You get a TA alert on a non altitude reporting target because the system ASSUMES the target is at your altitude and responds by warning you of its presence. The system doesn't KNOW what altitude the target is really at though so it can't give an RA.
If I got one I'd have a look around and put the lights on and hope I don't hit anything. A lot of the time it is caused by low level VFR traffic that are no real conflict and that you will never see.
yes thatīs right, it appears in yellow saying how many miles is from you "2.3 nm" but no altitude indication, so it should be normally below (in other airspace) but i was trying to find somewhere if i should deviate from the horizontal navigation.
My FCOM says quite clearly as a warning "DO NOT MANOEUVRE ON A TA ALONE!". If you visually acquire the traffic and it is obviously in conflict, then fine, get out of the way, but to manoeuvre horizontally around a target based on the TCAS display is fraught with danger because the TCAS display does not accurately present bearing information.