The Army goggles would have made it difficult to see the plane's colored lights, which might have helped the Black Hawk determine the plane's direction. The goggles also limited the pilots' peripheral vision as they flew near Ronald Reagan Washington National Airport.

The National Transportation Safety Board is holding hearings in Washington, D.C., this week to try to figure out what caused the deadly crash between the American Airlines plane and the Black Hawk helicopter over Ronald Reagan National Airport.

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.