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Friday 25 July 2008 (Updated at 5pm AEST)

RAPID ROUNDUP: Qantas plane depressurisation – Experts respond

Australian experts comment on media reports that a Qantas 747 has made an emergency landing in the Philippines capital Manila because of a large rupture in the plane's undercarriage.

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Dr David Newman is Managing Director of Flight Medicine Systems and Head of the Aviation Medicine Unit at Monash University

“When there’s a decompression of the airplane, what you’ve lost is the internal cabin pressure which is usually pressurised to a much lower altitude than outside the aircraft, usually 5-8000 feet is the internal cabin while you might have 35-40,000 feet outside the aircraft.  Obviously, when you’ve lost pressure, all that high pressure air in the cabin leaves the airplane and from a medical point of view you’ve got the risk of lack of oxygen which is compensated for by the oxygen masks that drop out of the overhead lockers and of course the aircraft then goes into an emergency descent to a much lower altitude.

It’s cold up there, at cruising altitude it’s about 56 degrees below zero, so obviously that may expose people to cold, but the descent is designed to limit how much time you spend up there and of course one of the other issues is that the mass flow of air leaving the airplane – if it’s an explosive decompression – will take a lot of loose objects and articles around the cabin and basically try and leave the aircraft via the hole.  So there’s a lot of noise, it’s cold, there’s an air blast effect, things go flying around the cabin, and in worst case situations, if there’s a major breach of the cabin, and the hole is sufficiently large, people can be carried outside with the high pressure air, basically as they say, get sucked out of the airplane.  Now in general terms, that’s the most extreme case, most of the decompression events that tend to happen are not as violent or as extreme as that.  The aircraft is usually structurally intact, the aircraft crew are able to initiate an emergency descent to a lower altitude and carry out an emergency landing.  And all it effectively means is that the aircraft has gone from being pressurised to being unpressurised, like a small, light aircraft which is usually unpressurised because they fly at lower attitudes.  So providing the aircraft is still intact, which is usually the case, then once the immediate effects have gone and people are on oxygen and the aircraft’s at a lower altitude, then there’s generally no cause for concern.” 

Dr David Wilkinson is Director of Hyperbaric Medicine at the Royal Adelaide Hospital.

“The most important aspect is the altitude of the plane when this incident happened. If it was at a cruising altitude you are talking about a pressure outside the plane that you get at around 35,000 feet. The interior hull of the plane is not at sea level pressure but is partly depressurised, the equivalent of a plane at 8000 feet. If you get a hole in the plane you will get a sudden movement of air from the higher pressure to the lower pressure - inside the plane to outside the plane. An analogy would be like shaking up a can of coke and opening it – the sudden depressurisation makes everything froth and bubble. When you make a hole in an airplane at an altitude like that you are going to get this torrent of air rushing around inside the plane and rushing out the whole. So anything inside the airplane that is not tied down or buckled down is going to be flung in the air like there has been a hurricane blow through the plane.  Big holes in airplanes have been known to actually suck people out through the hole which does further reinforce some of the safety messages they give you on the plane  - that when you are seated in your chair to always keep your seat belt done up. Partly the reason why they always suggest this is for sudden unexpected occasions like this. What would have happened is the hole would have then been followed by a rapid (over a couple of seconds) flurry of activity. Any loose bits of paper and cups would have been flung in the air and hurled around so there would have been quite a traumatic event happening around you.

The loss of air would have created a loss of oxygen and in all likelihood would have lead to all the oxygen masks dropping from the canopy above. At altitude, the lack of oxygen could potentially lead to loss of consciousness in a relatively short period of time. At 35,000 feet the lack of oxygen can cause you to lose consciousness in 30 seconds to a minute so you have that period of time for the pilots to lower their altitude to reduce the loss of pressure or for them to react and supply oxygen to the people in the plane.”