An exploding oxygen tank forced an emergency landing of a Qantas jet in Manila after it blew a hole in the fuselage, air safety investigators have found.
The tank failed and burst, blasting through the cabin floor from a storage area between business and economy class seats on a Hong Kong to Melbourne flight last month, a preliminary report by the Australian Transport Safety Bureau (ATSB) found.
Bureau investigators are yet to determine why the tank exploded, almost two months after the July 5 incident.
They could not rule out such an incident occurring again.
Safety investigators said passengers and crew were extremely lucky not to have been injured by the exploding oxygen tank.
All 346 passengers and 19 crew aboard the flight escaped injury when the Boeing 747-400 made the emergency landing after the mid-air explosion tore a three-metre hole in its fuselage.
"The cylinder had been propelled upward by the force of the discharge, puncturing the cabin floor and entering cabin adjacent to the second main cabin doors," the ATSB report says.
It said the cylinder hit the door, its handle and overhead panelling before leaving the aircraft through the ruptured fuselage.
A photo of the impact area showed the projectile came through the floor near the base of the door where a jump seat for a cabin crew member is located.
"The passengers were obviously very lucky," ATSB investigator Julian Walsh told reporters in Canberra.
"(The cylinder's) been propelled up with significant force to do the damage that it has, and it's happened very quickly so it probably would have been a matter of seconds," he said.
"These oxygen bottles are actually quite robust so it would take some significant force to do some damage like that.
"There's nothing at this stage that the ATSB can identify that could have been done to prevent this, we don't really know why the bottle failed - that's the key question for the investigation."
The inquiry is ongoing.
Mr Walsh said it would be difficult to learn what happened to the damaged cylinder because it was not available to examine.
He said he was not aware of a similar occurrence in aviation history but could not rule out such an event happening again.
"Anything's possible, it's certainly a rare event but no one can discount the possibility of those sort of things happening," Mr Walsh said.
The ATSB team is attempting to survey all of the passengers on board the flight about what happened.
During the incident, oxygen masks were deployed and the pilot initiated a rapid emergency descent, before making the trouble-free emergency landing at Manila's airport.
"As far as we can tell from all the information that's available to us the crew have pretty much done a textbook response," Mr Walsh said.