A radar-related technical fault in the UK’s air-traffic control system grounded flights at Heathrow, Gatwick, Stansted, Edinburgh and other airports on Wednesday, effectively closing London’s airspace for more than four hours. National Air Traffic Services said equipment at its Swanwick control centre malfunctioned and it limited aircraft movements "to ensure safety." Engineers switched to a back-up system and, by about 17:00 local time, declared the network "fully operational." Departures and arrivals have since resumed, although airports warned of residual delays while they clear the backlog of flights. Ryanair, which reported diversions and widespread delays, called the disruption "utterly unacceptable" and urged NATS Chief Executive Officer Martin Rolfe to step down, citing a similar outage in August 2023 that airlines say cost them over £100 million. The Civil Aviation Authority has previously pressed NATS to bolster its contingency planning.