What happened

On 17 October 2012, a Boeing 737-838 aircraft, registered VH-VXB and operated by Qantas Airways Limited (Qantas), was conducting a flight from Adelaide, South Australia to Canberra, Australian Capital Territory. The flight crew received an approach clearance into Canberra via a standard arrival route (STAR) and then an Area Navigation (Required Navigation Performance) (RNAV (RNP)) instrument approach to runway 35 at Canberra Airport. Just prior to commencing descent, at about 2030 Eastern Daylight-saving Time, air traffic control (ATC) cancelled the STAR and cleared the aircraft to track direct to the initial approach fix, HONEY, via a high-speed descent. As the aircraft approached 8,000 ft, ATC provided a descent clearance to 7,000 ft and also cleared the aircraft for the approach. As the aircraft approached HONEY it descended below the 7,000 ft altitude clearance limit. After being alerted to this by ATC, the flight crew climbed the aircraft back to 7,000 ft and continued the approach to land.

What the ATSB found

The ATSB found that, as the aircraft approached 8,000 ft, the auto-flight system vertical mode changed from a flight management computer‑derived and managed vertical navigation mode into the vertical speed mode. This was followed by a number of automated, but unnoticed, and crew‑initiated changes in the aircraft’s auto-flight system vertical mode. The combination of auto‑flight system mode changes and the management of the airspeed during the descent resulted in a high workload environment where the 7,000 ft altitude clearance limit was overlooked by the flight crew.

The ATSB also found that, on receipt of the approach clearance, the Qantas RNAV (RNP) approach procedures allowed the flight crew to remove the current limiting altitude from the auto‑flight system’s Mode Control Panel (MCP) and set the decision altitude. Application of this procedure by the flight crew removed the last automated safety system available to them to prevent descent through the current altitude limitation, well before the aircraft was established on the approach.

What's been done as a result

Following this occurrence, Qantas changed their RNAV (RNP) approach procedures to only allow the altitude on the MCP to be changed from the current limiting altitude once the aircraft was within 2 NM (4 km) of commencing the approach.

Safety message

This occurrence highlights the importance of paying continuous attention to active and armed auto-flight modes and the need to continually monitor descent profiles and airspace limitations in relation to the aircraft’s position, irrespective of the expectation that the descent is being managed by the auto-flight system. The adverse effect of workload and task focus on flight crew performance and the importance of robust procedures for high-precision approaches are also illustrated.