Jumbo jet oxygen tank blast would be a first: officials

An onboard oxygen bottle has never before exploded on a passenger jet in mid-air, airline and air safety officials said Monday, as investigators probed the cause of a huge hole in a Qantas jumbo.

The Australian carrier is carrying out urgent inspections of oxygen bottles on its entire fleet of Boeing 747s after the fuselage of a 747-400 was ripped open, forcing an emergency landing Friday in Manila.

"Boeing advises that no, they have not had one of their aircraft with an oxygen tank disintegrating," Qantas chief engineer David Cox told reporters.

Australian Air Transport Safety Board (ATSB) investigators are focusing on whether an oxygen bottle used for emergency back-up for the cockpit exploded mid-flight, tearing a three-metre (10-foot) hole in the Boeing's hull.

One of two such cylinders is missing from the plane, which was en route from Hong Kong to Melbourne and had 365 passengers and crew on board when it was forced to land, investigators said.

But Cox refused to speculate on whether an exploding oxygen bottle was to blame for rupturing the Boeing's hull in an incident that Qantas executives acknowledged ended in a lucky escape for passengers.

"We don't know that was the root cause so that's why we're not going to speculate. The fact that that has never happened may be relevant, it may not be relevant," Cox said.

Air safety officials confirmed that if an exploding oxygen bottle is proved to have blown a hole in the jet, it would mark the first time such an incident has been recorded in a large passenger plane.

"As far as we can determine this has never happened before on a passenger aircraft," Civil Aviation Safety Authority (CASA) spokesman Peter Gibson said.

"There's no reports of it anywhere, so it's very, very unusual and obviously understanding why that happened will be absolutely critical to making sure it can't occur again," he told the Australian Broadcasting Corporation.

"If it turns out that is the cause of the accident, the cause of the hole in the side of the aircraft, obviously that will be a key part of the investigation, working out why a bottle would suddenly give way," Gibson said.

Metal fatigue in the cylinder, a failure of the regulator valve, something hitting it and puncturing the bottle, or it overheating, were among possible causes the ATSB would look at, he said.

"Maintenance has to be looked at obviously, yes you can't rule that out, but at this stage you look at absolutely everything," Gibson told ABC radio.

Qantas has ordered checks on the oxygen bottles -- they are due to be completed this week -- after Australian investigators leading the probe in the Philippines revealed a cylinder was missing from the plane.

But outgoing Qantas Chief Executive Geoff Dixon defended the airline's "enviable" safety record and dismissed speculation that the stricken aircraft may have been improperly maintained.

"We have very strict maintenance and security in this company, I think we have a worldwide reputation for it," Dixon told a press conference in Sydney.

Some passengers have alleged that the oxygen masks that dropped from the ceiling when the plane began its emergency descent following depressurisation did not work and one claimed the elastic headband on his mask had deteriorated.

"There is no doubt that all of our masks are checked on a regular basis, yet there is every chance that the trauma suffered by the aircraft may have interfered with other systems," Dixon said.

"We believe that everything in that aircraft was in good shape when it took off," he said.