Australian Transport Safety Bureau (ATSB) investigators have not been able to determine why an oxygen gas bottle exploded mid-flight, ripping a hole in the side of a Qantas Boeing 747-400 flying high above the South China Sea on July 25, 2008.

The oxygen tank exploded at 29,000 feet, ripped a gaping hole in the fuselage and required an emergency landing in Manila. Fortunately, there were no injuries among the 369 passengers and crew aboard Fight QF30.

The ATSB's follow-up report on the serious incident found no evidence of systemic safety problems with oxygen bottles like those involved in a Qantas incident last year.

The ATSB's second interim factual report on the accident says " there is no evidence of systemic safety problems with oxygen bottles of the type involved in the accident. Various tests have not been able to replicate the cylinder failure that initiated the accident."

The report provides details of the wide-ranging and ongoing technical examination of five oxygen cylinders obtained by the ATSB from the same manufacturing lot as the failed cylinder. The original cylinder was lost in the South China Sea in the course of the accident.

Analysis of the factual information and findings as to the factors that contributed to the accident remain the subject of ongoing work, the ATSB said. Details will be included in the final report of the investigation.

"To date, all pressure tests of the cylinders met or exceeded the relevant safety specifications, with recorded rupture pressures being over twice the maximum working pressure of the cylinders," the ATSB added.

Other work is being carried out to determine the minimum size of mechanical flaws that could result in cylinder failure in service. The ongoing ATSB investigation will supplement that work with a program of rupture tests on cylinders that have had various sized 'artificial' flaws machined into the shell.

The ATSB expects to conclude its probe in early 2010 with a final report to follow.

The Qantas Boeing 747 depressurization event occurred 475 km northwest of Manila. An emergency oxygen cylinder (the number-4 cylinder) failed, rupturing the aircraft's fuselage while on a scheduled passenger flight from Hong Kong to Melbourne. The flight crew diverted to Ninoy Aquino International Airport, Manila, where an uneventful visual approach and landing was made.

A large portion of the failed oxygen cylinder shot upward through the cabin floor, impacting with the R2 door and the overhead ceiling panels, before exiting the aircraft through the fuselage rupture. Investigators failed to find the cylinder that blew a hole in the aircraft's fuselage

Early tests revealed no evidence of an external explosive event or the use of explosive materials around the rupture area. The oxygen valve from the number-4 cylinder, which was the only item of physical evidence recovered from the cylinder, was closely examined, with no evidence to suggest that an oxygen-promoted fire or an overpressure event had contributed to the cylinder failure.

No significant maintenance difficulties had been experienced with the passenger oxygen system prior to the occurrence. Investigators visited the Qantas Sydney Jet Base oxygen workshop, where the servicing and replenishment of all Qantas oxygen cylinders is performed. The inspections did not identify any significant issues or deviations from documented practice that had the potential to affect the integrity of the cylinder-valve assemblies.

The ATSB obtained two samples of the gaseous oxygen that was used to fill the number-4 cylinder. These samples were analyzed and compared against the required specification for aviators breathing oxygen. No anomalies were identified that would have contributed to this event.

The remaining cylinders from QF30, together with the aforementioned five additional oxygen tanks from the same manufacturing batch lot as the number-4 cylinder, were examined for evidence of any deficiencies or deviations from the certified design.