On 7 April 2009, at about 1210 Eastern Standard Time, the flight
crew of a Boeing 737-800 aircraft, registered VH-VYL, received an
enhanced ground proximity warning system alert during an approach
to land at Sydney Airport, NSW. At the same time, the autopilot
disconnected and the engine thrust levers moved towards idle. The
handling pilot corrected the engine thrust levers immediately and
conducted an uneventful landing.
The investigation determined that spurious data from the left
radio altimeter (RA) provided an indicated altitude of minus 7 ft,
resulting in the autopilot disconnecting and the thrust lever
movement.
An examination found that the left RA receive antenna displayed
rubbing wear adjacent to the attachment screw inserts. A bonding
check of the antenna indicated that the antenna's resistance was
outside the aircraft manufacturer's limits. The antenna was
replaced and the aircraft was returned to service.
Three months after the occurrence, a further RA warning flag
event was experienced by another crew in this aircraft. As a result
of that event, the left and right RA transceivers were removed and
tested with internal faults found on the left unit.