Originally Posted by PBL
Fact is that, had TCAS been available since 1950, it would have helped avoid four collisions in 36 years (Grand Canyon, Zagreb, San Diego, Cerritos). Since it was mandated (a matter of some 16 years in the U.S., less elswhere), use of TCAS has almost caused two collisions (Albany, NY, 1998, another three-aircraft situation; 28 June 1999 in Chinese airspace between BA027 and KE507) and been a contributory factor in one accident (Überlingen, as you point out). And that's just what I know about. Interestingly, the potential outcome at Albany was ameliorated through intervention of a controller, just the opposite of what is "supposed" to happen.
So, 4 in 36 years against 3 in 16 years. These figures are too sparse to draw any statistical conclusions. But they should suffice to indicate that the risk analysis is not at all obvious.
This is a rather simplistic analysis of the situation and it is at least reassuring that PBL does not try to draw any statistically based conclusions, albeit only for one of many reasons.
The reality is that the environment has changed significantly since 1950 and what may have happened then is not necessarily any indication of what might happen now or in the future (something that hazard analysts should think about). How much more traffic is around today compared to the 1950s? And how much have air traffic services changed? And how much have flightdecks changed? And the the operating procedures for modern aircraft? Making claims based on historical data in such a rapidly evolving environment is not for the faint-hearted!
Likewise, PBL's statistics are based on a sub-set of events in which TCAS may play a part. How many near misses occurred over the 36 years but were nothing more because of good fortune? And how many have been avoided in the 16 years because of increased situational awareness(although the accuracy of the information propvided by TCAS in this respect is often misunderstood) on the flightdeck?
Like so many other systems that we use today, TCAS provides benefits and creates hazards that would not otherwise have existed. Hazard analysis can help us to understand ways of managing risks - but it's not the panacea that some appear to think it is. It certainly doesn't guarantee that things won't be missed, that hazards won't evolve since the last analysis activity, or that the system will be used in the way that was originally intended.
The fact is that TCAS is here. With TCAS and countless other newly developed or evolving systems, we should learn lessons from previous (sadly sometimes tragic) experience. We should use it in the most effective way possible rather than trying to highlight supposed weaknesses and imply that this makes it a 'bad thing'.
By way of brief example,