All outbound flights were grounded until around 9am Eastern Time (2pm GMT) as the Federal Aviation Administration worked to restore its Notice to Air Missions (NOTAM) system, which alerts pilots of potential hazards along a flight route.As of 3pm GMT 4,948 flights within, into or out of the US have been delayed, according to flight tracker FlightAware.com, while 868 have been cancelled.Earlier it said "final validation checks" were being carried out and the system reloaded after "operations across the National Airspace System" were affected.She said there was no evidence a cyberattack was behind the glitch, "but the president directed [the US Department of Transport] to conduct a full investigation into the causes".Mr Biden told reporters earlier: "Aircraft can still land safely, just not take off right now."