Powerful signals from a secret U.S.-Australian navy communications base could have caused a Qantas Airbus jet to suddenly dive last month, injuring scores of passengers, Australia's air safety agency said on Friday.
But the agency said it was more likely the accident was caused by a glitch in an air data inertial reference unit, which feeds flight information to the Qantas aircraft's main computer.
The Airbus, with 303 passengers and 10 crew, was cruising at 37,000 feet (11,200m) from Singapore to Perth on October 7 when it suddenly gained altitude, then plummeted more than 1,000 feet in little over a minute.
In a preliminary report into the incident, Australia's Transport Safety Bureau said investigators were looking at the possibility the aircraft's flight computers were affected by a strong burst of electromagnetic interference.
"Possible external sources of electromagnetic interference are being explored and assessed, including from the Harold E. Holt very low frequency transmitter near Exmouth, Western Australia," bureau capability director Kerryn Macaulay said.
However, Macaulay said it was more likely the mishap was caused by something else, including an electronic device or laptop computer, as the aircraft was 200kms (124 miles) away from the naval base when the nosedive occurred.
She said the bureau hoped to establish whether the navy base was transmitting signals at the time of the accident.
Many people living near Exmouth have blamed the top-secret naval base, which transmits signals to U.S. and Australian navy ships, including nuclear submarines, in the Pacific and Indian oceans.
The station, on Australia's northwest coast, is the most powerful transmission station in the Southern Hemisphere and has been a frequent target of peace protests since it opened in 1963.
The air safety agency said it would begin detailed testing of the aircraft computers on Monday.
"A carefully prepared test plan is currently being finalized in anticipation of this complex work to ensure the investigation team has the best possible chance to understand what led to the pitch-down events," Macaulay said.
Last month the agency and Airbus issued emergency guidelines to airlines worldwide operating the Airbus A330-300 in the event of a similar emergency.
The ATSB's Safety Bureau director Julian Walsh then said an errant flight sensor led the computer to incorrectly determine the jet was climbing when it was actually in level flight, sending it into a corrective dive.
Many on board were flung around the cabin or crashed against rooftop luggage compartments before the pilots regained control and made an emergency landing. At least 13 passengers were seriously injured, with some airlifted to hospital in Perth.
Another 60 were treated for minor bruises.
There are 247 long-haul A330-300s used by airlines around the world out of total orders for 383 of the wide body aircraft, according to the Airbus Web site. They can hold a maximum of 335 passengers.
Qantas, the world's 10th largest airline by market value, has been hit by a number of incidents recently.
In one, Australian air safety investigators blamed an oxygen bottle for a mid-air explosion that blew a minivan-size hole in the side of Qantas jumbo jet on July 25, forcing the aircraft to make an emergency landing in the Philippines.