[kach22i]

Transponder's fate may prove key to solving Malaysia Airlines puzzle
By Tom Watkins and Steve Almasy, CNN
updated 1:49 PM EDT, Wed March 12, 2014
Transponder's fate may prove key to solving Malaysia Airlines puzzle - CNN.com

(CNN) -- As investigators search for clues to what happened to Malaysia Airlines Flight 370, the answer to one question may prove key: Why did the transponder in the Boeing 777-200ER stop transmitting information?

The fact that it happened at all is astonishing to John Nance, a broadcast aviation analyst and veteran pilot. "It is hard to conceive of a situation in which a triple seven would lose all ability to have its transponder on and the crew would not find some way to communicate," he told CNN.

A senior Malaysian air force official said Tuesday that the plane traveled hundreds of miles in the opposite direction from its original destination, and had stopped sending identifying transponder codes before it disappeared from radar screens.

http://www.cnn.com/2014/03/12/world/asia/malaysia-airlines-transponder/

Q. Why would you turn off a transponder during a normal flight?

There could be several reasons. One reason could be when airplanes get close to each other (perhaps they are approaching an airport). Air traffic controllers may then request pilots to turn the transponders off or to standby. Also, if the transponder is sending faulty information, the pilot might want to turn it off. Planes are still visible on primary radar until they get below the radar's coverage ability.