Difference between
- Mode A only transponder
and
- Mode A and C transponder selected to Mode A only.
No difference... IF thats really whats going on.
But it almost certainly isn't whats going on.
More likely, you have a mode A and C transponder with 'altitude reporting' turned off.
Note that in this case the transponder still replies to mode C interrogations (Tcas does interrogate Mode C all call, but not Mode A all call becasue it doesn't give a stuff what 4096 code you are wearing). However it sends an empty pulse frame, (no altitude date).
But, since it is still replying, TCAS can calculate time delay versus speed of light and calculate the range, and hence range rate, and issue a TA.
pb