On 12 February 2012, the flight crew of a Boeing 737 aircraft, registered VH-TJS and operated by Qantas Airways Limited, was conducting a scheduled passenger service from Sydney, New South Wales to Canberra, Australian Capital Territory. Due to scheduled maintenance the instrument landing system at Canberra was not available and the crew prepared for an alternate instrument approach that provided for lateral but not vertical flight path information. The flight was at night with rain showers and scattered cloud in the Canberra area.
Shortly after becoming established on the final approach course with the aircraft’s automatic flight system engaged, the flight crew descended below the minimum safe altitude for that stage of the approach. The crew identified the deviation and levelled the aircraft until the correct descent profile was intercepted, then continued the approach and landed. No enhanced ground proximity warning system alerts were generated, as the alerting thresholds were not exceeded.
What the ATSB found
The ATSB found that at the time of the occurrence the automatic flight system was in the level change mode rather than the vertical navigation mode specified by the operator for such approaches. While in that mode the flight crew had selected an altitude lower than the applicable minimum safe altitude, with the effect that unless the crew intervened, the aircraft would descend to that lower altitude. The flight crew then allowed the aircraft to continue descending in the level change automatic flight mode through the segment minimum safe altitude, reflecting a temporary loss of situation awareness.
During those phases of flight when terrain clearance is unavoidably reduced, such as during departure and approach, situation awareness is particularly crucial. Any loss of vertical situation awareness increases the risk of controlled flight into terrain. This occurrence highlights the importance of crews effectively monitoring their aircraft’s flight profile to ensure that descent is not continued through an intermediate step-down altitude when conducting a non-precision approach.