- TCAS doesn't consider the 1000 ft as separation, even 500 ft can be considered as enough ... TA and subsquent RA will only be made if there is a collision risk.
Tcas works on two different logics, the first one being time with the concept of the CPA and the second one being a safety bubble around the aircraft when the closing speed is low. As far as the vertical separation is concerned in the 2nd case it depends of the altitude of the aircraft, but it cannot go (at least to my knowledge) below 600 ft. If an aircraft ( with C mode of course) is flying 500 ft below or above a Tcas equipped plane, it will trigger a RA.
To avoid this kind of situations there is a rule stating that aircraft shoul make no more than x ft/min when less than y ft to be leveled and reduce to x2 ft/min when less than y2 ft to be leveled. This is a recommendation and each company has its own values for x and y ...
Yes, but the trouble starts when both a/c are evolving vertically, ie one climbing and the other one descending. Indeed the two rates add up and may create a RA. Where I work, we have that type of configuration with outbound traffic climbing to FL XX and inbound descending to FL XX+10, and I can tell you that I've had some RAs more than once, even with a good traffic information and even traffic in sight...
I have been taught regarding warnings relevant to times to closest point of approach (CPA) the following:TA (traffic advisory) given when intruder is 35-45 seconds from CPA.
RA (resolution advisory) is given if the intruder is within 20-30 seconds of CPA.
Which means 20-30 seconds of a possible collision ! Not much time isn't it ?
The last point I'd like to add about the Tcas is that the information is renewed every second . Compare that to the speed of you radar and think of it !
If you want some more precise info :
http://minilien.com/?AFDkYWEA7k