Air safety investigators say there was an "irregularity" in the onboard computer equipment of a Qantas plane involved in a mid-air incident over Western Australia.
A second team of investigators was travelling to Learmonth, in the state's north, to find the cause of the incident which injured about 20 passengers and crew on board flight QF72 travelling from Singapore to Perth.
The pilots sent a mayday call shortly before making an emergency landing at the regional airstrip, 40km from Exmouth on WA's Gascoyne coast.
Qantas today said the cause of the "sudden change in altitude" was speculation.
Australian Transport Safety Bureau (ATSB) director of aviation safety investigation Julian Walshe said the plane was travelling at 37,000 feet when the incident happened.
"The pilots received electronic centralised aircraft monitoring messages in the cockpit relating to some irregularity with the aircraft's elevator control system," he told reporters in Canberra today.
The aircraft then climbed about 300 feet before "abruptly" pitching nose down.
Passengers arriving in Perth last night told of their horror as the drop threw them and their personal belongings across the plane.
Jim Ford, of Perth, said he thought he was about to die as he watched people being thrown around the cabin.
"It was horrendous, absolutely gruesome, terrible, the worst experience of my life," he said.
Ben Cave, of Perth, said for a few seconds he had feared for his life and "saw a bit of a flash before me".
"We had a major fall and another fall shortly after.
"I hit the ceiling but I was OK, I only got a few bruises and strains. I just remember seeing that the plane was a mess."
Henry and Doreen Bishop, of Oxford, England, said it was one of the worst experiences of their lives.
"People were screaming but they cut off any panic that might have started...", Mr Bishop said.
"I put it down to life. The titanic hit an iceberg, we hit an air pocket."
A spokesman for Perth's Sir Charles Gairdner Hospital said one of the 20 passengers it treated last night was in a serious but stable condition. Eight people were under observation and 11 other patients were discharged, he said. Injuries included fractures, lacerations and suspected spinal injuries.
WA Police Commissioner Karl O'Callaghan was forced to activate the state crisis centre because of the number of injuries.
"It seems that there might have been some sort of systems failure, we're not sure yet, we're still waiting for further information," Dr O'Callaghan told ABC Radio.
A further five ATSB investigators were on their way to Learmonth to join their two colleagues already on the ground.
The flight data recorder and cockpit voice recorder have been quarantined and sent to Canberra for testing.
The Australian and International Pilots' Association today said turbulence was not uncommon on that flight path.
Captain Ian Woods said most modern passenger planes were built to cope with changes in altitude.
"When you cross those jetstreams as you do from Singapore to Perth ... you run across the transition boundary," Capt Woods, also a Qantas pilot, told ABC Radio.
"It's at that point where you're crossing from smooth air to fast-flowing air, that there can be quite unexpected and significant turbulence."
This could cause a "jet upset", Capt Woods said.
"So if you're unfortunate enough to run into that, and it sounds like that's what's happened, then certainly it's unexpected and you can get outcomes like this."
Capt Woods said turbulence was nothing pilots "can't cope with".
"Aeroplanes have been ... refined over the years and if we go back to the 50s, then these kinds of events were worse than they are now."
The incident is another blow to Qantas which is still dealing with several problems this year including an exploding oxygen bottle which punched a huge hole in the side of a Qantas Boeing 747-400, forcing an emergency landing in Manila.
A Qantas Boeing 737-800 returned to Adelaide after a landing gear door failed to retract, and a Manila-bound Boeing 767 was turned back to Sydney after developing a hydraulic fluid leak.