Karrank,
Our manual is unambiguous - don't set standby when airborne unless specifically instructed by ATC.
If this causes untenable problems for ATC, it needs addressing.
The concern for us is the integrity of TCAS. If an intruder has transponder at standby, then our TCAS is blind to him. Even worse, if we are at standby, our TCAS does not function at all (i.e. can't track any intruders), and has to go back to square 1 tracking all the intruders when the transponder is back on.
Correct me if I'm wrong on this next bit - you clearly know more about looking at radar screens then I do. From an SSR point of view, if I'm wearing the wrong code (temporally) at least you can still see a blip, so you know there is an aircraft where I am. You can still see my altitude. Depending on the radar type you can still see the speed value / vector, and presumably your STCA still works. In otherwords, my incorrect code is inconvenient, but not a total show stopper. So we could say perhaps ATC is 'X'% effective instead of 100%. But our TCAS is still working (100%).
If I set standby though, surely you can't see me at all? (accepted - unless you have raw data coverage). I.E. your effectiveness is degraded. Maybe its not as bad as the wrong code, but your effectiveness has got to be less than 100%. Shall we say 'Y'%
As a controller, you are better placed to put a value on the relative hassle of (no data) versus (wrong code but other data present).
Onviously it all boils down to the relative degradation in performance.
Normal operation (i.e. no code change req'd):
ATC efficiency = 100 %
TCAS efficiency = 100%
New code, we don't set standby:
ATC efficiency = X% (<100%)
TCAS efficiency = 100%
New code, we do set standby:
ATC efficiency = Y% (<100%)
TCAS efficiency = 0%
I guess what we really need is transponders to have a mode which inhibits them from replying to Mode A but still allows replying to Mode C. We could set that when changing code. That would solve everything:
- No code confusion.
- You still get a blip + altitude and speed tag whilst I change codes.
- TCAS doesn't even interrogate Mode A so will work quite happily.
Unfortunately that isn't going to happen. In the mean time, remember that TCAS is designed to be an independant safety net. It won't work if we turn it off!
On the possibility of accidentally setting 77,76,7500. It does not take a great deal of effort to avoid turning though these settings.
The bottom line with this is that we operate the equipment the way the manufacturers tell us too. If it looks like that method is degrading the ATC system, then the solution is to get the regulators to tell the manufacturers to modify the kit or the operating techniques.
CPB