INVESTIGATORS are focusing on the possibility that exploding oxygen tanks were responsible for blowing a gaping hole in the fuselage of a Qantas Boeing 747 plane at 29,000ft.

A team from the Australian Transport Safety Bureau has found debris scattered through the cargo hold from two oxygen tanks holding a two-hour emergency supply for the pilots. The roof above them appears to have been pushed up by the force of an explosion.

Investigators are working on the theory that the explosion was caused by impact from loose cargo or the corrosion of the tanks themselves.

A source close to the Civil Aviation Safety Authority (CASA) in Australia said: “It is looking extremely likely that one or more of the oxygen tanks exploded. It’s never happened before so nobody knows what caused it. It could be as simple as some cargo shifting and smashing into it, which shouldn’t be able to happen.”

The investigators added, however, that it is too early to discount metal fatigue, whereby the fuselage is weakened over time by the expansion and contraction of the plane during take-off and landing. This creates tiny cracks in the fuselage which are invisible to the eye, but which can eventually give way.

Flight QF30 from London to Melbourne was an hour into its leg from Hong Kong to Melbourne on Friday morning when a gaping hole appeared in the front right side of the plane. Part of the ceiling in the passenger cabin collapsed, some of the flooring gave way and debris flew across the first class section.

Some passengers recorded the drama on their mobile phones. Footage showed passengers sitting with their oxygen masks on as the 19-strong crew continued to serve food, showing no sign of panic.

Captain John Francis Bartels, a former pilot with the Royal Australian Navy, managed to put the aircraft into an emergency descent from 29,000ft to 10,000ft where the passengers and crew could breathe without oxygen. He landed at Manila airport in the Philippines an hour later.

Investigators are analysing debris in the cargo hold from the two oxygen tanks. The tanks were bolted to the walls of the hold near where the fuselage ruptured.

There were unconfirmed reports last night that the pilots had experienced difficulties with their oxygen supply as they made their descent. They have a backup in the cockpit in the event of the main supply failing.

According to Bill Hamilton, a former pilot who flew Qantas 747s for seven years, the bottles are made of toughened aluminium or carbon fibre and would have been difficult to break. He believes corrosion could have caused the tanks to rupture.

“There’s a saying in the engineering field that if you don’t find corrosion you are not looking hard enough. The structural failures I’ve heard of in the past have been corrosion-related or damage-related and because of inferior repairs,” he said.

As well as the debris, investigators also found that the floor above the tanks had been pushed upwards, which is more consistent with an explosion.

The CASA source said: “In a normal depressurisation, if a fuselage had metal fatigue and a hole was suddenly ripped in it, everything would be sucked down and out. But in fact the floor above the bottles has been blown upwards, which is more consistent with an explosion.”

Geoff Dixon, chief executive of Qantas, denied that corrosion caused the hole. According to the CASA, corrosion was identified during a month-long maintenance check in February, but the damage was minor and further back in the plane. It had previously undergone a more thorough check in 2004.

Although the plane is 17 years old, it is still considered relatively young by aviation experts because long-haul aircraft make relatively few take-offs and landings, which put strain on the fuselage.

If the oxygen tanks are to blame, the repercussions for the rest of the 700-strong fleet of 747-400s are likely to be limited. Most do not carry the large tanks used on flight QF30, which are standard issue on planes whose path takes them over the Himalayas. They need larger supplies before a plane can be sure of dropping to a safe altitude of 10,000ft. The majority of 747-400s in service use chemical oxygen generators, which are less volatile.

Explosive decompressions such as that suffered by the Qantas jet are rare, with about 20 incidents in the history of commercial aviation. The most recent took place in 1989, when nine people died on a United Airlines 747-122 when the cargo door blew out after it left Hawaii for Australia. Qantas has never lost a jet aircraft to an accident. Its record was emphasised in the 1988 movie Rain Man, where an autistic character played by Dustin Hoffman insists on flying with the airline.

The delayed passengers finally arrived in Melbourne yesterday. For Jane and Dermot Charlton, from Fareham in Hampshire, QF30 had been their first flight.

“It was a real good adventure and certainly very interesting, but three days to get here is a very long time,” said Dermot.

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