After listening to the recently released audio tapes from 9/11, I was struck how the very simple maneuver of switching off the planes’ transponders prevented a rapid and coordinated response. But what is/was the rationale of having transponders that can even be turned off? What situation might arise where the pilot would actually want them disabled?
ETA: I would bet (hope) that they cannot now be switched off.
While delivering a plane this spring, I was informed by ATC that our transponder had essentially, gone nuts. It was dropping out, and randomly reporting altitudes all over the place. We were asked to change from Mode C to Mode A, and then to simply turn it off. Additionally, on rare occasions electrical equipment in the plane will overheat, shortout, trip breakers, or otherwise misbehave. It’s imperative in either of these cases that pilots have control over the power supply to any equipment in the aircraft.
Doubtless there are other examples I haven’t encountered.
Oh, I see. Thanks.
Then let me restate my addendum from the OP: I would hope they can now be turned back on remotely (i.e. by some airport/airline authority). Technologically, it must be easy (if my car maker can unlock my car remotely, I’m confident Boeing or AA can turn on/off transponders remotely).
Transponders can still be turned off and they cannot be turned on remotely.
The pilots of the plane have a sizable bank of circuit breakers which they can use to selectively switch parts of the plane’s electrical systems on and off. This is important. It prevents the plane from catching on fire in case of an electrical short. Having the pilots able to access these circuit breakers, see when they have tripped and manually turn them on or off is a vital part of troubleshooting when something is wrong with the plane.
You do not want to give some external signal the ability to turn circuit breakers on and off, potentially overriding the pilots. That would be horribly unsafe and might give someone able to hack the signal or access the control codes the ability to crash a plane without being on it.
Thanks for that thoughtful reply.
Nothing is ever simple, is it?
A pilot will also normally switch the transponder to Standby or Off, so that they don’t respond to queries, while on the ground. That “declutters” the screens at ATC, and on aircraft with TCAS (which allows aircraft to query each others’ transponders), so they can focus on aircraft that are actually airborne.
I’ve also been *told *( 
) that, if you want to bust minimum altitudes for some hypothetical reason, you turn/leave Mode C off, so that ATC has to depend on much-less-accurate skin-paint radar returns to determine your altitude - and if you’re outside radar coverage anyway, they have no other means to nail you. No, really, I’ve never done it or seen it done; only heard about it.
It also clutters the screen for large numbers of small planes such as fly-ins.
In the case of 9/11 it was an easy way to tell who was NOT interacting with ATC. It doesn’t really affect the military’s ability to hunt down a wayward plane.
But - but that would be wrong! :eek: