You prove my point for me that some people are operating under a misconception. There are people out there who will use their ACAS system (which includes TCAS and everything else in the market) in the horizontal plane. The system is not designed for that and is not accurate enough in providing bearings. They could be up to 30 degrees out.
I think you're also operating under a misconception, PPrune Radar, about the alternative to TCAS and TCAD. You're thinking like a controller.
There's no doubt that ACAS systems are unsuitable for traffic
management. To have traffic jumping around the screen while trying to work a busy sector would be madness, and so the sorts of azimuthal accuracies that are obtainable with ACAS are orders of magntitude less than the requirements of radar.
But the pilot's situation is a little different. Imagine that, as usual, we're in a situation where, because of controller workload, traffic density, or both, a radar advisory service is unavailable or impractical. So some helpful controller is calling traffic under a RIS.
And he calls "G-CD, traffic 12 o'clock, 3 miles, opposite direction, height and type unknown". At a typical closing speed of 300 kt, we have a little over 30 seconds to the potential impact. So we look for a few seconds, in the 30 degree arc -- that's the resolution of the clock code too -- ahead of us. And we see nothing. "G-CD nothing seen, update please?" "G-CD say again please, I was on the landline..." "Update on the traffic you called for G-CD please?"
A number of possibilities now emerge:
"G-CD he's now in your 10 o'clock, half a mile, passing down your left hand side"
"G-CD he's now in your 2 o'clock, half a mile, passing down your right hand side"
In either of those, I hold course. I never saw the traffic and had no way to manoeuvre to increase separation. If I turn left in the first case, or right in the second case, I make matters worse.
"G-CD he's still in your 12 o'clock, half a mile, opposite direction"
Course of action now? Prayer, perhaps?
Now compare that to ACAS. I find an aircraft at 12 o'clock, 3 miles, no altitude indication. I look out, but see nothing. I look again at the display for a few seconds. The relative bearing is increasing, it's now at about 12.15, 2 miles.
What do I do? Turn to the guy next to me and say "well the azimuthal resolution of TCAS is only 30 degrees so let's just sit here and see what happens?" Of course I don't. I commence a gentle turn to the left to increase the separation, looking out all the time. And I do that because as a pilot I'm a safety manager, and for every occasion that the manoeuvre takes me closer to the other aircraft, there are 99 occasions when it really does increase the separation. Those are good odds. And without guaranteed separation from ATC, odds are all I have when the notoriously unreliable human eye fails me.