E/GPWS warning involving a Boeing 747-438, VH-OJD, Kinglake, Victoria, on 3 July 1993

Overhead Kinglake at 5,500 ft on approach into Melbourne the crew received a terrain proximity alarm and applied emergency climb power, climbing to 7,000 ft.

This is one of a large number of spurious warnings occurring to the world fleet of Boeing 747-400 aircraft. In broad terms, the problem is in the logic of the ground proximity warning system (GPWS) which allows a warning to be registered when the landing gear is retracted, and the number 1 radio altimeter registers an altitude of 2,500 ft (i.e. 2,500 ft above terrain).

The aircraft manufacturer has devised a modification to the antenna and the number 1 radio altimeter that has since been incorporated into the operator's fleet and these spurious warnings no longer occur.