FullWings, pursues the more practical and simpler route (#60 onwards), which often the regulatory segments of the industry choose to follow, albeit hidden in the paperwork. This practical approach has been most valuable in persuading the industry of the relevance of new equipment in the threat environment, and the need for standardised (world wide) procedures.
ACAS, like EGPWS and windshear warning systems, has enabled the industry to visualise a particular threat to safe operation, which previously many people did not believe existed (numerically), or did not whish to believe posed a significant risk. We now have growing evidence of the magnitude of the threat and a means (amongst others) of counteracting it.
I suspect that ACAS will continue to evolve (refinable) as did GPWS. I further suspect that any significant ‘failures’ in the overall ACAS safety system will originate from human error at the point of operation and not at the point of design, again compare this with EGPWS and the many incidents of incorrect crew action (again a suspicion, not provable).
The greater problem for the industry is how to get pilots to follow the ‘instructions on the tin’; something which applies to many systems and is fundamental to human behaviour.
ACAS may not assist our quest for correct behaviour in its use of terminology, e.g. using ‘advisory’ and ‘resolution’ (legal/engineering bias) when describing situations which warrant much higher or urgent levels of alerting and warning. These terms may mislead non English speaking pilots or the regulators (often with government/ legal structures) who may apply ICAO recommendations from their perspective and not from that of the crew. But such is the outcome of the worldwide adoption of a system based on a single national culture, perhaps without the necessary depth of understanding and debate on its design philosophy and operation.
ATC Watcher identifies a significant issue with the ATC/ACAS system, perhaps the issue; communications. The ATC based anti collision avoidance depends on communications at all times. Communications are fallible; even the alleviating action of the aircraft informing ATC of an ACAS manoeuvre depends on communication. Until this problem is resolved then there is a role for ACAS.
From the descriptions above, I see ATC anti collision avoidance as the strategic activity, generally proactive, but reactive when necessary and if the conditions (time) allow. ACAS is the tactical, reactive system; the last line of defence and thus at the time of warning it must have precedence.
There are similarities here with CFIT. ATC can provide early defences with safe altitudes and alerts, but still suffer problems of communication in the reactive sense, e.g. delays in transmitting MSAW warnings. EGPWS again provides the tactical, last line of defence.
For both ACAS and EGPWS, the success of these systems depends entirely on the crew following the instructions.
Peter Re PhD; x,y,z, = the 3 aircraft, but lets use your coordinate system. Am I correct in assuming that the 3D solutions to the quartic equation are inside the ACAS alerting area, and thus are realistic solutions for the risk of collision and not just imaginary space-time zones?