Air safety investigators continue to probe the serious July 25 incident in which a Qantas flight en route from Hong Kong to Australia made an emergency landing in Manila after a large gaping hole developed on the fuselage of the Boeing 747-438 (VH-OJK), prompting the plane to lose cabin pressure.

All 346 passengers and 19 crew aboard Qantas Flight 30 were safely deplaned after landing in Manila. The aircraft had departed Hong Kong Chek Lap Kok International (HKG/VHHH) and was bound for Melbourne-Tullamarine Airport, VIC (MEL/UYMML). The incident occurred over the South China Sea.

The Australian Air Transport Safety Board (AATSB) said the crew was forced to conduct an emergency descent from 29,000 feet after a section of the fuselage separated and resulted in a rapid decompression of the cabin. The crew descended the aircraft to 10,000 feet in accordance with established procedures and diverted the aircraft to Manila where a safe landing was carried out. An inspection showed that a section of the fuselage has separated in the area of the forward cargo compartment.

Investigators quickly discovered that one of 13 oxygen cylinders was missing from the bank of 13 that are responsible for supplying oxygen to the passenger mask and cabin crew in a decompression emergency.

And a number of passengers said that some of the oxygen masks appeared not to function correctly when they deployed from the overhead modules.

The ATSB said oxygen cylinder parts traveled through the passenger cabin and impacted the Number 2 right doorframe handle, thereby moving the handle part way towards the open position. However, the door handle mechanism had been sheared as it is designed to do if an attempt is made to open the door in flight.

The investigation team said the door latch remained engaged. Additionally, the door is of the plug-type that first needs to be pulled into the cabin, rotated 90 degrees then pushed out to open. So there was never any danger of the door opening.

The Associated Press quotes Australian Transport Safety Bureau ATSB Director of Aviation Safety Julian Walsh saying part of the oxygen tank blasted into the passenger cabin through the floor, smashed into a door handle and embedded in the ceiling. "The ATSB can confirm that part of an oxygen cylinder and valve entered the passenger cabin. Clearly the valve has traveled vertically through the floor of the aircraft, glanced with the door handle and impacted with the ceiling of the cabin. This is a unique event. It’s not happened before that we’re aware of."

Meantime, investigators found that most of the oxygen masks deployed correctly from the passenger modules and had been pulled to activate the flow of oxygen to the mask. Inspection by the ATSB shows that 484 masks had deployed, that is, dropped from the ceiling. Of those, 418 had been activated by pulling on the mask to activate the flow of oxygen. A small number of masks did not deploy from the passenger modules. Investigations into this aspect of the accident are continuing.

The investigation team is still examining the oxygen system, including liaising with the manufacturer to determine if the flow of oxygen was adequate for the five and a half minute descent to 10,000 feet, where the masks were no longer required.

Investigators said the aircraft’s three Instrument Landing Systems (ILS) and anti-skid system were not functional for the emergency landing in Manila. However, all the aircraft’s main systems, including engines and hydraulics were operating normally.

The approach to Manila airport was conducted in visual conditions. Other navigation instruments (VOR and NDB) were still available to the crew should the conditions not have been visual. Additionally, controllers could have provided radar assistance if the crew had required it.

The ATSB is also examining maintenance records for the aircraft, to include any airworthiness directives (AD) or alert bulletins that may have been issued by the regulatory agencies or Boeing.

It appears that an AD finalized by the Federal Aviation Administration (FAA) in May is unrelated to what happened to the Qantas jumbo jet.

The FAA had revised an existing airworthiness directive that applies to certain Boeing Model 747-400 series airplanes. That AD currently requires inspecting the support bracket of the crew oxygen cylinder installation to determine the manufacturing date marked on the support, and performing corrective action if necessary.

The new AD retains all the requirements of the existing AD and expands the applicability of the existing AD to include certain airplanes that are not on the U.S. Register. This AD results from a report indicating that certain oxygen cylinder supports may not have been properly heat-treated.

The FAA said it was issuing this AD "to prevent failure of the oxygen cylinder support under the most critical flight load conditions, which could cause the oxygen cylinder to come loose and leak oxygen. Leakage of oxygen could result in oxygen being unavailable for the flight crew or could result in a fire hazard in the vicinity of the leakage."

The Qantas incident is not the first involving an oxygen tank explosion aboard a jetliner.

On May 11, 1996, ValuJet Flight 592, a McDonnell Douglas DC-9 (N904VJ), crashed into the Florida Everglades about 10 minutes after takeoff from Miami International on a flight to Atlanta, GA. All 105 passengers and five crew died in the accident. In the minutes before the crash, the flight crew reported to air traffic controllers that they were attempting to return to Miami because of smoke in the cabin.

Evidence from the wreckage recovery showed that there was an intense in-flight fire in the forward cargo compartment, which contained, among other things, more than 100 expired, but still active, chemical oxygen generators.

The National Transportation Safety Boarddetermined that, sometime between when those materials were loaded on the aircraft and when the aircraft took off, one or more of the oxygen generators activated, initiating a fire that eventually brought the aircraft down.

As hazardous materials, those generators should not have been loaded onto the passenger aircraft . In addition, the Safety Board determined that the generators were not properly protected from inadvertent actuation by the installation of safety caps.

The NTSB said the fatal accident was caused by the failure of SabreTech (a contract maintenance operation in Miami) to properly prepare, package, and identify unexpended chemical oxygen generators before sending them to ValuJet for transport to its Atlanta Hartsfield.

According to the ATSB, "from the evidence gathered to date it appears that the flight crew responded to and managed the emergency situation extremely well. It is apparent that they followed the procedures they have trained for in simulators, which ensured the best possible outcome for the aircraft, the passengers and crew.

"The investigation will need time to review and analyse the evidence collected to date and to plan and undertake further evidence gathering and analysis. It is difficult to say how long an investigation such as this will take.

"However, a preliminary factual report will be released by the ATSB within about 30 days and, should the need for urgent safety action by any agency be identified, the ATSB will immediately notify the relevant agencies who are best placed to address the issue," the ATSB added.