NigelOnDraft
Skywatch has some limitations.
It will issue a traffic alert but it will not provide an RA.
The operating manual warns that the sytem should be used to visually acquire the traffic and then adjust track and height to ensure seperation.
AOPA conclude that pilots are far more likely to be able to acquire traffic with TCAS, than without.
You raise an interesting question.
In the event it has not proved possible to acquire the traffic what should the pilot do? With a system that initiates an audible RA is it not the case the pilot is expected to recognise and enact the RA within 5 seconds whether or not he has acquired the traffic? Response to a further RA is expected within 2.5 seconds. Of course TCAS have been developed primarily around the needs of commercial operators in a CAS. Examples outside CAS therefore seem to be rare. I have read the report of the incident between a Boeing 737 and a Tobago outside CAS in 2003. In this case the Boeing had the advantage of dual crew. The co-pilot was the handling pilot. The commander was solely concerned with "see and avoid". TCAS indicated traffic below and the descent was arrested (although the traffic had not at that time been visually acquired). Subsequently TCAS issued an RA to climb. The commander took over and complied. At no time did either crew members visually acquire the traffic. The Tobago pilot saw the traffic which he said appeared to be crossing his track but he thought this was probably an illusion and did not take further action.
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? If you do nothing at best a few seconds before the collision you will get "traffic, traffic, traffic" - you will never get an RA. If you turn away from the traffic you acted before visual acquisition. If you turned away and climbed or descended you have also acted without visually acquiring the traffic as did the boeing crew.
We can debate the risks involved in operating light aircraft in IMC outside CAS. The fact of the matter is that pilots do, and the law permits such operations. Often no RIS is available. Once again the pilot is never going to visually acquire the traffic but Skywatch will for sure paint the traffic on the MFD. As the risk of a collision increases in the event no avoiding action is taken no RA will be issued other than "traffic, traffic traffic". Please assure me in such circumstances you will do something to avoid the traffic before you visually acquire the traffic.
Systems such as Skywatch are being used in a far less controlled enviroment than I suspect their manufacturers in the States may have envisaged. It maybe I need to give a deal of thought to how such sytems are best used. In these examples I find it difficult to rationalise the traditional wisdom that no action should be taken until the target is visually acquired or an RA is issued when the sytem is incapable of providing an RA.