For generations who have been fed a steady diet of fantasy in movies and television, the thought of an armed Air Marshal or armed pilot firing a gun through aircraft skin may be horrifying. From Airport with Van Heflin exploding a briefcase bomb, to Auric Goldfinger being sucked out of a tiny window of his private jet, the uninitiated believe rapid decompression sucks everything out of an aircraft.
My first experience with rapid decompression came during the altitude chamber phase of physiological training as a crew member. The chamber, about 15 feet long and 10 feet in diameter, was brought to a low pressure rapidly to simulate decompression. The chamber clouded up as all the moisture inside condensed, and we donned oxygen masks as quickly as possible. That was it, except for changes in ear pressure for some.
The movies always show gale force winds lasting for many seconds as the higher pressure air escapes through whatever hole has been created. Never is the source of this hurricane explained. It’s as if there is a huge reservoir of air at the other end of the cabin which must somehow find its way out the hole.
As an engineer in the airplane business, I’ve had to do some calculations on precisely this contingency. Interior doors must be capable of opening to allow equalization of pressure between cabin areas (which is why airliners have curtains rather than doors) to preclude bulkhead failure. The volume of air is actually negligible, and it escapes at, roughly, the speed of sound. Which means it’s all over rather quickly. No more than one second to completely evacuate an entire airliner cabin.
Consider a football or basketball. The pressure in the ball is about 13 pounds per square inch. That is actually the gage pressure, or differential between internal pressure and atmospheric pressure. If you put an inflation needle into the ball, it returns to something close to atmospheric pressure pretty quickly.
Airliners fly at altitudes of about 35,000 feet as a general rule. Some can fly higher, but 40,000 feet is the limit established by the FAA unless specific conditions are met in construction and operation. Above 40,000, the air pressure is low enough that at human body temperature, water boils. This causes problems with the bloodstream!
At 35,000 feet, the atmospheric pressure is 3.5 psi. I don’t know what interior pressure airlines maintain in their cabins now, but they used to keep it close to, but below 10,000 feet. That is because cabin pressurization requires compressed air to be extracted from engines, which lowers the efficiency of the engine, which costs more to operate. Any altitude above 10,000 feet is bad physiologically because available oxygen isn’t adequate to support human respiration for more than about 30 minutes. At a cabin altitude of 9,000 feet, the air pressure is about 10.5 psi.
What this means is that the difference in air pressure from the cabin to the exterior while flying at 35,000 feet is about 7 psi, or about half the pressure in a basketball. Not enough of a difference to cause a huge rupture of the aircraft skin if pricked.
The real danger of a bullet penetrating the skin of an aircraft is crack propagation. Modern airliners are designed with Damage Tolerance features. Manufacturers spend a great deal of time and money doing structural analysis and testing, and FAA requirements are very stringent. For Damage Tolerance compliance, skin stresses and fatigue vulnerability are validated first by calculation, and then by testing, and must exceed the requirements by a factor of 1.5, which means at least 50% stronger than it needs to be. In addition, there are techniques for stopping cracks from reaching a critical length which are built into the airframe. Critical length for a crack is determined to still be flyable.
What this all comes down to is that the danger of a bullet penetrating the skin (pressure vessel) of an aircraft is minimal. There will be no gaping hole through which all unsecured passengers are sucked out, and no Force 10 gale blowing down the aisle. So if an Air Marshal or pilot carrying a gun worries you, you are worried for the wrong reason.
Now, a bomb is a different matter…
texas hold’em
One of the saddest lessons of history is this: If we’ve been bamboozled long enough, we tend to reject any evidence of the bamboozle. We’re no longer interested in finding out the truth. The bamboozle
Trackback by texas hold'em — February 23, 2005 @ 6:30 pm