ITCZ,
> Can someone perhaps resolve this as 'so many seconds to
> closest point of approach' for RA? I cannot remember off the
> top of my head what CPA triggers an RA. Maybe our answer
> lies there.
depends on sensitivy level e.g. altitude. tau_CPA is 25 seconds for this altitude range
All the values are in that booklet on TCAS:
http://www.smartcockpit.com/operations/TCAS.pdf. Great reading...
7300'-5700' at 2 x 3000 fpm would give you a tau of 16s, e.g. trigger the red lights on the VSI.
Btw, much more details on the TCAS inner workings right down to engineers and software level are in here
http://sunnyday.mit.edu/papers/intent.pdf. Beware - 840 p. of technical stuff.
E_o_D