Long-standing concerns about military helicopters flying in crowded airspace and worries over short staffing among federal aviation workers have emerged 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 questioned witnesses Friday on collision avoidance technology and organizational systems to manage risk. It is the third and final day of investigative hearings probing January’s midair collision between a US Army Black Hawk helicopter and an American Airlines regional jet,