[miratrix]

Transponders have to be turned off while you're at the airport and while the plane's being serviced on ground. I think 777 has 2 transponders, so you also need the ability switch from one to the other. Lastly, I believe all electronics on the plane must be on a breaker circuit in case of electrical shorts so that pilots can isolate and turn off faulty circuits.

But you're right, ACARS (the system through which the engine information was routed - http://en.wikipedia.org/wiki/Aircraft_Communications_Address...) probably should be updated to add more information...

[TomGullen]

I get that, but what about just simple rules?

if(Plane.Altitude > 0 && Plane.FlightTime > 10 Minutes){ TransponderOn }

[kurige]

The altitude of the "ground" varies by thousands of meters:

https://www.google.com/search?q=highest+airport+altitude

Although, I guess you could just check if the landing gear is up. I'm sure there are corner cases I can't think of, but it seems like that would pretty much cover it.

[mburst]

While this makes sense it may have to do what the poster above you said

Lastly, I believe all electronics on the plane must be on a breaker circuit in case of electrical shorts so that pilots :can isolate and turn off faulty circuits.

[verroq]

What if the altitude meter breaks? Does the transponder go off?