How many escape manoeuvres ACAS equipment can calculate simultaneously?
 

ATPL H New Zealand syllabus. Can someone answer this?

I am slightly surprised that none of the experts out there have answered in all this time.

It sounds like a trick question. The obvious answer is "one" because ACAS can only offer a single RA instruction - albeit one which may modify as the situation develops. However the fact that, under some circumstances, the RA may be reversed does suggest that a second manouevre is being calculated in the background and then the reversal occurs as the system decides this is the preferred one.

However ACAS logic is not my strong suite and I offer this only as conjecture.

Thanks i think your right. At one point I considered two because if each aircraft has acas they need to respond to opposite instructions. One flies up the other down to avoid a collision.

Just found this other thread which seems relevant...

http://www.pprune.org/tech-log/571329-tcas-threats.html

TCAS II has specific algorithms to deal with cases where two or more other aircraft are simultaneously considered to be threats – ‘Multi-aircraft logic’.
Because TCAS only issues resolutions advisories in two senses (‘up’ or ‘down’’) the coordination and resolution of multiple threats cannot be ideal.
Generally, one aircraft will be instructed to pass above all the other threats, one aircraft will be instructed to pass below all the other threats, and the other aircraft will be instructed to fly level.

Some information

http://i67.tinypic.com/mncjp.jpg