An exploding oxygen bottle was to blame for a mid-air blast which last month blew a gaping hole in a Qantas jet travelling from Hong Kong to Australia, safety investigators said on Friday.

The emergency caused the cabin to depressurise and forced the pilots to land the Boeing 747-400, carrying 365 people, in the Philippine capital Manila.

In a preliminary report, the Australian Transport Safety Bureau (ATSB) said that one of seven passenger oxygen cylinders failed and then exploded in the aircraft hold, rupturing the fuselage.

The explosion punched through the cabin floor, driving the malfunctioning cylinder into the cabin before it fell back through the floor and out of the aircraft, it said.

"The cylinder had been propelled upward by the force of the discharge, puncturing the cabin floor and entering the cabin adjacent to the second main cabin door," the report said.

"The cylinder had subsequently impacted the door frame, door handle and overhead panelling, before falling to the cabin floor and exiting the aircraft through the ruptured fuselage."

The report said that the incident occurred about 55 minutes into the July 25 flight as the plane was cruising at 29 000 feet.

It said "a loud bang was heard by passengers and crew, followed by the rapid depressurisation of the cabin."

Passengers and crew donned oxygen masks and the pilots commenced a descent to a lower altitude and issued a mayday distress radio call before landing safely in Manila. No one was injured in the emergency.

The bureau said its inquiries into the accident were continuing and that a survey of all passengers was also being conducted.

Series of glitches dogged airline

The incident, which left a T-shaped rupture in the 1991-manufactured plane measuring 2.0 by 1.5 metres, is the most serious in a series of glitches which have beset the airline in recent months.

Just days after the Manila incident, a Qantas 737-800 was forced to return to Adelaide after a landing gear door failed to retract. And in early August, a Qantas 767 was forced to return to Sydney shortly after take off when the pilot detected a hydraulic leak in the wing.

Qantas welcomed the report into the incident, saying its own investigations were consistent with the bureau's initial finding that one of the aircraft's passenger oxygen system cylinders failed.

"We will continue to assist the ATSB to ensure that factors that may have contributed to the incident are understood and that any corrective actions ultimately identified are implemented," chief executive Geoff Dixon said.

In a statement, Dixon said Qantas completed an inspection of the oxygen systems across its B747-400 fleet in early August to confirm there were no safety issues.

He said despite the seriousness of the incident, Qantas was proud of its safety record. The "Flying Kangaroo" has never lost a jet plane to an accident.

Dixon said the B747-400 involved in the mid-air blast would be repaired for less than A$10-million and be back in service in November.

AFP