B773, vicinity Melbourne Australia, 2011

B773, vicinity Melbourne Australia, 2011

Summary

On 24 July 2011, a Thai Airways International Boeing 777-300 descended below the safe altitude on a night non-precision approach being flown at Melbourne and then failed to commence the go around instructed by ATC because of this until the instruction had been repeated. The Investigation concluded that the aircraft commander monitoring the automatic approach flown by the First Officer had probably experienced  automation surprise in respect of the effects of an unexpected FMS mode change and had thereafter failed to monitor the descent of the aircraft with a selected FMS mode which was not normally used for approach.

Description

On 24 July 2011, a Boeing 777-300 being operated by Thai Airways International on a scheduled passenger flight from Bangkok to Melbourne descended so far below the vertical profile for the night non precision approach procedure being flown in Visual Meteorological Conditions (VMC) at destination that ATC, observing that the aircraft was below the minimum safe altitude for the range from touchdown, instructed the aircraft to fly a go around. However, despite acknowledgement, this instruction only resulted in the descent being arrested and the go around did not commence until a further instruction to do so was given nearly a minute later. The subsequent approach using the same procedure was uneventful.

Investigation

An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). It was established that the aircraft had been on a VOR/LOC approach to runway 34 (see procedure plate below) with the First Officer as PF and the AP engaged.

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