However I note that you have not responded to the specific example of two aircraft in open FIR in IMC. You will never acquire the traffic visually
Surely any aircraft in IMC is receiving a minimum of RAS? And so the question is not relevant - you have to rely on ATC for separation? Please don't tell me people fly in IMC without such a service
A RIS would for me, be risky, but anyway, the service provider should be giving the traffic information.
I do not tend to fly IMC outside CAS, and on the rare occasion I do, would always have a RAS - but that is my flying pattern. The thread to date I had treated as applying to VMC outside CAS..?
NoD