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Mode a is completely useless to TCAS it does not report altitude as I understand
Not so. TCAS will still plot the target in azimuth and issue a T/A. You certainly should not take avoiding action based soley on this, but it is a tool to try and aquire the aircraft visually.
Lots of Mode A only aircraft in the UK, and you quite often get T/As from aircraft that are seperated vertically from you.
TCAS does not interrogate mode A. It doesn't care what the intruders 4096 code is so interrogating it is a waste of bandwidth. It only sends Mode C interrogations (plus Mode S calls to the aircraft on its roll call).
What people call "Mode A" transponders are in fact mode A and C transponders, but without an altitude encoder (or if it's turned off). i.e.
they still reply to mode C all call, but they just send an empty pulse frame. But this allows TCAS to perform a time versus speed of light calculation for range.
So, if you have an A+C aircraft whose mode C has actually
failed, i.e. it is not responding to mode C all call, then TCAS will
not see it.
pb