TCAS indication for Mode A traffic
 

hi, would like to clear my 'simple & stupid' doubt .

What will the tcas II indicate if the traffic is on Mode A. When will it announce 'traffic'?

thanks for your inputs.

Hi, we had this happen today. The TCAS displays the appropriate symbol without any altitude information. It treats the intruder aircraft as being at all altitudes so it'll give a "TRAFFIC" warning when the intruder is calculated to be a threat regardless of the actual height of the intruder (obviously the TCAS unit doesn't know how high the Mode A aircraft is and so acts on the worst case scenario.)

We had a TRAFFIC alert today from a Mode A helicopter 4,500" below us.

Strictly speaking it won't, because inspite of what most books say, TCAS doesn't interrogate Mode A.

What you probably mean is a Mode A & C transponder with altitude reporting not fitted or deselected.

In this case TCAS will issue a TA if the flight past is assessed to be a threat horizontally. This assessment is based on looking at closing speed, and the rate it is changing, to estimate the miss distance. If the miss distance is small then the alerting condition is met. (The value for the miss distance depends on closing speed but also bnacked up by an absolute minimum value that is related to altitude / radio altitude)

When the horizontal condition is being met, a TA is generated 20-48 seconds prior to the predicted closest point of approach.

pb

Capt Pit Bull is spot on. One other point is that since TCAS determines the closest intercept by the extrapolating from current range and rate of closure, if you are reasonably high above the traffic that is not reporting altitude you will sometimes see it be alerted as "traffic" and then the alert clears before you actually catch up to the traffic, because as the slant range moves to more vertical than horizontal the rate of closure reduces towards zero.