Canadian airline WestJet has begun canceling some flights in anticipation of a strike by aircraft maintenance technicians, saying it wants to avoid having passengers and planes stranded if there is a walkout.

WestJet said it expected to cancel about 40 flights from Tuesday through Wednesday, affecting 6,500 passengers. By Wednesday afternoon Eastern time, the airline had canceled 20 flights, or 4% of its schedule, after canceling five flights Tuesday, according to tracking service FlightAware.

The low-fare airline said it was trying to find alternate arrangements for customers whose flights were dropped.

WestJet and the Aircraft Mechanics Fraternal Association are locked in negotiations for an initial collective-bargaining agreement covering about 680 mechanics.

Earlier this week, WestJet asked the Canadian Industrial Relations Board to order both sides into binding arbitration. The union responded by notifying the airline of its intention to strike as soon as Thursday night unless the company returned to the bargaining table this week in Calgary.

Diederik Pen, the airline's president, said the company's most recent contract offer would have made WestJet maintenance engineers the highest paid in Canada and raise their take-home pay 30% to 40% within a year.

The union says the airline is trying to impose a contract that its members rejected by a 97.5% vote, and that low pay is preventing WestJet from filling open jobs.

WestJet was founded in the 1990s and modeled after Southwest Airlines in the U.S. It is Canada's second-largest airline.