TCAS philosophies

[PBL]

alf,

I don't necessarily agree that *the* issue with ACAS/ATC interaction is communication. The communication issue would theoretically be solved by having secondary radar interpret and display the Mode S interactions between two TCAS-equipped aircraft undergoing an RA sequence.
Under a TCAS RA manoeuvre, the aircraft involved can well depart cleared airspace. That means that the controller has suddenly to replan, and that may neither be desirable nor, in some circumstances, possible, perfect communication or not.

And since TCAS may (and does) propose manoeuvres when aircraft are still legally separated, (and in the case of "zoom climbs" the crews likely will not bust their clearance) that means this replanning problem may be artificially introduced.

I must say that it does surprise me that, even at this late date, many line aircrew seem to be unaware of the problems of TCAS/ATC interaction. (For example, FullWings suggested he wasn't aware of problems; ATC Watcher implicitly pointed out that he was aware of lots.) I could have understood it, say, a decade ago, but I don't understand it five years after Überlingen.

I think some of it may be due to Eurocontrol's and others' attempts to impose uniform RA behavior on aircrew. If you are trying to get all pilots everywhere to follow an RA, it works against that message to publicise all the incidents in which TCAS might prompt inadvisable or unnecessary manoeuvres. So the incident narratives are suppressed (to which phenomenon I can attest, having tried to get data on TCAS incidents).

I originally said that I am not taking a stand on whether this is a "good" or a "bad" thing; that I was simply noting a phenomenon. That is not quite true; I do take a stand. I am for transparency of information on all safety-relevant systems in which there is a public interest. Just as the UK has an Airprox Board that reports publically on all designated near misses, I think data on TCAS manoeuvres should be publically collected and publically analysed and displayed.

Then maybe we would not see such travesties as Eurocontrol's declaration in their EUR-RVSM Safety Case that they had not identified any anomalous ACAS-RVSM interactions and that therefore they did not need to address such potential interactions in the Safety Case (even after I pointed such out to them, they maintained their "view").

PBL