Non PC-Plod,
TCAS, TAS, FLARM, ADS-B et al are all valuable SA tools. None, however, can replace a good, thorough look-out scan. In fact, they can degrade lookout by giving the operator a false sense of security or causing distraction (eg, looking at a contact that has already declared and therefore not scanning the arc where the danger is coming from). None of these systems are 100% effective - far from it in Open FIR where the vast majority of non-electronically co-operative traffic is (ultra/microlights, GA, paragliders etc). Crews can also forget to make the appropriate settings (especially on Xpdrs) or be unaware that the kit isn't working as advertised - a C130J and C27J collided at night in the US thinking they were both protected by TCAS (one of the systems was faulty). GAPAN (as was) commissioned a study to identify the MAC risk, and therefore S2A requirements, for RPAS operating in UK airspace, but it has relevance for manned operations as this was the measure. One comment is revealing - "Flying in VMC around aerodromes and glider sites in Class G airspace below 3000ft was much more risky than flying at night, in IMC and in controlled airspace. It was not difficult to see why. The sky at night was much less crowded than it was by day; and GA aircraft, gliders and micro-lights were rarely flown then. Even in the military, those pilots flying at night did so more procedurally, more sedately, using a visual-instrument flying mix even when flying under VFR. Through both regulation and sensible practice, in general, pilots flying at night, and in controlled airspace, and in IMC, tended to be more experienced, and tended to fly more extensively equipped aircraft, more predictably and often more procedurally, to tighter tolerances, under higher levels of radar service, with better self- illumination (both by lights and by electronic means) than pilots operating under VFR by day." The full study is here - it contains some interesting facts and statistics.
https://www.airpilots.org/file/737/s...d-aircraft.pdf