[bobby.223]

Question pfflier about the transponders. They can be turned off in the cockpit by the pilot, (or someone else), is what I do not understand. Why? Why have this transponder be even capable of being turned off? Why not have it located deep in the plane’s innards somewhere where it would continue it’s ‘work’ and not be available to in flight shenanigans ? Thanks in advance.

[pfflier]

ATC Transponders are assigned a specific code number by air traffic control (ATC). The flight crew must set the code into the transponder on the airplane when ATC tells them the code as part of routine departure information. That then becomes the planes unique ID to radar. (that explanation is a little simplified)

Turning off the transponder in the cockpit (yes it can be done)is a moot point. If an improper code is set or the transponder is turned off, the effect is the same. The airplane is not as visible to ATC radar. The plane can be seen but is more difficult to see.

The second type of transponder in question is the Emergency Location Transponder (ELT) sometimes called an ELT beacon. It is powered by it's internal battery and must be disabled by manual disconnection. No cockpit switch for this one. Only someone who knows the airplanes unique systems and configuration can do this. This transponder is typically not easily accessible in flight. In most plane it is located in the tail section because it is most survivable structurally in a crash.

[CFIIIMEIATP737]

Bobby, the pilot needs to control the transponder at all times. The transponder has four digits. Those digits run from zero to seven. Air traffic control has a finite number of codes that can be assigned. It is a little unusual to change codes during a flight, but not too unusual. I have had as many as seven code changes during one flight. ATC’s computer gets ‘full’ and as I transition from one airspace to another, that local controller asks me to change codes.

Transponders have a four position switch: Off, Standby, On and Altitude (for reporting your altitude to ATC). The pilot needs control because the altitude encoding function might have a problem. He still wants to be identified by ATC, but he has to sacrifice the altitude encoding capability.

And finally, you need to control the power to ANY piece of electronics from the cockpit, either with a switch or a circuit breaker, in case of a malfunction leads to arcing, or overheating and thus an inflight fire.