On the evening of 25 February 2013, a Boeing 737-838 aircraft, registered VH-VYE and operated by Qantas Airways Limited, was conducting a scheduled passenger service from Canberra Airport, Australian Capital Territory to Brisbane Airport, Queensland.
At 2110 Eastern Daylight-saving Time, about 115 NM (213 km) from Brisbane, and as the aircraft approached the descent point for Brisbane Airport, the aircraft’s autopilot unexpectedly commenced climbing the aircraft. The crew disconnected the autopilot and descended the aircraft.
During the descent to an air traffic control-cleared level the aircraft rolled left and deviated laterally from the flight plan track. The autopilot was subsequently re-engaged and the aircraft was manoeuvred to re-intercept the flight-planned track. The remainder of the flight was uneventful and the aircraft landed on runway 01 at Brisbane Airport.
What the ATSB found
The ATSB found that the auto-flight system allowed the capture of the Brisbane Airport runway 01 instrument landing system (ILS) glideslope and the aircraft unexpectedly climbed to intercept the signal, which had itself been affected by atmospheric refraction. The preconditions for the occurrence were that the Canberra and Brisbane ILS frequencies were the same, this frequency remained active in the aircraft’s navigation system and, shortly after reaching top of climb out of Canberra, the auto-flight system’s approach mode was inadvertently armed. This meant that when the aircraft was within range of the Brisbane ILS signal, the glideslope would become the active vertical flight mode. The configuration of the auto‑flight system logic on the operator’s Boeing 737 fleet allowed the aircraft to capture and follow a glideslope signal despite not being on the localiser.
The ATSB also found that contrary to the flight crew’s intent, after dis-engaging the autopilot, it was not re-engaged, resulting in a lateral deviation from the planned flight path.
This occurrence highlights the human performance limitations with respect to monitoring and detecting mode reversions and flight mode annunciator (FMA) changes in automated aircraft. Flight crew are reminded of the importance of regularly identifying and confirming the flight modes displayed on the FMA.