PCAS on Saturday
I have a Zaon PCAS unit which I find works very well . It is the more expensive one which gives approximate heading of the other aircraft, relative altitude and altitude trend, and audible warning in the headset and display flash when there is a 'threat'.
Clearly it only picks up transponder equipped aircraft but does mode C, S and A (no height on A of course). As there is good SSR coverage across the UK I have not found any obvious areas where it does not give a return for transponding aircraft.
Saturday near Norwich the viz was very poor and cloud base lunchtime just below 2,000ft. When about 10 miles from the airfield the PCAS went off. It first indicated an aircraft at 0.5nm distance from me then 0.3nm, 300ft below and climbing, to my rear left quadrant. As I was IFR climbing through 2,500ft and in IMC I could not see the other aircraft. I turned directly away and levelled for a speed increase: the PCAS soon indicated an increasing distance. Shortly afterwards I heard the other pilot give a position report, so I know it was not a spurious alert.
PCAS isn't perfect, and it is unlikely that we would have hit each other, but I never saw him and I doubt that he ever saw me, but having the PCAS allowed me to take easy action to avoid getting closer than I would have liked.
I have had a fair number of other like examples (perhaps a 1/2 dozen) in the past 15 months flying with the unit; so from my experience I am in favour of 'the more transponders the better' and of using PCAS.
Yes, it won't do everything and won't give 100% protection (but nothing would) but it is a useful aid.