Wow, seems to be a lot of interest in this subject... Shame I can't post from the jet (yet!).
ATC Watcher,
ATC is responsible to provide separation , and anti collision avoidance at all times
Absolutely. I have stated that quite positively.
The notion that an RA only occur when ATC has failed is wrong.
Not what I said. What I
am saying is that the penalty for assuming that TCAS has got it wrong (and being wrong in that assumption) is much higher than for assuming ATC has got it wrong (and being wrong in that assumption). Apply a bit of logic to that and you can see why I'd follow an RA. It's a sort of Pascal's Wager for aeroplanes...
If ATC instructions differ from what TCAS is telling you , there you have a dilemna. Do I have , as a controller, the right picture, with every player, moving as my radar tells me, or does TCAS has the correct solution ? . You will never know until you try , but either way, it is not a 100% guarantee that what you chose is the correct action.
At the end of the day it's down to probabilities, isn't it? In order to hit an aircraft during an RA (assuming you're following the guidance correctly) there would have to be another aircraft in addition to the one(s) that gave you the RA that a) Doesn't have a working TCAS or transponder and b) just happens to be in exactly the right place at the right time. I would suggest (although I haven't published a paper on it yet) that you are much more at risk from the *known* target(s) you are avoiding because of a *definite* risk of collision than the *unknown* ones that *might* be there (or not)?
alf5071h,
The voice of reason, as always.
joernstu,
Originally Posted by FullWings
If I'd set two aircraft on a collision course in error I'd want something to manipulate the outcome of my instructions!
You assume that the instructions inevitably would have lead to a collision.
If you (as a controller) have sent two aircraft directly at each other and there is no further intervention (from you, pilots, TCAS, etc.) then they're going to collide. ATC in Africa have plenty of practice at setting this up!
Controlling the airspace by radio communications takes time. A controller telling one aircraft to descend, the other to climb or maintain altitude can make these instructions only sequentially. In this scenario the controller's instructions would have prevented a collision.
We don't know. ATC voice instructions have a very high potential latency compared with TCAS interactions. If there are other aircraft on the frequency and one transmits for, say, 20 seconds then there is no chance to issue any avoiding instructions until too late. The situation becomes worse because if a potentially serious conflict is noticed by pilots, one of the first things that often happens is they check with ATC about the situation, thereby blocking information flow from ATC to the pilot(s); Standard VHF R/T is only half-duplex.