So we were watching "Seconds From Disaster" on NGC last night, which
happened to be on AA 77. When they mentioned that seconds after gaining
control of the aircraft the hijackers turned off the transponder (along with
a brief explanation of what the transponder does), my wife simply says:

"That's stupid, why does something like that even have an on/off switch?"

I honestly never thought about it and couldn't answer her question. Does
anyone know?

Regards,

John K.