For Relax Max: "Explosive" Decompression

>> Sunday, August 22, 2010

Relax Max asked: I'm researching an article on high altitude airplane collisions in which the integrity of the pressurized passenger cabin is suddenly compromised. I would like a more detailed explanation on the term "explosive decompression" than I am finding on wikipedia. I'm guessing it means the person can't expel the air in his lungs fast enough and so his lungs explode. Probably preferable than living two minutes before he is killed by impact with the earth. Is that what it means?

No. I've talked before about decompression on Rocket Scientist. In general, with regards to high altitudes, the effects of high altitude alone is insufficient to cause any part of a person to "explode." When movies show eyes popping out of the head or body's spontaneously exploding in vacuum, well, it's just not real. Explosion, per se, is not going to happen (unless someone insists on holding their breath). A single atmosphere is just not enough differential pressure to tear a body apart. Usually, when experts think of explosive decompression, it's a deep sea situation instead, where differential pressure can be many atmospheres instead of one and such horrible events as the Byford Dolphin calamity are possible. (Read with caution; it's gruesome)

When people describe "explosive decompression" with regards to high altitude, they are talking about the plane, not people, experiencing a pressure differential it is not designed to withstand. Not to say low pressure doesn't have effects on people. Anoxia is quite debilitating and harder vacuums can swell body parts as they did for Kittinger when during one of his high altitude balloon jumps. High enough, and the saliva can boil in one's mouth. One can be subject to the bends as a result of sudden high altitude and some forms of lung trauma or altitude sickness.

When I was a test subject, we actually trained through a simulated instance of rapid decompression, complete with fog and tests to see our reaction (and how long it took symptoms to show up) for anoxia. Great fun, that.

Decompression, however, is not a guarantee of a crash, depending, of course, one what caused the decompression. There's a nice little table in Wikipedia that lists notable aircraft incidents involving decompression. Though many ended badly, a surprisingly high number involved few if any casualties. Often the most dangerous results were debris affecting engines or stress destroying control or hydraulic lines. The pressure differential can be acerbated by the high speed of a plane so that, even after the plane's pressure has equalized (which rarely takes long), there is still a suction issue because high speed air (like that flowing outside a rapidly flying plane) is at a lower pressure than still air (like that inside the plane). It is, in fact, this differential pressure between fast and still air that allows a plane to fly by providing lift.

Additionally, anything that might extend outside the plan, like, say, a limb, will produce drag in that swiftly moving air and tend to yank the object out for aerodynamic reasons. And that ties nicely into your next question.


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