When the USA closed its airspace at 09:42EST 197 aircraft were crossing or were cleared to cross the North Atlantic westbound.
"It occurred at the worst possible time," says UK National Air Traffic Services (NATS) operations director Keith Williams.
At the time that oceanic controllers at Prestwick, Scotland, broadcast the news, it was unclear whether Canadian airspace was open, leaving crews at or around their point of no return unsure where they could land.
One crew immediately transmitted a Mayday and three others declared fuel emergencies, all of which were rescinded when Canada affirmed that it would accept diversions. But with more than half the aircraft out of radar and VHF radio coverage, controllers faced a huge challenge.
There was no contingency plan for the USA closing down. "I would not mind speculating that nobody had even considered it before," says Williams.
Immediately the most critical cases were turned back to Europe or onwards to Canada and the Caribbean. Aircraft still in UK or French airspace also diverted, generally to their home bases.
Then one by one the remaining flights selected their new destinations: about 80 returning to Europe and the rest continuing to the west. A European airline source says 85 aircraft landed in Newfoundland alone - 47 at St John's and 38 at Gander. The aircraft are thought to have had around 17,000 people on board.
The turnbacks created an unprecedented situation on the North Atlantic Organised Track System. Aircraft either turned on to reciprocals immediately, changing level as they went to avoid traffic following them, or flew 60nm (110km) north or south to the next set of tracks and changed level as they turned eastwards.
Controllers were hampered by the hitherto successful reduced vertical separation minima in the region, which left them less flexibility, but helped by the number of aircraft equipped for datalinking rather than voice messages.
A prompt Russian decision to call off a military exercise in the far north freed up more airspace, and off-duty Scottish controllers volunteering to work ensured maximum ATC capability was available.
Although a handful of aircraft reached Glasgow or northern England, most in Europe returned to overcrowded hubs.
Source: Flight International