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 NTSB is holding three days of hearings to uncover insights into what caused the fatal crash between the passenger jet and the army helicopter over Ronald Reagan National Airport in January.

An investigative hearing into a deadly mid-air collision of a US Army helicopter and a passenger plane that killed 67 people in Washington has revealed a discrepancy in the chopper's altitude displays.