"Neither helicopter was equipped with a traffic
collision avoidance system, nor was such a system required. Had
such a system been on board, the Board noted, it likely would have
alerted the pilots to the traffic conflict so they could take
evasive action before the collision. However, according to Federal
regulations, ultimately the pilots are responsible for maintaining
vigilance and to be on alert and avoid other aircraft at all
times."
Source: From Probable Cause statements by
the NTSB in regards to a fatal mid-air collision that occurred
between two Bell 407 EMS helos over Flagstaff, AZ, on June 29,
2008.