TCAS has limitations as you suspect. Below 1000 feet it doesn't make Resolution Advisories (RA) because of the very real risk that it could steer a pilot into t

TCAS has limitations as you suspect. Below 1000 feet it doesn't make Resolution Advisories (RA) because of the very real risk that it could steer a pilot into terrain. Below 500 feet it doesn't deliver aural warnings, only the visual ones. These are both limitations of the tech as it exists right now, and in theory pilots should be aware of this and be extremely cautious. Personally I'm in favor of the "Swiss cheese" model of accidents, in which a lot of errors might routinely happen, but they're blocked from leading to an accident by other measures. When however the holes in the cheese line up... boom. There's evidence that there was a culture of accepting near-misses along this route, there's evidence that the training flight ignored best practices, there was a single ATC person managing rotary and fixed wing in that airspace, it was night time, the jet was making an approach to the shorter runway, etc etc. On any given night one or two of these factors might have lined up, but not all of that. That night however they all lined up, and people died. This notion of failure in depth is the driving force between the total nature of an NTSB investigation, they don't want to find the LAST failure, they want to find ALL of the failures.