Investigators are focusing on a potential irregularity in the flight control system of a Qantas Airways Airbus A330-300 that yesterday experienced a sudden nose pitch down mid-flight resulting in around 74 people on board being injured.
The pilots received centralised aircraft monitoring messages to say there was "some irregularity with the aircraft's elevator control system", the Australian Transport Safety Bureau (ATSB)) says in a statement today.
This statement is referring to an accident yesterday afternoon while the aircraft was at 37,000ft (11,280m) en route from Singapore to Perth.
The aircraft "climbed approximately 300ft...during which time the crew had initiated non-normal check-list and response actions", says the ATSB.
Then the aircraft "abruptly pitched nose-down, [and] during this sudden and significant nose-down pitch, a number of passengers, cabin crew and loose objects were thrown about the aircraft cabin, primarily in the rear of the aircraft".
This resulted in "a range of injuries to some cabin crew and passengers" including broken bones.
ATSB says the pilots first made a 'pan pan' emergency broadcast to air traffic control but a few minutes later called 'mayday' and requested clearance to land at Learmonth rather than continuing to Perth.
"The aircraft landed at about 15:30 local time, about 40min after the start of the event."
ATSB says the event has been classified as an aircraft accident because of the nature of the injuries.
It says of the 303 passengers and 10 crew on board, 14 people had serious - but not life threatening - injuries.
An additional group of up to 30 had serious enough injuries to receive medical treatment in hospital and up to a further 30 required first aid treatment, it adds.
ATSB says it currently has seven investigators examining or about to examine the aircraft plus Australia's Civil Aviation Safety Authority, France's Bureau Enquetes-Accidents and Airbus are involved in the investigation.
It says it plans to issue a preliminary factual report within 30 days.
Qantas says in a separate statement that the severely injured people on board the aircraft were airlifted from Learmonth to Perth by Australia's Royal Flying Doctor Service. Other passengers were flown to Perth on Skywest Airlines, it adds.
Learmonth is an airport and airbase south of Exmouth, a remote town near Western Australia's northwest coast.
Qantas' A330-300 remains in Learmonth because the ATSB is inspecting it, says the airline.
The airline's CEO, Geoff Dixon, says the carrier is assisting with the investigation and the aircraft's flight data recorders and cockpit voice recorders have been removed and will undergo assessment.
According to Flight's ACAS Daily Alert the A330-300 involved has local registration VH-QPA.