Hello Capt Lim,
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.
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.