SYDNEY - INSTRUMENTS aboard a Qantas airliner warned of a glitch in its stabilization system when it suddenly rose and plunged, tossing unbelted passengers to the ceiling and injuring more than 70 people, Australian investigators said Wednesday.

The A330-300 was carrying 303 passengers and 10 crew from Singapore to the Western Australian city of Perth, and was nearing its destination on Tuesday when it experienced the sudden altitude changes while flying at 11,000 m.

The plane made an emergency landing in Learmonth, Western Australia.

Air Transport Safety Bureau investigators quarantined the flight data recorder and cockpit voice recorder and planned to interview crew and passengers.

Mr Julian Walsh, director of the bureau's aviation safety investigation, told reporters that the pilots received electronic messages 'relating to some irregularity with the aircraft's elevator control system.'

That system helps keep the plane stable and level in flight.

The aircraft then 'departed level flight,' and climbed approximately 90 m, he said.

'The crew had initiated the non-normal checklist response actions. The aircraft is then reported to have abruptly pitched nosedown,' Mr Walsh said.

It was unclear how far in altitude the aircraft dropped during the incident.

Passengers who were not wearing seatbelts flew into the air, some hitting the ceiling of the plane.

Mr Walsh said it was too soon to draw any conclusions about the specific cause of the accident, but that a preliminary report would be released within 30 days.

The ATSB investigation will examine the flight data recorders, on-board computer systems, air traffic control and radar warnings and weather conditions, he said. - AP