The board focused on air traffic control and heard it was common for pilots to ask to use visual separation — relying on eyesight — just as the Army Black Hawk's pilot agreed to do the night of the crash.

NTSB Chairwoman Jennifer Homendy has urged the Federal Aviation Administration to improve safety measures, pointing to ignored warnings.

Investigators have found that a helicopter involved in a deadly crash with a passenger plane over Washington was flying above its height limit. The helicopter's altitude-measuring instrument was also inaccurate.