Moving air traffic controllers means retraining them on the quirks of a new facility, and the process can take years.

More than 450 flights were delayed Monday causing chaos at Newark and Atlanta international airports following the weekend’s ground stop order over air traffic control failures. The Federal Aviation Administration (FAA) issued a warning that a second ground stop order was possible given the situation at Hartsfield-Jackson Atlanta International — the busiest airport in the US — which has seen more than 370 delays as of Monday afternoon,

The FAA has put a ground delay in place at Newark airport. The ground delay comes a day after a 45-minute-long ground stop.