Of course Ian Shoesmith's comment that one does not want to rely exclusively on TCAS is both responsible and appropriate.
I continue to be surprised at how often TCAS and people's opinions on its trustworthiness or not come up in this forum. It seems that there are plenty of people who want to say that of *course* TCAS is the best thing since sliced bread, but some must also be aware that there are weaknesses, otherwise it wouldn't keep coming up.
TCAS is a sociotechnical system that not only has its weakness in that the human part does not consist of perfectly reliable robots but also inter alia that the algorithms are not yet known to be adequate for other than two-aircraft conflict (and as far as I can tell might be inadequate for three), and in that it can present pilots with decision problems which do not have an adequate solution. If you talk to ATCOs who work on TCAS issues you will also learn of more issues.
I am afraid that 757 Driver's wish that
Originally Posted by 757 Driver
the issues with some nations pilots being trained to give ATC precedence over TCAS were cleared up after that event
remains that, a wish. Things have gotten better, but they are by no means uniform. All kinds of different organisations still give somewhat differing guidance on actions in response to an RA.
Let me direct people towards the Tech Log thread
TCAS Philosophies for discussion.
Those wishing for some engineering-scientific detail may care to look at my paper on
Causal Analysis of the ACAS/TCAS Sociotechnical System, an invited paper in a safety-critical systems conference in Brisbane in 2004, and the paper on TCAS at the same conference by Ed Williams,
Airborne Collision Avoidance System
Originally Posted by morbos
The RA in that TU154 case was overridden by the crew choosing the controller command rather than the TCAS RA. There really is only one case in the vertical where you cannot obey, a GPW takes precedence. Flying at max altitude is the other case.
The crew did follow the controller's descent advisory, but we don't in fact know what their reasoning was. The commander was concerned that they were involved in a three-aircraft conflict and was searching for another aircraft that they did not see, and which he was concerned was the traffic in the controller's advisory.
Originally Posted by morbos
There really is only one case in the vertical where you cannot obey, a GPW takes precedence. Flying at max altitude is the other case.
Apart from the fact that these two sentences contradict each other, I should point out that you don't in fact know that. The complete situation with three-aircraft conflicts remains unknown. For some three-aircraft conflicts, the TCAS algorithms will suffice to avoid collision, but it is not known for all.
PBL