Replies

There's a gulf between the movies, "probably," and actual facts, my friend.

Boeing Country: Guns in jets: Don't believe movie image
Jetliner cabins are pressurized. The air inside is compressed to the same density as air at 8,000 feet of altitude. Airliner fuselages at 30,000 feet frequently are compared by the press to inflated balloons. The implication is if you puncture it, it will blow.
That's plain wrong. First, the difference in pressure between the inside and outside air at 40,000 feet is 8.6 pounds per square inch, and the cabin wall is tested to withstand 18.2 pounds, more than twice as much. Second, airliner fuselages are not airtight -- they leak air all the time.
Maintenance people told me that in the days when smoking was allowed, you could find leaks in the fuselage simply by noting the starting point of the brown streaks of cigarette tar.
One engineer told me that Boeing pressurizes each new plane to look for and seal leaks. If the plane passes this ``low blow'' test, it's subjected to a ``high blow'' test at even higher pressure to proof the fuselage for pressure changes.
During the tests, assembly workers outside feel for leaks. When they find one, workers inside apply a sealant glue that is sucked into the leak and seals it. But here's the key point, he said: They don't necessarily have to find all leaks; they just have to find enough.
The cabin atmosphere is continuously pressurized using air from the engines' compressor stages, cooled and mixed with cabin air. The pressure is controlled automatically. If it needs to be increased, more is drawn from the engine. If it needs to be decreased, an outlet valve opens to draw out cabin pressure.
And -- talk about holes in the fuselage -- a retired airline captain told me that the outlet pipe may be 15 inches in diameter, though it's rarely opened fully.
In a twin-engine plane, there are two sources for the compressed ``bleed air,'' and with that air at 40 pounds per square inch, the engines are capable of supplying far more air than they ordinarily have to.
So if a small hole -- say bullet-sized -- opened in the fuselage, the air conditioning packs would supply more bleed air, easily keeping up with the air loss. ``All you'd have is a whistling noise in the cabin,'' a structural repair specialist said It would be no worse than when you open the lavatory sink drain or flush the vacuum toilet.
The oxygen masks, which drop from the ceiling automatically when the cabin air pressure drops below about 12,000 feet, wouldn't even deploy.
But wouldn't the air rushing out open the bullet hole further?
Only in the movies, the structural guy indicated. All jetliners incorporate ``tear straps'' in the fuselage wall. If a hairline crack or bullet hole opened in the skin and started to grow, it couldn't progress far before it reached one of these reinforced points and turned, making what he called a ``controlled flap.''
OK. But what if the slipstream catches a projecting piece of the plane's skin torn out by a bullet? Wouldn't that pull more metal away?
Possibly, but again, not as it does in the movies. A Boeing engineer told me that at 400 knots, the air passing over the plane exerts only 4.11 pounds of pressure per square inch. At 450 knots, it's 5.33 pounds. So if a bullet opened a 4-square-inch projection outside, the slipstream would exert about 21 pounds of pull on it.
``I have no problem at all ... If you shot 50 holes in the fuselage, structurally it would be no problem at all,'' the structural expert said.