In order to enable safe flight, airspace is split into categories ranging from very tightly controlled areas around airports to other zones – mostly rural – where rules are much more relaxed.

The most stringently controlled zones are Class A airspace, such as the area around the Ronald Reagan airport in Washington.

Pilots must obtain clearance from Air Traffic Control (ATC) to enter and, except in an emergency situation, must follow ATC instructions to the letter, such as on heading, height or clearance to land.

If reconstructions of the situation in Washington last night and ATC instructions are correct, a passenger aircraft seems to have been given clearance to line up and land on runway 33. (The runway is laid out on heading 330 degrees – with the opposite direction being 150 degrees.)