•  The cause, from Duffy’s White House briefing: the trouble began at 3:36 p.m. Eastern, when radar and communications became unstable at the air traffic control centers and terminal radar facilities. The system’s switches and routers “got overloaded with data” and began giving inconsistent information, and the system was shut down because of it. Duffy called it a legacy network problem and urged Congress to pass a $50 billion funding bill to modernize the system.

•  Mullin: the newer technology was “overwhelming the older system.” No evidence of a cyberattack so far, but nothing has been ruled out — the investigation is ongoing and the FAA has not announced a final cause.

•  The scale: about 3,500 airplanes were in the air when the stoppage was ordered; fewer than 400 remained airborne by the time it was lifted. Cirium counted 827 diverted flights.

•  Recovery: gradual — planes and crews are out of position, some aircraft won’t move until Saturday morning. Delta issued a travel waiver for Friday and Saturday.