Well, having PAPI in place won't prevent a pilot from flying low and impacting terrain.
The Citation X has
EGPWS that must have warned the pilots about the inappropriate flight envelope in relation to the terrain. The interesting question is, what on-board events derailed pilot awareness and lead to this devastating event? The answer to this question would certainly be answered with the evaluation of the flight recorder and cockpit voice recorder that have been salvaged.
Fact is, there must have been an unusual cyclone of activity surrounding the flight configuration which eventually lead to this disaster.
