Australia’s aviation safety watchdog is investigating the possibility that interference from a defence communications base caused a Qantas flight to suddenly plunge, injuring 44 passengers and crew.

A preliminary report into the incident, released today by the Australian Transport Safety Bureau (ATSB), said there was a possibility that transmissions from the Harold E. Holt naval communications station interfered with aircraft onboard systems.

The ATSB is also investigating the possibility that passenger electronic devices aboard the aircraft caused the problem.

ATSB regards both as unlikely but it cannot rule either out at this stage.

The watchdog is focusing on a flight computer system component called an ADIRU – air data inertial reference unit.

Examination of flight data recorder information indicated this was producing erroneous information, possibly from a software or hardware fault.

It appeared the spurious information was sent to the flight control computer, causing the autopilot to shut down.

“The three ADIRUs will be subject to comprehensive testing at the manufacturer’s facility in the US,” ATSB official Kerryn Macaulay told reporters in Canberra on Friday.

That testing is planned to start on Monday.

The incident occurred on October 7, as a Qantas Airbus A330 flew off the West Australian coast en route from Singapore to Perth.

Forty-four of the 313 people on board required hospital treatment after the aircraft suddenly plunged, hurling passengers around the cabin.

The aircraft was cruising at 37,000 feet when the fault occurred, causing it to descend up to 650 feet in seconds.

The pilot made an emergency landing at Learmonth in north-western WA.

The submarine base, which transmits very low frequency signals at very high power, is about 200km from where the plane plunged.

Macaulay said ADIRUs were usually highly reliable and there were only a few that had experienced some form of failure.

In this case, the ADIRUs were manufactured by US firm Northrop Grumman.

“A carefully prepared test plan is currently being finalised 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 in order to provide a basis to eliminate the problem at its source,” she said.

In a 2005 incident involving ADIRU problems, a Malaysian Airline Boeing flight travelling between Perth and Kuala Lumpur plunged suddenly. That was blamed on faulty accelerometers feeding erroneous data to the ADIRU and flight control computers.

Macaulay said it was unlikely external electromagnetic interference was the cause, especially if the problem was clearly identified during testing of the ADIRU.

She said ATSB needed to establish whether Harold E. Holt was transmitting at the time.

“Our preliminary analysis is that it is unlikely to have been the source of the problem here, although we haven’t absolutely ruled it out at this point,” she said.

“It emits very low frequency transmissions and from a significant distance from the aircraft flying overhead. So our feeling is it’s unlikely to have caused sufficient currents in the system to have caused problem with the A-330 systems.”