Air Canada has asked the Canada Industrial Relations Board to intervene in what the airline perceives to be "industrial action to disrupt our operations," following mass delays and flight cancellations during one of the busiest air travel weekends of the year.

A statement from Air Canada spokesperson Peter Fitzpatrick attributed the disruptions to factors such as foggy weather and a fire on the airfield of Toronto's Pearson Airport. But he also blamed "a higher-than-usual" amount of "pilot bookoffs" — an apparent reference to reports on Saturday that several pilots had called in sick that morning.

"While Air Canada supports the right of its employees to book off when they are unwell or otherwise unfit to work, we cannot condone such activities as part of industrial action to disrupt our operations and we have asked the CIRB to intervene," the airline said in a statement on Sunday. "Customers should be reassured that the vast majority of Air Canada employees are hard at work to get them safely to destination.‬"

Passengers using Toronto's Pearson airport on Sunday were experiencing a second day of flight delays and cancellations, but officials said that by the afternoon, fog that slowed operations had lifted and a crippled runway had reopened.

Air Canada had reported a "significant reduction in runway operations" on Sunday because of the weather and a small fire on the airfield that damaged an underground electrical maintenance system and forced the shutdown of one of the runways.

The four other runways in Toronto remained fully operational throughout the morning. But Fitzpatrick said the fire late Saturday greatly contributed to the flight delays and cancellations on Sunday.

"There was a fire in a manhole last night that has taken power from some runway lights. The GTAA reduced the number of landings allowed to 12 per hour for all carriers this morning, where normally it would be 50 or more, although this may be relaxed and more flights permitted," he said.

"Our operation is currently affected by a significant reduction in runway operations at Pearson which is impacting all airlines on a proportional basis," Fitzpatrick said.

The problems caused a ripple effect at airports in other cities, including Montreal, where most Air Canada flights from Toronto and one to Toronto have been cancelled, while most other arrivals were delayed.

Fire temporarily closes runway

Runway 05/23 at Pearson airport remained closed overnight before reopening late Sunday morning because of a small, isolated fire in an underground maintenance area on the airfield. The fire shut down the electrical system that feeds runway signs and lighting. Officials say it was quickly contained and there were no injuries.

Stranded airline customers, meanwhile, were running out of patience at Pearson airport.

"We tried to go to Calgary, but the flight had been cancelled, so we had to wait here for another six hours," said Tuan Tran.

Matthew Jennings said he was "overwhelmed" after being passed from person to person to file a complaint. Jennings told CBC News he waited in a line so long he missed his flight out, and has had to come up with a backup plan so he can make it to work on Monday.

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Airline customers stranded at Toronto's Pearson Airport waited hours for alternative flights after their original bookings were delayed. (CBC)

"Instead of flying to Halifax, which is where I'm going, I'm flying to Moncton and my father's driving seven hours to pick me up," he said. "So, it's been an adventure."

The first Air Canada flight of the day left Pearson Airport on time, at 5:45 a.m. ET. However, a few hours later, airport officials said around 15 per cent of the arrivals and departures have been cancelled or were experiencing delays.

"We have some 32 Air Canada flight departures already cancelled today, and 33 arrivals from a variety of destinations cancelled. Other airlines have a number of cancellations as well. There are fog problems persisting," CBC's John Northcott reported early Sunday.

Air Canada advisory

The airline is asking customers to check its automated flight information system prior to leaving for the airport at 1-888-422-7533.

The latest problems follows reports that fog over the past two mornings was largely to blame for delays and cancellations, although there were also unconfirmed reports of a possible job action on Saturday by Air Canada's pilots' union amid an ongoing contract dispute.

Air Canada would only say the airline was experiencing "operational challenges" on Saturday, one of the busiest travel days of the year, marking the end of March break.

A source at the airline told CBC News that a number of pilots had planned to call in sick, but couldn't say if that was a factor in the cancellations. The pilots' association was not immediately available for comment, but some pilots took to Twitter on Saturday to say the weather was the cause of any service disruptions, not sick calls by co-workers.

Nearly two dozen Air Canada flights were cancelled or delayed on Saturday. Websites for both the airline and the airports listed a number of delays and cancellations for flights between Montreal and other cities including Toronto, Halifax, Saint John, N.B., and some American destinations.

The flight problems come just days after the Senate passed back-to-work legislation that would prevent the airline from locking out pilots and the two unions representing pilots and other workers from striking.