Fuji.... Some thoughts / replies...
The operating manual warns that the sytem should be used to visually acquire the traffic and then adjust track and height to ensure seperation.
Good
In the event it has not proved possible to acquire the traffic what should the pilot do?
The $64M question
To begin to answer it, we could look at TCAS. Range, or rather Range + Rate of change in Range + Height Difference => Collision possibility assessment. As IO540- says above, a miss is OK - you do not need a large safety margin. I have had an RA, with 200+ people down the back, and the system demanded we "maintain V/S" (which was zero). We had the traffic visually (Biz Jet), and went very closely laterally to it, and 500' above it. It's not a lot, and a pilot might visually try and get more... but it was safe and all TCAS wanted
TCAS assesses range VERY accurately (since it can actively demand and time returns), and Height quite accurately (to whatever the 2 x Mode C device work to - in some GA applications, or even commcercial non-RVSM, I would not want to trust it within a few hundred feet?) Other systems assess range on signal strength (?) - fairly dubious to assess range accurately, and very for rate of closure (this is TCAS' strong point). IMHO TCAS and other ACAS systems are nowhere near up to assessing bearing accurately enough for either automatic, or visually judged vis screen, lateral avoidance - as the article above stated, it is too easy too reduce separation...
The other factor on "what you do" depends on judging the "target" is/might be doing. He might actually have seen you, and judged (visually) he is going to miss you, when you take some inappropriate avoiding action and reduce or even eliminate the separation. Again, a TCAS strong point is that where possible, avoiding action is co-ordinated - hence the compulsory need to follow it. Ditto - it might be taking avoiding action based on an ATCO, who has a more accurate picure...
So given the limitations of the Skywatch system a fast contact is approaching you from directly behind. (you might just as well be descending with traffic below as in the example above). The only way you are going to visually acquire the traffic is to turn away from the traffic. What action would you suggest?
You could run the argument that if you are concerned about your 6 o'c, you should always be weaving and looking behind you! What you do is again a difficult call... the traffic behind might have seen you and gets somewhat upset when you turn into his path.
Please assure me in such circumstances you will do something to avoid the traffic before you visually acquire the traffic.
I would probably do "nothing", because my training is (military) to trust my life on my lookout, and accept that without the latter, I am in trouble, and in my later life that you do NOT avoid on a TA, and await an RA - even though that trained for environment is not applicable to GA. In practice it is academic, I do not have such a system... and whilst following this debate is interesting, will await buying anything until a lot of these questions are resolved.
What you do is up to you, and see best at the time. The "problem" we are discussing is what we as a community advise you do, both in actions, and in equipment terms... and that carries the liability of ensuring the advice is truly fact based and risk free. TCAS has had a tough learning curve, and nobody wants to repeat that in a far less trained for / regulatory environment.
NoD