Air Canada warned passengers Monday that “ongoing runway repair and maintenance” could delay flights and force cancellations at Canada’s second-busiest airport Monday.

It’s rare for such a warning to be issued at Canadian airports, with most delays in the summer caused by thunderstorms or other severe weather. Airports generally schedule runway work for overnight periods, when air traffic is low.

YVR’s online arrivals and departures page showed numerous delays Monday. As many as a quarter of arriving flights through the noon hour posted delays, most of them only a few minutes, but some, such as an Air Canada flight from Winnipeg, delayed almost three hours. A handful of cancellations had also been posted.

Vancouver has two parallel east-west runways that handle the vast majority of flights. Its South runway is generally used for departures, while the North runway is generally used for arrivals. There is also a shorter, rarely used, cross-wind runway.

The South runway, the older and longer of the two, is routinely closed overnight for maintenance, as many as four nights a month through the summer, though rarely do the closures affect daytime flights.

The airport posts the planned closures on its website, and advises airlines months in advance, in case they were planning overnight departures that may be too heavy for the shorter North runway. The latest planned South runway closure was announced for Friday night.

Travel Alert – August 19: Vancouver (YVR). Details, Flight Status & rebook online. Change fees waived: https://t.co/LkJG413LFk

— Air Canada (@AirCanada) August 19, 2019

A number of Canadian airports – such as Saskatoon, Winnipeg, and Prince George – have been resurfacing runways and taxiways this summer without incident. Vancouver is not undertaking such disruptive work this year, though it has spent months, and $150 million, extending the Runway End Safety Areas on the North runway to bring it up to international standards.

In rare circumstances, a Canadian airport is forced to close to complete runway upgrades. Dawson City airport closed for a number of weeks this spring, so it could pave its only runway. The airport reopened in May, suddenly able to accommodate a wider range of aircraft, the first newly-paved runway to open in the North in a quarter century.

It was not immediately clear what circumstances led to Air Canada’s warning Monday. Efforts to contact the Vancouver airport authority were not returned.

Flights from Westjet and Pacific Coastal – the second and third-largest users of the airport – were also affected. Air Canada waived change fees for affected passengers.

Vancouver International handled 296,000 aircraft movements last year and 25.9 million passengers, making it the second-busiest airport in the country after Toronto’s Pearson International.