Originally Posted by markjoy
. If the TCAS produces an RA, either someone is not where they are supposed to be, the air traffic services have failed, or it is a false alarm.
Not necessarily true : the 3 cases you mention are of course correct, but for instance, many RAs currently are caused by pilots keeping a very high vertical rate in the last 1000 ft of their clearance. ( ATC provides separation, but TCAS asumes that a/c will not stop where it is cleared to and issue an RA )
Another problem for instance, is that at high altitude CPAs are such that RAs will start with distances over 6NM , affecting among other perfectly separated aircrfat with 5NM.
In BRNAV and RVSM airspace those 2 cases for instance are causing may RAs that are not falling into your 3 categories. These are not false alarms .
And this is part of the problem surrounding TCAS : not every RA is a " real " one and some people, both ATCOS and Pilots, are tempted to select which one they like.
Back to the topic : with " 40 seconds to disaster" ( that should be 35 seconds to be correct ) with 90 degree angle, those 2 a/c could have been 5 NM appart, so no big deal. What would be interesing to know is what would have been the separation if both a/c had followed the ATC clearances they received . Then you can debate.