVBEG 5A standard arrival route (STAR). This deviation caused the aircraft to turn south, crossing restricted areas R430A, B, and C.
Because the controller was unaware that the STAR included a specific descent restriction to avoid these restricted areas, they issued an altitude instruction to maintain separation rather than alerting the crew to the track deviation. This intervention caused the aircraft to remain approximately 1,300 ft above its intended descent profile.
As the aircraft approached the instrument landing system (ILS) waypoint MENZI, it was roughly 680 ft above the glideslope. In an attempt to descend, the crew entered a holding pattern at waypoint MOMBI. However, the holding pattern was flown incorrectly, resulting in the aircraft maneuvering significantly below the minimum safe altitude. The approach was subsequently restarted from an altitude approximately 700 ft below the required minimum.
The investigation
The investigation examined the sequence of events leading to the altitude excursion and the controller's response. It was found that the controller's decision not to issue a safety alert regarding the initial track deviation contributed to crew confusion regarding airspace classification. Furthermore, the investigation looked into the technical aspects of the MOMBI holding pattern, noting that a 2022 obstacle evaluation error had increased the holding pattern minimum altitude to 5,600 ft, creating a difficult transition to the approach glideslope.
Findings
- The flight crew deviated from the cleared track by commencing the AVBEG 5A STAR without authorization.
- The air traffic controller did not notify the crew of the deviation or issue a safety alert, which led to confusion regarding whether the aircraft was in controlled or non-controlled airspace.
- The crew failed to select the Canberra common traffic advisory frequency (CTAF) and did not make necessary radio broadcasts, preventing them from receiving safety alerts from Canberra Tower.
- The holding pattern at MOMBI was not flown correctly, leading to the descent below minimum safe altitude.
- The operator's change management processes were ineffective in mitigating risks associated with the new Denpasar to Canberra route.
- The crew had not completed all required CTAF training prior to operating flights into Australia.