Several equipment outages over the past two weeks at the Philadelphia-based air traffic control center that guides planes to and from Newark Liberty International Airport have raised concerns about how safe it is to fly at one of the nation’s busiest airports.

Technology and staffing problems have plagued the airport, one of the nation’s busiest hubs. Officials say they are trying to resolve the issues, but concerns remain.

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