US flights grounded because engineer accidentally 'replaced one file with another': Official

With the Federal Aviation Administration's Notice To all Air Missions, or NOTAM, system back up and running, staffing remains high and systems monitoring is at an urgently high level this morning, a senior official told ABC News Thursday.

Computer traffic on the NOTAM system is at super-high levels as airlines, pilots and airports start the day with normal flight operations while also trying to make up for delays and cancellations yesterday. At the same time, public and media computer traffic on the NOTAM system is running high because of global interest in the antiquated system that crashed on Wednesday.

The ground stop order that paused all airplane domestic departures and the FAA systems failures Wednesday morning appear to have been the result of a mistake that occurred during routine scheduled systems maintenance, according to a senior official briefed on the internal review.

An engineer "replaced one file with another," the official said, not realizing the mistake was being made. As the systems began showing problems and ultimately failed, FAA staff feverishly tried to figure out what had gone wrong. The engineer who made the error did not realize what had happened.

Engineers and IT teams are working to keep the system from crashing again today while they also try to figure out if there are any similar systems that could fail without redundancies.

The official equated this morning's situation in the FAA networks to the unprecedented strain that was placed on residential internet systems in the first days of the COVID shutdowns in 2019. The systems were just not built for that level of sustained traffic, the official said.

Canada's NOTAM system was also disrupted Wednesday. FAA teams in contact with the Canadians do not know what happened to the Canadian NOTAM system Wednesday or whether it was connected to the U.S. chaos, the official who spoke with ABC News said. Canada did not have to initiate a ground stop and its system had backups and returned to normal function much more quickly than the U.S. operation. Canada also has a more modern system, the official said, and a fraction of U.S. air traffic.

ABC News' Sam Sweeney, Jon Haworth, Kevin Shalvey and Meredith Deliso contributed to this report.