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

The shortage on Monday forced the F.A.A. to delay flights to the busy airport for up to nearly seven hours, the latest problem to plague the airport.

Over the past two weeks, several equipment outages at the Philadelphia-based air traffic control center that guides planes to and from Newark Liberty International Airport, have raised questions on just how safe it is to fly at one of the nation’s busiest airports.