Boeing 737-376, VH-TAX

The Departures North radar controller was radar vectoring a Boeing 737 (B737) for a right downwind leg to runway 34R and had instructed the crew to descend to 7,000 ft, which was correctly read back. Subsequently, the controller observed the mode C altitude radar display for the aircraft indicating that it was descending through 6,400 ft. The controller requested that the crew confirm the assigned altitude and was advised that they had been assigned 5,000 ft. The controller instructed the crew to climb the aircraft to 7,000 ft. There was no infringement of separation standards; however, the B737 was being vectored to overfly a De Havilland Twin Otter, and vertical separation standards were being applied.

An investigation by the operator established that the pilot in command (PIC) had selected 5,000 ft in error, and that a cross-check of the altitude setting by the co-pilot had been obscured by sun reflections on the instrument panel. The PIC believed that he may have overheard the assignment of 5,000 ft to the crew of another aircraft at the time he was resetting the altitude indicator. At the same time, the purser entered the cockpit and this action may have distracted the PIC so that he only remembered the last altitude transmitted, and not that actually assigned to his aircraft.