Perceptive comments as usual from alf.
Originally Posted by alf5071h
one of the alternative views of [the Überlingen] accident was that it involved ‘poor’ worldwide/industry communication.
I wouldn't characterise the view as "alternative"; it was thoroughly investigated and all but proved in the report.
Originally Posted by alf5071h
ACAS / TCAS evolved from a US concept (to solve a specific US problem) into something that the industry found to be of great value and thus was approved by ICAO
While this is generally true, I am not sure that TCAS was introduced to solve a specific problem. I was around in California at the time of the 1978 PSA midair and already flying around the state at the time of the 1985 Cerritos accident. TCAS had been under development then for quite a while, by the U.S. research contracting corporation Mitre, on government money. The precise history eludes me at the moment, but Honeywell claims on their WWW site (where there used to be a history) "over 40 years of TCAS development experience". That would make at least 18 years up to Cerritos. In other words, there was a system there waiting to be sold.
So there are two polar-opposite ways of construing developments. One is that some far-sighted industrialist knew that an opportunity would arise, developed a system, and grasped the opportunity with aid of the appropriate lobbying. Another is that some far-sighted industrialist anticipated the future need for some such system two decades before it became necessary. The difference between the two views is only that of whether one believes that such a system was/is necessary. The evidence was obviously lacking at the time of Cerritos; I don't know that there is any way to decide the question now.
The feature of TCAS which most disturbs me, and which I believe should continue to disturb anyone, is that we now have two institutions whose (main) function is collision avoidance, TCAS and ATC, and the former works *against* the latter. With 2,000 ft vertical separation, 1,500 fpm vertical manoeuvring (at least on first RA) and 30 seconds to CPA, the math says that there is no obvious conflict. However, with 1,000 ft vertical separation, a TCAS-manoeuvring aircraft could get potentially within just over 100 vertical feet of another, thus initiating a further RA. This used not to be true at cruise altitudes, but is so now with the introduction of RVSM.
Eurocontrol did a study, called ACASA, which amongst other things looked at simulated RVSM/TCAS interactions. They massaged about two days' worth of radar data to give aircraft RVSM-type vertical separation, and scanned it for potential TCAS interactions, and saw none. Eurocontrol then claimed in their safety case for RVSM that there were no identified risks with TCAS and RVSM (citing the ACASA results as proof, in personal communication). The problem is that they were aiming for a far higher TLS than looking at
the statistical equivalent of two days' radar data would allow . (General considerations from statistical theory suggest that it will be impractical to assess TCAS interactions statistically to currently-required levels of safety.)
In contrast, EGPWS, while requiring a manoeuvre, does not operate in conflict with ATC, because an aircraft responding to an EGPWS warning is already well below minimum altitude and therefore well out of airspace in which there are other controlled aircraft.
PBL