At their closest points, helicopters and planes routinely flew within 75 feet near the airport, according to the NTSB.

The pilots of a Black Hawk helicopter that collided with a passenger jet near Washington Reagan National Airport in January likely didn’t know how high they were flying or how close they were to an airliner before the deadly crash,

Investigators testifying at a hearing into the January's mid-air crash in Washington DC suggest faulty altitude data may have led to a miscalculation.