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 on Wednesday said the cause of the "sudden change in altitude" was speculation.
Australian Transport Safety Bureau (ATSB) director of aviation safety investigation Julian Walsh 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 on Wednesday.
The aircraft then climbed about 300 feet before "abruptly" pitching nose down.
Passengers arriving in Perth on Tuesday 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 on Tuesday 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.
The flight data recorder and cockpit voice recorder have been quarantined and sent to Canberra for testing.
Two ATSB investigators were immediately flown to WA, while another five were on their way to the site where the plane landed to interview passengers and crew, Mr Walsh said.
An officer from the Civil Aviation Safety Authority has joined the team, as well as an investigator from the Bureau d'Enquetes et d'Analyses (BEA), which is the French counterpart to the ATSB.
The BEA officer has been assigned to the investigation because the plane was manufactured in France.
An investigator from Airbus is also travelling to Australia and will also assist the investigations.
"It is obviously very early in the investigation and too soon to draw any conclusions as to the specific cause of the accident," Mr Walsh said.
The ATSB investigation would explore all aspects of the aircraft's operation, including the flight's black boxes, on-board computer systems, air traffic control and radar warnings and weather conditions, he said.
"The ATSB will also be conducting a range of interviews with the pilots and cabin crew and will also speak with passengers to examine the cabin safety aspects."
The investigation was likely to take some months but the ATSB would release a preliminary report within 30 days, Mr Walsh said.
It was unclear how far in altitude the aircraft dropped during the incident.
The Australian and International Pilots' Association on Wednesday 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 jet streams 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.
Passengers tell of horror aboard QF72 - Yahoo!7 News