The Federal Aviation Administration has blamed a computer glitch for flight delays across the US today.
Flight plans which are normally entered onto the system automatically are not being loaded.
A spokeswoman for the FAA told the Register: "We're experiencing a system wide outage. It started at 5.06am and we're still working out what caused it. The problem means that air traffic controllers are having to enter flight plans manually. We have radar coverage and radio contact with planes as normal and we have contingency plans in place."
The FAA is not sure when the problem will be fixed.
New York's John F Kennedy is working at half capacity, not helped by poor weather, ABC News reports.
The FAA map of how different airports are coping is here.
The FAA is now saying the problem has been fixed, although the backlog of flights is likely to lead to delays for some time. ®