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.

Incorrect altitude readings on the Army helicopter that collided with a passenger plane over Washington, D.C., in January contributed to the aircraft getting too close, but air traffic controllers warned about the hazards helicopter traffic presented years before the crash.

The National Transportation Safety Board will question witnesses Thursday about air traffic control training, a key part of the investigation into January’s deadly midair collision at Ronald Reagan Washington National Airport.

The acknowledgement came during the second day of testimony held to discuss the collision of a commercial plane and Army helicopter in January that killed 67 people.