An aviation safety expert tells AP that the nationwide ground stop looks like a repeat of a September air traffic control communications outage in the north-east of the United States, but on a national scale.
Steve Arroyo, a former United Airlines pilot, said the problem appeared similar to the September regional outage, which AP says snarled hundreds of flights across the north-east.
Arroyo said the stop differed from the one imposed on 9/11 because planes were not being told to land immediately, and pilots were likely still able to communicate with ground controllers at airports.
"If you don't have that ability, whether it's because the old wires that they have are just run down or for whatever reason, you've got to stop airplanes on the ground," he told AP.
AP says the outage may be linked to outdated technology used by controllers. That is the view of an outside expert, not a finding from the FAA, which has so far cited only "extraordinary circumstances". The cause of the outage has not been confirmed.