[upchuck]

"We still have a fundamental problem about having ballistics in a pressurized cabin," McAuslan told British Broadcasting Corp. radio.

I'm neither a pilot nor air marshal. But I remember reading a thread, I think it was here, shortly after 9-11 where a pilot, who also carried, played down the danger of shooting a gun in a plane, possible puncturing the outer skin. Anybody remember this?

[night reader]

...I remember reading a thread, I think it was here, shortly after 9-11 where a pilot, who also carried, played down the danger of shooting a gun in a plane, possible puncturing the outer skin. Anybody remember this?

Yep! Like you, I believe it was here. If I recall correctly, it was asserted that in a large modern airliner, there is already the equivalent opening of over a square foot to the outside and the pressurization equipment was capable of compensating for this a the much smaller opening that a bullet might make. The writer also asserted that control lines, whether electrical or hydraulic, have multiple redundancies.

Again, if I recall correctly, that poster claimed credentials which would make such a claim credible. I, myself, make no such claim.

[x1stcav]

I don't remeber the specific reference you're referring to but found this of interest:

Home I Introduction I FAQs I B777 Images I Web Links I Aviation Links l Airlines


Will a gunshot depressurize the airplane cabin?


Question:

Could you please resolve an after dinner discussion on the effects of firearm discharge in a pressurized commercial aircraft - assuming that the hull is penetrated.

One point of view is that quite rapid depressurization could occur, depending on the size of the hole, requiring oxygen and descent to 10000ft. The James Bond viewers believe that the aircraft would disintegrate.

We would appreciate your advice.

Merry Christmas

David Purton


Hi David,
Thank you for the very interesting question. This controversy must probably have arose from the new anti-hijacking measures of having sky marshals on board commercial aircraft and the talk of equipping cockpit crew with guns. The effects of firearm discharge in a pressurized commercial aircraft is dependent on the size of hole caused by the bullet. If the hole is clean and of the size of your finger, it would not caused a rapid depressurization. Let me discuss this scenario in the context of a Boeing 777. (Other pressurized aircraft are almost similar). Inside this aircraft, there are already two existing 'holes' for regulating the cabin pressurization. They are called the outflow valves, one located in the front and the other is at the aft belly. Their function is to modulate and maintain a desired cabin pressure of around 8.5 psi (pounds per square inch), and it varies with the aircraft altitude. This operation is performed automatically. It is never possible to fully seal the aircraft doors and hence there are very minute spaces where some pressurized air may already be leaking out. They are hardly noticeable. At the same time, the interior of the airplane is always being pressurized and recharged by a constant flow of pneumatic or bleed air from the engines. If a gunshot creates a clean hole through the skin, it is not going to be disastrous because air will just whistle out of the hole. The outflow valves will automatically response to this sudden loss of air by closing the valves a little to compensate for the air leak. My estimate is that it would probably take quite sometime to fully depressurize the aircraft cabin. It only takes about less than 5 minutes to carry out an emergency descent from, say 35,000 to 10,000 feet assuming the aircraft is descending at about 5000 feet per minute. If the size of the hole of the firearm discharge is big then it may depressurize quite rapidly. Think of the aircraft cabin as if it is a balloon. The bigger the hole, the faster the air would leak out. The hole with a size of a finger will not have any major or significant effect on a big commercial aircraft. What is more worrying is that, a gun shot hitting the electrical cables, hydraulic lines or control cables may cause some headaches, but not altogether critical because the aircraft has many backup systems. Even that, it is quite remote because such vital cables or lines are generally well protected and are securely located away from possible damage. In real life, a Boeing 737 in Hawaii had the front roof section of the First Class cabin ripped off due to a depressurization caused by some undetected cracks in the airframe at 24,000 feet. There was only one fatality, apparently sucked out of the cabin due to the rapid depressurization but the aircraft landed safely. The aircraft would certainly not disintegrate unless there is a bomb on board. I hope I have been able to answer your question. Merry Christmas to you too. KH Lim

[snopercod]

Safe ... Or Free?. The article was written by John Deakin, the man with more hours in the left seat of a B-747 than any other living man. Also flew for "Air America" in SE Asia.

The real issue with the air marshalls as I see it is "who's in charge of the aircraft?". Historically, the pilot is, just like the captain of a ship. But these air marshalls answer to someone else, not the captain. I can see why pilots might be concerned with dilution of their authority.

[Criminal Number 18F]

Yeah. Nothing to the bullet-depressurising-the-plane theory. There have been planes that lost cargo doors and landed safely, and planes that had a bomb go off and blow a sizeable hole in the aircraft, that still landed safely (apart from those pax blown overboard by the bomb)

At other time both of those events, bombs and lost cargo doors, have lead to loss of the plane and all aboard, so there is no guarantee.

There have been many incidents in which a firearm was discharged on an aircraft. In the only ones I am familiar with where this led to a crash, one in Asia on CAT decades ago and one in the USA in 1987, the firearm was used to kill the pilots. The airplane was still pressurized when it hit the ground.

d.o.l.

Criminal Number 18F