Contrary to the "Cowboy Pilot" fantasy that some opponents have put forth (neglecting to notice that a high percentage of airline pilots were trained in the military, where discipline and a familiarity with firearms go hand in hand), the cockpit crew is no longer going to leave their stations in flight other than to go to the head or to check out a mechanical problem. After 9-11, the cabin crew knows that they are on their own should someone attempt to take over the plane.

To prevent damage to windows and other aircraft systems, the firearms use frangible ammo like that carried by the Air Marshalls. This ammo has been on the market for years.

Next time you board an airliner, take a look inside the cockpit if the door is open. Even on very large planes, the cockpit is pretty damned small. That doesn't leave the crew a lot of room to move around or to swing the crash ax. Unlike home defense, a handgun with frangible ammo is the best choice for use in the cockpit. (For home use, a short shotgun with bird shot is a better choice than a handgun, IMO).

Like I said, the cockpit is the last line of defense in the plane. Aircraft defense begins long before the plane, such as airport screening and ensuring the ground crew is reliable. Once everyone is on the plane, the cabin crew and the other passengers are another line of defense. There simply aren't enough Air Marshalls to adequately cover the thousands of flights per day in the US. After that, reinforced cockpit doors provide the final barrier. On some planes, I've watched the cabin crew place a heavy piece of equipment like the beverage card in front of the cockpit door after takeoff. However, if all else fails and an intruder makes it into the cockpit, then the crew must be prepared to kill him if necessary to protect the lives of the other passengers and innocent people on the ground.

Lest you think cockpit intrusions are unheard of, here are a few incidents other than the 9-11 hijackings.

Intruder incident in England

Intruder on a flight near Chicago

Co-pilot's ax foils cockpit intruder

Man stabs two in Australian jet hijack attempt (check the date on this one)

If that isn't enough, this article lists 16 separate attempted (and sometimes successful) cockpit intrusion incidents from 1997 through 2000, including that British article I linked above.

There was an earlier incident in 1994 where a disgruntled Fed Ex employee broke into the cockpit of a DC-10, attacked the crew, and attempted to crash the plane into FedEx headquarters. These incidents don't even count the number of hijackings over the years, like this one.

On a percentage basis, the threat of cockpit intrusions is pretty small. Every day, there are somewhere thousands of scheduled airline flights in the US alone. Still, we saw on 9-11 what damage an airliner can do.