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 on 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.

As few as three air traffic controllers were working at Newark Liberty International Airport Monday evening, according to a published report. That’s 11 fewer than the targeted amount for that time period, The New York Times reported.