Timzsa :
quoting you :
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TCAS II is there for when ATC gets it wrong
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Not true , in fact most RAs I have seen are un-necessary or nuisance RAs cause by pilots elected to fly the last 1000 ft of their asigned altitude with a vertical rate exceeding 1000 ft/ min and causing RAs between otherwise perfectly separated aircraft.
Level busts are also prime candidates for TCAS RAs.
The point Stanley Eevil makes is a very valid one, especially in the lower altitudes. We must not forget than a VFR without a functioning mode C ( and there are quite a few of those below 10.000 ft ) will only trigger a TA, and indeed if you make visual contact and you judge it is at the same altitude, you should manoeuvre to avoid...