Boeing Co 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.