Long-standing concerns about military helicopters flying in crowded airspace and worries over short staffing among federal aviation workers emerged Thursday as key factors in investigators' inquiry into the fatal midair crash between an Army helicopter and a commercial airliner earlier this year.

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.

The National Transportation Safety Board wrapped up a three-day investigative hearing into the mid-air collision between a helicopter and passenger jet in Washington that killed 67. New information confirms the crew of the Army helicopter faced several problems,