US air traffic controllers must follow new procedures as officials try to contain the fallout from an incident earlier this week in which two airliners landed at Washington without assistance because the lone controller on duty was asleep.
Regional radar facilities are now required to alert controllers working alone at night in an airport tower that a plane is approaching, said Randy Babbitt of the Federal Aviation Administration.
The radar controllers are "to confirm that there is a controller prepared to handle the incoming flight", he said.
Regional controllers have also been reminded that if no controller can be raised at an airport tower, they must offer pilots the option of diverting to another airport.
Controllers at a regional FAA radar facility in Warrenton, Virginia, about 40 miles from Washinton's Reagan airport, did not offer that option to the pilots who were to unable reach the tower between 12.04 and 12.28am on Wednesday.
Repeated phone calls from the regional facility to the tower also went unanswered.
The planes -- an American Airlines flight from Dallas and a United Airlines flight from Chicago with a combined 165 people on board -- landed safely.
The controller on duty in the tower -- a veteran air traffic supervisor -- acknowledged to investigators who interviewed him that he had dozed off, the National Transportation Safety Board said.