Ugly Facts About Shooting a Gun on an Airplane
Steven Seagal didn’t quite manage to make it aboard Kurt Russell’s 747-400 in Executive Decision (1996). It was unfortunate – we know. However, one thing the movie managed to skim over quickly is what happens when bullets get exchanged aboard an aircraft. It’s not a new thing. Since aircraft have been in the air, they’ve been getting shot at and on. With the advent of the jet turbine, the game changed. Why?
In a Nut Shell: This really is a question of thermodynamics. If the pressure system outside an aircraft is different than inside the aircraft, air will violently move towards the low pressure system.
There was a previous How Stuff Works article that glossed over the four major things that occur after a weapon is discharged on an airplane. It was obviously aimed at trying to assuage anyone’s worry about what happens when bullets fly through a cabin – as if the concept of having bullets flying through the cabin of an airplane isn’t scary enough.
Air Pressure Inside DOES NOT Equal Air Pressure Outside
A single puncture hole in an airplane’s fuselage isn’t a big deal unto itself. However, that hole becomes a big issue when one considers the speed at which the plane is flying, the volume of pressurized air within it, and the altitude. All three of these things – alongside the diameter of the hole – actually play a very big role in deciding the level of emergency maneuvers that pilots will undertake. Confusing? Let’s use an example.
Case In Point: Boeing 747-400
A Boeing 747-400 is one of the most common commercial sized jets in the air. It can carry anywhere between 400 to 600 passengers depending upon the configuration. Cruising speed is somewhere around 500 mph and cruising altitude of around 36,000 ft.
At 36,000 ft, atmosphere pressure is roughly 25% (187 mmHg) that which is found on the surface (~760 mmHg). Altitudes above 12,500 ft require a pressurized cabin due to the availability of oxygen. While cabin pressure may vary, it stays roughly approximate to surface pressure.
- Single Bullet Scenario
If one .45 ACP bullet were to rip through the cabin and perforate the fuselage – the hole would not be big enough to dangerously depressurize the cabin. Violent depressurization is where the hole is big enough where the gas inside makes a mad dash for the lower pressure outside. In order for that to occur, an expanse of the cabin would have to be exposed – like if the emergency hatch is activated at 36,000 ft. That would certainly do it. So a bullet, in of itself? Not an immediate threat. Air will still be leaking and the pilots will likely need to divert and, at some point, drop down into the 12,500 ft altitude range.
- Multiple Bullet Scenario
If multiple bullets strike through the fuselage, then the cabin begins to quickly depressurize. Oxygen levels and temperatures plummet. Without emergency action, most of the crew will be at severe risk of exposure, hypothermia, and hypoxia. In commercial airliners, it’s not uncommon for the pilots to seal the cockpit to keep their end pressurized. They do, after all, have to try to land the bird. But the more holes in the fuselage equals the faster gases can escape. And because of thermodynamics, the faster the rate of escaping gases – the faster the temperature drops.
- Hardware Malfunction due to Ballistic Damage
Surprisingly, commercial airliners and most cargo jets are very robust in terms of wiring and electronic systems. The threat of a small arms round – such as a 9mm Parabellum or even a .357 JSP – penetrating towards a vital spot like a fuel line is next to nothing. The greater threat is striking electrical systems that will short circuit the cabin’s lighting and environmental controls. Anything that can jeopardize the pressure of the cabin and temperature is a serious threat.
- Cockpit Damage by Small Arms Fire
The last caveat that need be covered is the scant possibility that bullets find their way into the cockpit. This is the most sensitive area of the plane because it carries the pilots and their instrumentation. Small arms fire is extremely dangerous. Thankfully, the cockpit is a small target and most pilots keep the cockpit doors secure. The door sealing the pilots into the cockpit is, in most cases, very resistant against most forms of small arms fire.
