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Investigations into the sudden plunge that injured more than 80 people on a Qantas Airbus A330-300 on Tuesday appear likely to centre on the aircraft's computer system.
Aviation sources yesterday discounted reports that passenger laptops could have interfered with the aircraft's electronics, launching the Perth-bound Airbus into a dive from normal level flight at 11,200m near Carnarvon, in northern Western Australia.
There have been cases where passengers trying to connect to the internet by modem have affected navigational instruments including some in New Zealand and some studies indicating laptop use could cause aircraft to veer off course.
But sources said civil aviation around the world operated conservatively and with a very large margin of safety, including the use of laptops.
"The likelihood that it had anything to do with a laptop computer is pretty insignificant," one said.
Instead, initial investigations are focusing on an irregularity in the computer system that issued warnings of a problem with the aircraft's elevator control system.
Although the Australian Transport Safety Bureau yesterday declined to comment on possible causes, the Sydney Morning Herald reported that a Qantas source had blamed the aircraft's autopilot system.
The ATSB said after electronic centralised monitoring messages reported problems with the elevator control system, the Airbus had performed of its own accord and climbed 300ft before abruptly pitching nose down.
The sudden drop injured 74 passengers and 10 crew, 14 seriously, including back injuries, broken bones and lacerations.
Before an emergency landing at an RAAF base, the pilots reported problems with the flight control computer.
ATSB investigators, with specialists from the manufacturer and the bureau's French counterpart, have removed the flight data and cockpit voice recorders and were yesterday continuing their inspection of the aircraft.
Investigators will also be interviewing passengers, cabin crew and pilots.
Concern grew yesterday that a passenger may have caused the emergency by using a laptop, following reports that investigators were following this lead.
The report, in Brisbane's Courier Mail, said a bureau report in July had said a Qantas Boeing 747 had been diverted off course on a three-degree bank by a passenger clicking a wireless mouse.
But an ATSB spokesman said yesterday: "I don't know where that came from."
He said the investigation was continuing with the aim of identifying and analysing all factors that may have contributed to the emergency.
The use of laptops is widely permitted on the world's airlines, including New Zealand aircraft, provided passengers do not try to connect to the internet.
An Australian source said generally they were allowed when aircraft reached the top of their climb or when the seat belt sign was switched off, although civil aviation regulations give pilots legal control of their use.
A New Zealand Civil Aviation spokesman said links between the use of laptops and aircraft electronics were inconclusive, although there had been cases where navigation equipment had experienced problems when laptops had been in use.
These had occurred when passengers had been trying to connect through their modems.
In one case an aircraft's instruments had reported the flight was well off course, but after passengers were told to turn off laptops the instruments returned to their correct settings.
The Australian Civil Aviation Safety Authority said yesterday that while numerous studies around the world had shown such electronic equipment as radios, mobile phones and bluetooth connectors held the greatest risk, the results for laptops had been inconsistent or inconclusive.
It said some case studies had shown use of a laptop can interfere with the aircraft systems with an immediate and obvious effect of making the aircraft veer off course.
With ascent and descent being the most critical phases of flight, interference caused by laptop use may have serious safety implications, the authority said.
The authority said laptop screens and microprocessor circuits could radiate electromagnetic energy, which, depending on a range of environmental factors could set up resonances within the fuselage that could affect some aircraft systems.
"Whether [a problem caused by a laptop] is a 1 per cent chance or a one-in-10,000 chance or a one-in-a-million chance, you can never eliminate these things entirely," a source said.
"But there are a lot of other reasons which are more likely."
