Yes, and after many years of trying to resolve the difficulties created by the (lack of) precision in azimuth information led to the industry giving up on horizontal RAs. Given that such an up upgrade, possibly mandated by regulators, would be a huge gravy train for the avionics supplies, I suspect they did not give up lightly. Now with mode S transponders transmitting location and ADS-B, they might try again.
I am not sure there is a rational case for that, given that there are few, if any, cases of collisions after an RA was issued and followed. The only places where a horizontal RA capability would be good is near ground, where current systems do not issue any advisories below around 1,000ft above ground (radar altitude).
why you are so sure the UL was not transponding?
I didn’t say that, I even put the word “IF” in uppercase.
It isn’t an SOP, it is how the entire TCAS system is designed
That is just a circular argument…
Actually the RA system was technology-limited when introduced many years ago.
Peter wrote:
Airline SOPMaybe @chrisparker will have an explanation.
It isn’t an SOP, it is how the entire TCAS system is designed. All TCAS II does is tell you to climb, descend or level off and to increase or decrease the rate. There is no pilot decision making involved.
But TCAs only works if both have transponders.
I am interested Peter, why you are so sure the UL was not transponding?
A specialist from FIS Langen, doing support for VFR flights, who posts in a German forum, says that both planes had their transponders on. FIS Langen has the radar tracks.
I understand this for the jet world where all participants have TCAS but are not in a position to negotiate which way to turn (especially if somebody does something ambiguous / complicated to resolve, like a level bust in a holding stack – I’ve seen ATC radar videos of that and in fact then the pilots do their own actions and disperse laterally, regardless of SOP) but in a light GA proximity scenario prob99 I am the only one with the system and the other traffic is most likely non-TXP / broken TXP / broken encoder (so Mode A) / Mode C OFF / using some box which almost nobody else uses but which was evangelically plugged in a forum / etc and they most likely have no idea I am there, so it is down to me to look after myself 
The azimuth in active TAS is not accurate but is plenty accurate enough to decide which way to turn – unless the other traffic is right where you are, in which case you have possibly left it too late, but it happens sometimes if a conflict is created when turning. I’ve had a few of those, probably caused by traffic climbing up vertically under me for some time. Got one departing from Zell am See last time.
Whether this is relevant in this accident I have no idea. The SR20 might have had TAS but the UL may or may not even have carried a TXP. IF it turns out the SR20 had TAS but the UL was non-TXP, that might “draw some comments” especially from the family of the SR20 pilot whose life will have been totally pointlessly wasted for the sake of “civil liberties” for the UL pilot, but nothing will be done in the long run.
According to the TCAS manufacturer the azimuth of the conflicting traffic is not reliable – therefor the SOP of diverting vertically is a must. And it is easy when you have trained it in the simulator. And one must follow the TCAS resolution advisory even if a human controller tells you something different … see DHL midair collision NW Friedrichshafen in 2002. The TCAS in both aircraft work together (coordinate) without the controller on the ground knowing which way.
In all my flying I never saw an improper azimuth of traffic ahead but I’m convinced that the vertical resolution is the only way. Otherwise a turning radius away from traffic in the horizontal plane would be very difficult to estimate.
Airline SOP
Maybe @chrisparker will have an explanation.
Peter wrote:
airline pilots can see the azimuth also.
So why do you think that they strictly only avoid up or down unless they have visual contact?
you get a azimuth with even mode A/C? Wow.
That is normal for active TAS.
TCAS will never give left/right resolution advisories but only up/down
It is true that the Resolution Advisories are only up/down, but the traffic is displayed for the pilots in the same way as on mine, so airline pilots can see the azimuth also.
Peter wrote:
I get azimuth to within about 20-30 degrees which makes it pretty clear which way to turn. But that is a £12k active system…
you get a azimuth with even mode A/C? Wow.
TCAS will never give left/right resolution advisories but only up/down. That is why the airliners have this procedure. I find that azimuth nonwithstanding, it works quite well with Powerflarm too, for the very reason that it is the one criteria which puts distance between you and him rather fast. I mostly get range rings anyhow (altitude only) and Flarm Targets in the mountains. ADS-B targets usually stay well outside alert range.
