Air Canada customers should expect further delays on Friday as the airline continues to work to restore service to normal after a technical issue on Thursday.

In its daily travel outlook, the carrier said that while its computer system is now stable, flights could be affected at nine of Canada’s busiest airports, including Toronto Pearson International Airport, Montreal International Airport -Trudeau and Vancouver and Calgary airports.

Thursday’s outage caused more than 500 flights to be delayed or canceled, more than three-quarters of its trips that day, creating what the company called a “spillover effect” that could lead to further delays Friday.

The source of the disruption was in the system the airline uses to communicate with planes and monitor their performance, which Air Canada is in the process of upgrading.

On May 25, she was forced to ground her planes for about an hour when the system experienced a separate issue, causing delays for nearly half of Air Canada’s flights that day.

As of 11 a.m. Friday, a total of 81 Air Canada flights, or 15% of the airline’s scheduled flights, had been delayed, while another 30 had been canceled, according to tracking service FlightAware.com.