Loss of separation involving a British Aerospace PLC BAe 146-300, VH-EWR and Boeing 737-377, VH-CZL, Yango, New South Wales, on 14 May 1996

A British Aerospace 146 (BA146) was established in the holding pattern at YANGO, 60 NM NNW of Sydney, maintaining Flight Level 190 (FL190). A Boeing 737 (B737) entered the holding pattern at FL200, and was given traffic information on the BA146, which was directly below.

Holding was cancelled for the BA146, which was cleared to descend to FL180. The Arrivals North radar controller subsequently observed the BA146 height readout indicating FL200, and was about to query the level of the aircraft, when the crew of the BA146 advised the aircraft had climbed to FL200 but was now on descent to FL190. The lateral separation between the two aircraft was reduced to 1.53 NM, with a vertical separation of 800 ft. The minimum required separation was 5 NM laterally, or 1,000 ft vertically.

The Captain of the BA146 subsequently reported that the aircraft was being controlled by the autopilot at the time of the occurrence. FL180 had been correctly set on the altitude controller, but the aircraft had inadvertently climbed without warning. The captain, who was the non-flying pilot, had earlier given permission to permit three children to stand near the flight deck door. As the aircraft commenced descent to FL180 he had turned around briefly to say goodbye. When he turned back, he saw the aircraft was climbing at about 1,500 fpm, approaching FL200. He immediately instructed the co-pilot to descend to FL190, and alerted ATC.