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.