Canada's two busiest airports — Toronto Pearson International and Montréal-Trudeau International — issued ground stops on Friday after a Federal Aviation Administration air traffic control outage forced a nationwide halt to flights in the United States.

The Canadian stoppages, logged in an FAA airspace advisory with the cause listed as "other," signal how quickly a domestic US systems failure propagates across the border. Toronto's advisory noted that the "probability of extension is high (greater than 60%)," indicating airport operators expect the disruption to last well beyond the initial order.

Why did Canada ground its own flights?

NAV CANADA, the privately run nonprofit corporation that owns and operates Canada's civil air traffic system, said Friday night that its own systems are "operating normally." The ground stops, the organization said, were issued to "safely manage traffic volumes" — not in response to any Canadian technical failure.

"Travellers should expect delays and check with their airline," NAV CANADA said in a post on X.

The operational logic is one of capacity, not malfunction. With the US airspace system frozen, aircraft bound for American destinations must either hold, return to their origin, or land at the nearest suitable airport. Canadian hubs sit directly in the path of transatlantic and domestic North American routes feeding US cities.

Mark Stephens, a retired Delta Air Lines pilot, told Business Insider that Toronto and Montreal may be preparing for US-bound international flights that need to divert. Aircraft already on the ground at the two airports will return to their gates, he said, which makes traffic management imperative to handle the potential increase in volume.

In plain terms: the airports are clearing aprons and taxiways now so they have physical space for incoming diverted aircraft later.

How large is the disruption?

The numbers as of 6:20 pm Eastern time on Friday, according to FlightAware.com:

• More than 2,700 flights canceled within, to, or out of the US
• More than 10,500 additional flights delayed
• At least 600 flights diverted across the system, per aviation analytics firm Cirium

The diversion count is the figure that matters most for Canadian and secondary US airports. Cirium reported that hubs including Denver, Chicago, Los Angeles, and Cleveland are already taking on additional flights beyond their scheduled loads.

Each diverted wide-body arriving at an unplanned airport cascades into knock-on effects: gate shortages, crew scheduling conflicts, and displaced aircraft that were themselves due to operate subsequent departures.

What does this mean for carriers and travellers?

For airlines, a nationwide ground stop of this scale is among the costliest single-day operational events in aviation, exceeding the disruption profile of severe weather because it affects every route simultaneously rather than one region at a time.

For passengers, the practical guidance from NAV CANADA is direct: expect delays and confirm status with the airline before travelling to either Toronto Pearson or Montréal-Trudeau. With Toronto's own advisory flagging a greater-than-60% probability of extension, travellers should plan on a disruption measured in hours, not minutes.

The FAA outage also drew responses from its own technology vendors. Microsoft and the vendor behind the FAA's SMART AI system both said the ground stop was not their fault, Business Insider reported, pointing to an ongoing dispute over the root cause of the systems failure.

What comes next?

The recovery sequence is standard but slow: the FAA must lift the national stop, US airports must work through departure queues, and only then can diverted aircraft be repositioned to their intended destinations. NAV CANADA will manage the release of its own ground stops in coordination with that timeline.

Until the US system restarts, Toronto and Montreal remain in a holding pattern — with their gates full, their advisories open-ended, and the probability of extension running above 60%.

Elena Vasquez is a correspondent for CEO Frontier covering aviation and logistics.