Originally Posted by
BlueMoon
Just as a correction above...mode S transponders aren't required...TCAS works with Mode-C as well. The TCAS system figures out collision hazards and not the transponders. If one aircraft has TCAS and the other doesn't the aircraft with TCAS will still get an RA.
this is correct. the only time tcas 2 doesn't work is if the other guy either has no transponder, or he doesn't have an altitude encoding transponder. you will still get a TA if the other guy has a transponder but no altitude encoding. you will not get an RA in this situation. all the system will show you is a yellow circle indicating the possibility for collision exists, but the TCAS system doesn't know the altitude of the intruding aircraft,, they could be several thousand feet below you.
all mode s transponders talk to each other, and send each other airplane data, such as altitude, so the system can analyze the threat level.
here is another example of what can happen when you dont follow an ra
http://www.faa.gov/about/office_org/...Y_BULLETIN.pdf