Canadian Union of Public Employees, or CUPE, which represents the flight attendants, refused to voluntarily submit to arbitration. “The appropriate course of action is for Air Canada to return to the table and resume good faith bargaining,” it said in a statement.

Their union says one of its core demands, to receive pay for groundwork, is unlikely to be introduced in arbitration. The airline began canceling flights ahead of the potential strike.

More than 10,000 flight attendants are poised to walk off the job around 1 a.m. ET on Saturday, followed by a company-imposed lockout if the two sides can't reach an eleventh-hour deal.