A loss of situational awareness coupled with the selection of an improper mode in the automated flight system contributed to an incident where a Qantas Boeing 737 descended below the minimum safe altitude while landing at Canberra airport last year.
The 737-400, registered VH-TJS, was operating on the Sydney-Canberra route on 12 February 2012 and conducting its landing approach at night with scattered cloud and rain in the area. The instrument landing system at the airport was inoperable at the time due to scheduled maintenance, so the crew conducted a very high frequency omnidirectional radio range instrument approach towards Runway 35.
The investigation found that an unexpected tailwind affected the aircraft and prompted the captain to switch the automatic flight system from vertical navigation (VNAV) mode to level change mode. The crew had intended to put the system back into VNAV mode to control the descent profile, however both pilots overlooked the fact that the system was still in level change mode.
"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 report stated. "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."
When the crew realised that the system was in the wrong mode, the captain took action to level out the descent until it intercepted the proper approach path, at which point the vertical navigation mode was engaged and the approach continued and landed on Runway 35.
The report adds that the aircraft was above the minimum clearance from terrain of 1,100ft and did not exceed the threshold to activate enhanced ground proximity warning system alerts.
The ATSB says that the report highlights the importance of situational awareness during phases of flight when terrain clearance is unavoidably reduced.
"Any loss of vertical situation awareness increases the risk of controlled flight into terrain," the report noted. "This occurrence highlights the importance of crews effectively monitoring the flight profile to ensure that descent is not continued through a segment minimum safe altitude when conducting a non-precision approach."