A midair explosion of an oxygen cylinder that blew the side out of a Qantas jumbo jet also fractured wiring and control cables to the emergency oxygen system, some flight control systems and shrapnel hit an engine, air safety investigators have found.
But after 2½ years of investigations, investigators could not identify why the fourth oxygen tank, out of a bank of seven, ruptured and rocketed upwards, through the cabin floor, striking the cabin door handle and smashing overhead panels, wiring and cabling.
Miraculously, no one on board Qantas flight QF30 was injured. The plane was carrying 350 passengers, 16 crew and three pilots.
The incident occurred 55 minutes after the Boeing 747-400 had taken off from Hong Kong, bound for Melbourne on July 25, 2008.
The plane had levelled off at 29,000 feet when, without warning, the bottle exploded, blowing a hole measuring two metres long by 1.5 metres high, causing the cabin to depressurise rapidly.
The pilots reported they heard a "loud bang or cracking sound", warnings went off, they donned oxygen masks and made an emergency decent to 10,000 feet, issued a mayday distress call and diverted to Manila, touching down 55 minutes later.
But passengers variously reported ear pain, popping, temporary hearing loss (typical of a rapid depressurisation), anxiety, racing pulse, faintness, light-headedness and tremors, though investigators could not determine if these symptoms were due to oxygen deprivation or the general anxiety brought on by the situation.
The oxygen masks dropped from the overhead compartments but some passengers had to be instructed how to use them.
Cabin crew, too, were affected. "Several crew members had become very distressed during the depressurisation and were initially unable to carry out emergency tasks," investigators reported.
The gas bottle was never found - investigators presume it was lost through the two-metre hole it punctured in the fuselage, falling 29,000 feet to the South China Sea below.
The Australian Transport Safety Bureau today released its final report into the blast, concluding it was a rare and unique event.
"Given the widespread and long-term use of this type of cylinder, it was clear that this occurrence was a unique event," bureau chief Martin Dolan said.
Investigators examined other oxygen tanks from the plane and others from the same manufacturing batch as the one that exploded, but were unable to find any manufacturing or metallurgical defects in a representative sample.
The tank that exploded had been inspected just eight weeks earlier.
"The investigation found that the manner of cylinder failure was unusual and implicated the presence of a defect, or action of a mechanism that directly led to the rupture event. However, despite the extensive exploration of the available evidence and the study of multiple hypothetical scenarios, the investigation was unable to identify any particular factor or factors that could, with any degree of probability, be associated with the cylinder failure event," investigators concluded.
"In light of the investigation's findings, it is our view that the risk of a similar rupture and consequent aircraft damage remains extremely remote," Mr Dolan said.
An examination of the number three engine found it had been struck by structural honeycomb from a wing panel from the blast, though the engine had not ingested metal shards. But a boroscopic examination of the engine's internals found damage to the turbine area, "though the nature of the damage suggested that it was unrelated to the depressurisation event".
- with Dan Harrison
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