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

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.

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,