[TomGuy]

thinking about installing the equivalent of "911" phones on planes

Sadly, that is laughable. A phone isn't going to help stop a homicide bomber/hijacker.

It is just another dumb idea to create a false-sense-of-security without putting any defensive weaponry within reach of those who might try to use them to stop a homicide bomber/hijacker.

[daviddennis]

This sort of thing already exists, actually. You can switch your transponder to a certain combination of digits and it will notify ATC that something's up. (Transponders identify planes in the air with a four-digit code assigned by air traffic control (ATC).)

Unfortuantely, there's not a lot that can be done for an airliner in danger from an internal threat. The captain and crew are almost entirely on their own.

D