A report released by the US National Transportation Safety Board (NTSB) into an unresponsive controller incident which closed Washington National airport's control tower for approximately 30min, early on the morning of 23 March, appears to raise questions as to whether two airlines that landed during that period violated their own operating rules - a suggestion one of the carriers rejects.
Unusual conditions preceded the landings: the lone controller in the tower cab that morning had fallen asleep for an estimated 24min, leaving terminal radar approach controllers at a distant facility - and the pilots themselves - to decide out how to handle the radio silence.
An American Airlines Boeing 737-800 initially performed a go-around after not receiving a response or landing clearance upon contacting the tower, but ultimately decided to land, treating the airport as "uncontrolled" and following approach control's guidance.
A United Airlines A320, arriving minutes later, did the same.
In the final report on the incident, released on 17 October, the NTSB cited the probable cause of the incident as "the tower controller's loss of consciousness, induced by lack of sleep, fatigue resulting from working successive midnight shifts and air traffic control scheduling practices."
Investigators determined that the controller "had the necessary preconditions for the development of fatigue at the time of the event, specifically acute sleep loss in the 24 hours before the event and circadian disruption as a result of working the midnight shift."
In the wake of the incident, the Federal Aviation Administration made a series of changes, including scheduling of a second controller on the midnight shift at National and several other airports.
Other measures taken included ensuring nine hours scheduled "time off" following working a midnight shift and not allowing controllers to swap duty for a midnight shift while taking their "regular days off".
In relation to the pilots landing at an airport without an operational tower, the NTSB said both American and United have guidance in their company manuals which allow for landing at such a facility - providing the status of the airport is obtained before initiating an approach.
Sources of such information can include arriving and departing aircraft, although terminal radar approach control is not specifically mentioned.
However, the NTSB said the FAA-approved airline operations specifications for both airlines do not include National in a list of airports where the carriers are approved to land without an operating control tower.
An American Airlines spokeswoman said the FAA has "taken no action" against the airline, and the pilots "did what should have been done".
She added that operations specifications cover an airport in normal operation, which did not apply to National that morning.
"The pilots will not be disciplined," the spokeswoman said. "They did what they were trained to do."