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. :uhoh:

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