Re: the second plane ... can they turn off the plane's radar signal, so no one can track it?
You can turn off the transponder. You can't make that much aluminum invisble.
Yes. The radar and IFF could be disabled.
Yes. What the air traffic controllers see is actually a return signal from a box called a transponder...it receives the radar signal and sends back a coded burst of information such as altitude, airspeed, etc. The pilots set a four-digit code on the transponder as assigned by ATC.
If they turn off the transponder, then all that controllers will see is the "primary return" of the radar hitting the skin of the plane...just a blip without other information, and maybe not even that if the plane is far from a radar site.
The 9/11 hijackers turned the transponders off on all four aircraft after seizing control. It made it harder for ATC to track them.
}:-)4
They can turn off the transponder which is the primary tracking device for air traffic control. However they can't turn off the reflected radar signal from the skin of the aircraft. These are two separate signals, often on quite different frequencies. The transponder, aka a beacon, transmitts a signal when "interogated" with a coded signal transmitted from the "radar". The "skin return" is just bouncing a radar signal off of the skin of the aircraft and as I stated, cannot be turned off. Military radars tend to do better at tracking via skin return than civilians ones, but the civilians ATC radars use both methods as well.