How Dangerous Is It To Fire A Gun On A Plane, And What Would Happen?

This is a topic which has been oft debated – and even become a subject of an opinion piece published earlier this week.  As a recent study has concluded, Federal Air Marshals are extremely sleep deprived and over-worked.  What would actually happen if a gunman were able to gain access to the passenger cabin?  More importantly, what sort of effects can we see from the impact of bullets inside the passenger cabin?

A big, direct threat in an onboard incident of gunfire is the rapid depressurization of the cabin due to multiple perforations.  That’s not one bullet or two or even three.  While any perforations between the fuselage and the passenger cabin is bad – it’s not nearly as dangerous as shooting out an aircraft’s window at 25,000 feet.

But we’re going to limit this article to looking at the possible effects of just one bullet.

The Effect Of One Tiny Bullet… What’s The Deal With Cabin Pressurization?

When traveling at altitude, the inside pressure of an airplane is kept artificially higher than the outside pressure.  Why is that?  Well, at cruising altitude for many passenger jets, there simply isn’t enough air pressure outside to keep a person conscious.  As one travels further above the earth, air pressure precipitously drops.

The fuselage of most passenger jets (Boeing 747 for example) is lined with aircraft-grade aluminum.  This is because aluminum is an extremely durable and light metal – thus decreasing the jet’s base weight but still keeping all the contents safely inside.

The pressure is kept at no more than 0.75 atm (atmospheres) – 3/4ths the air pressure at an elevation of 0 feet.

When a bullet is able to penetrate the passenger cabin and push out past the fuselage, there is now a hole of the rough dimensions of that bullet.

Taking the total volume of a passenger cabin and then the pressure differential caused by the hole into consideration, the proportion of that hole to the volume is not likely great enough to cause any major disturbances immediately.  The effects will be instantly noticeable, though.

In thermodynamics, two vessels containing different air pressure (P1 ≠ P2) will attempt to reach equilibrium (P1 = P2).  Thus, if the pressure in the cabin is greater than the external pressure outside the cabin, the cabin will leak air until the air pressure inside the cabin equals the outside.  Based upon the second law of thermodynamics, this means that as the cabin pressure (at P – pressure, T – temperature, and V – volume) decreases, the temperature inside the cabin will also decrease.

At 25,000 feet in altitude, the ambient temperature is approximately -30.2°F.  The air pressure is also 376 hPa — 0.37 atmospheres.  So, the outside pressure of air is roughly equivalent to almost a third of what you would normally experience on the ground.

This means that this tiny little puncture hole is passing gas as fast as it possibly can until the inside of the cabin gets close to the outside air pressure.  But, at the end of the day, it’s just one tiny hole.

Outside of that bullet striking any critical electrical systems, damaging an aircraft window, hitting a fuel line or anything else of that nature (all extremely unlikely – but then again, so isn’t a gunfight in an airplane) – one bullet hole won’t destroy the aircraft.

In most cases where a depressurization of the cabin is taking place, the pilot will attempt to decrease altitude to something with a far less difference in pressure.  Just dropping down from 25,000 feet to 10,000 doubles the outside air pressure and increases the temperature to approximately 23°F.  That’s a lot more manageable than -30.2°F.

If the pilot descends to 6,000 feet, ambient outside temperature is usually above freezing.

This action slows the rate of air being sucked out of the passenger cabin.  Because the cabin is constantly circulating air inside the cabin, it may be possible that the cabin is able to significantly reduce its loss in cabin pressure.

What a bullet does effect is efficiency of the aircraft.  The second the fuselage is punctured, there is an increase in drag which reduces the optimal output of the aircraft for the fuel it has.

If the aircraft has to descend to a lower altitude to maintain cabin pressure, it will also result in a decrease in fuel efficiency.

Fuel is something that the airline and the pilot calculate for a somewhat optimal trip.  While there are reserves, for especially long flights (like San Francisco to Tokyo) – a bullet puncturing the fuselage would definitely impact the entire flight.  But then again, an exchange of bullets of any kind on a flight would certainly do that as well.

Now if someone were to shoot out, say,  one of the passenger windows… that’s a whole other story.