Oxygen bottle behind Qantas blast

The hole resulted in rapid depressurisation at 29,000ft

Australian investigators have confirmed that an exploding oxygen cylinder caused the blast which blew a large hole in a Qantas plane last month.

But they admit they are no closer to explaining why the cylinder blew up.

The incident happened on a Boeing 747 that was flying from Hong Kong to Melbourne on 25 July.

The explosion caused a sudden loss of cabin pressure, forcing an emergency descent to a point where oxygen masks were no longer needed.

The pilot diverted the plane to the Philippines, and it landed safely in Manila despite having a 5 foot (1.5m) hole in its fuselage.

None of the 365 passengers and crew were injured.

Key question

WHY QF30 DESCENDED 20,000FT Planes are pressurised as cruising altitudes are freezing and lack sufficient oxygen to breathe Hole causes decompression, rapidly reducing air pressure and risking exposure Oxygen masks are deployed and pilot makes emergency descent to breathable altitude

The Australian Transport Safety Bureau (ATSB) released an interim report on Friday, as part of an ongoing investigation into the incident.

The report confirmed early findings that one of seven emergency oxygen cylinders had exploded, blowing a hole in the fuselage and sending the bottle up through the passenger cabin floor.

Julian Walsh, acting executive director of the bureau, said the challenge now was to pinpoint the cause of the explosion.

"There's nothing at this stage that the ATSB can identify what could have been done to prevent this because we don't really know why the bottle failed - and that's the key question for the investigation," Mr Walsh told reporters.

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A passenger films the Qantas forced landing from inside the plane

Since the emergency, Qantas has inspected the oxygen cylinders on all its aircraft without identifying any problems.

But the incident has raised questions about the much-praised safety record of Australia's flagship airline.

Qantas planes have recently experienced a number of other problems. In early August a Qantas 767 was forced to return to Sydney shortly after take-off because of a fluid leak. In another incident, a landing gear cover failed.

Australian aviation authorities have announced a special review of the company's safety standards.