It’s a difficult situation.
Another way of looking at the change altitude/turn decision is the possibility that the other aircraft has you in sight, and determined that you’re not a risk, so divert their attention elsewhere momentarilly. But then you start a quick turn/climb/descend and create a conflight that didn’t exist and they now miss.
I don’t think there is a correct solution for this.
It bothers me that TCAS still isn’t cheap and ubiquitus in GA, especially with the current advances in electronics. But I suppose RF is as hard as ever, and you still have certification.
I work in the marine technology and I’m similarly bothered with the lack of a marine equivalent to TCAS or transponders altogether. There is AIS, but I don’t think it would be safe to produce resolution advisories from that data, nor from ADS-B.
It is almost impossible to make safe horizontal conflict resolution solely with a simple TCAS / TAS system.
This is a video (unfortunately mostly in French, but I think you can understand just by looking at the movie) where a AAL crew does not comply with ATC instructions but rather tries to resolve the “conflict” by looking at their TCAS leading to a loss a speration.
You can see the difference between the AAL crew point of view and the ATC point of view on the video.
This happened when TCAS were introduced in commercial aviation some 20+ years ago.
Thanks for posting that video, Guillaume. Just watched it.
I see a lot of things wrong. Poor terminology on both sides. Why, when AAL62 asked where the other plane was going, ATC told him “Fuertaventura”? That’s like this 
For me, the lesson is that if you see a target which is converging (distance reducing) on a constant bearing, it is better to turn on his inside i.e. AAL62 should have turned left instead of right, because turning right prolonged the conflict. But, according to airline SOPs, they should have only changed altitude, not turned.
The problem, and I have watched a few ATC videos like this (shown with a “do not post on the internet” during ATC presentations) is that the pilot needs balls of steel to follow the SOP.
Did either of the two ever get an RA?
No RA in this case (video says so in french at the end).
