Originally Posted by
jondc9
I have seen airplanes visually prior to getting any indication on TCAS. HOW/WHY?
A TCAS alert is, triggered by either a rate-of-closure limit (Tau) or a proximity limit (DMOD and ALIM). So you could have two aircraft maintain a reasonable separation within visual range, not come any closer (i.e., maintain a large time-to-convergenec) and not have a TCAS alert. However, the intruder should still appear on the display.
Between FL200 and FL420, the tau envelope will be around 48 seconds for a TA and 35 seconds for an RA. Also, a TA or RA will be issued if the aircraft are within ±850feet & 1.3nm, and ±600feet & 1.1nm, respectively. The relative altitude boundaries also apply to the Tau protection envelope (so if one aircraft was transmitting an altitude off by more than 850 feet, an alert might not happen when it would have otherwise).