Originally Posted by
vegacat
Why is it possible on a civil aircraft to turn off the transponder whilst the plane is in the air?
Because it is a piece of electronic equipment.
Like most electronic equipment, it has on and off switches. One of the reasons to turn one off is for flight safety: if one of its wires shorts out and begets an electrical fire, killing the box can sometimes prevent a mess from turning into a disaster.
If you can't turn it off, you maybe can't isolate the cause of a fire, and fire in an airplane is a serious emergency that can get a whole lot of people killed real fast. (See recent UPS crash due to fire in the Middle East).
Another reason to turn it off is as simple as your PC. Sometimes, a piece of electronic gear needs to be turned off, and then back on, to get it to work right.
If it wasn't we would know where the plane had gone (and this thread) would be considerably shorter.
Only if the appropriate kind of radar interrogated it and received a reply.