Probe at a loss on Qantas blast flight

But after a 2-year probe into the midair explosion on the flight on July 25, 2008, the Australian Transport Safety Bureau cannot conclusively say why an alloy-steel oxygen tank, one of 13 on board, ruptured in such dramatic fashion.

The plane was 55 minutes into a flight from Hong Kong to Melbourne and was carrying 350 passengers and 19 crew.

"Given the widespread and long-term use of this type of cylinder, it was clear that this occurrence was a unique event," said bureau chief Martin Dolan.

The tank holed the fuselage and rocketed vertically from the cargo area, punching through the cabin floor, smashing into a cabin door handle and striking overhead panels. It damaged wiring and control cables to the emergency oxygen system and some flight control systems.

Not all of the oxygen masks dropped from the overhead compartments; on some the tubing became disconnected and some passengers struggled to fit them because elastic straps had perished, passengers told air investigators.

A recorded emergency decompression-procedure message for passengers did not activate automatically leaving cabin crew to take off oxygen masks to shout instructions to passengers as a cold wind whistled through the cabin.

"Several crew members had become very distressed during the depressurisation and were initially unable to carry out emergency tasks," investigators reported. Some crew struggled to determine if their masks were supplying oxygen.

The pilots steadily brought the plane from 29,000 feet down to 10,000 feet where oxygen masks were not required, issued a mayday and made an emergency landing at Manila.

They landed the craft safely despite the blast knocking out instrument landing systems, a flight management computer and impairing the right-side anti-skid brakes.

"The investigation was unable to identify any particular factor or factors that could, with any degree of probability, be associated with the cylinder failure event," investigators said. The tank was lost overboard in the blast.

Qantas said the report found "the event was unique and without precedent" and the bureau had made "no findings in relation to Qantas's engineering and maintenance operations".