How does ACAS II work if one aircraft is using the standard altimeter setting and the other QNH?
ACAS II (TCAS II) always utilises pressure altitude information which relates to the standard 1013.2 hPa. It does not affect ACAS II operation if aircraft are flying Flight Levels on the standard pressure setting, altitude on QNH, or height on QFE. The pressure setting input by the flight crew does not affect the ACAS system at all. Additionally, below 1750 feet, ACAS also uses radio altimeter data.
