The Australian Transport Safety Bureau (ATSB) is investigating an incident on board a Qantas Airways 737-800 where an inadvertently activated system caused the aircraft to deviate from its flight path.

The aircraft, registration VH-VYE, was operating a scheduled passenger service on the Canberra-Brisbane route when the incident happened on 25 Ferbuary.

The ATSB said in its initial report that after the climb phase, when the jet was at 41,000ft (12505m), the autoflight system approach push button (APP) was "inadvertently selected".

"With the autopilot engaged, the arming of the APP meant that, when associated with a pre-selected instrument landing system frequency, the autopilot would command the aircraft to intercept and capture the associated localizer and glideslope beams when within range," says the ATSB.

At about 200km from Brisbane airport, some 45 minutes after the APP was activated, the autopilot commanded the aircraft to climb.

Responding to the unexpected climb, the pilots disconnected the autopilot and manually descended to the assigned flight level. Following the altitude recovery, the aircraft deviated to the left of the assigned flight path, and the crew identified that the autopilot had not been re-engaged.

The jet was subsequently flown to re-intercept the planned flight path and continued without further incident, notes the ATSB.

"The investigation is continuing with a focus on the human factors aspects associated with autoflight system mode awareness and a technical analysis of the aircraft systems in relation to glideslope and localizer capture criteria," it adds.

The investigation is expected to be completed in January 2014.

Flightglobal Pro data shows that the airframe was built in 2005 and is owned by SMBC Aviation Capital.

Source: Air Transport Intelligence news