MONTREAL - Disgruntled Air Canada employees plan to hold a rally at Pearson International Airport in Toronto, the country's busiest hub, on Thursday, a day before one of the airline's union potentially serves a strike notice.

The Canadian Auto Workers union hopes thousands of its members and supporting unions will converge at the Terminal 1 domestic arrivals starting at 1 p.m.

It wasn't immediately clear where the rally may move from there or if the gathering will cause any disruptions to airport passengers.

The CAW, which has 3,800 Air Canada customer service and airport workers across the country, said union members will stage "a mass rally of solidarity and convey to management that enough is enough."

The union and Air Canada are meeting behind closed doors trying to reach an agreement.

The CAW could serve 72-hours notice on Friday for a strike beginning midnight June 13.

If talks fail to reach a settlement with the CAW, the airline said it plans to maintain full operations by implementing a contingency service plan at airports and call centres to minimize impact on customers.

The union said Air Canada delivered its first all-encompassing offer last week, which included economic and non-economic items.

"The comprehensive package was riddled with concessions and deemed unacceptable by the union," it said in an update Tuesday.

On Monday, the union representing 6,800 Air Canada flight attendants asked for a federal conciliator to assist in contract negotiations with the airline.

The Canadian Union of Public Employees said it filed the request with the Federal Mediations and Conciliation Services after reaching an impasse with management on several key issues.

Air Canada is demanding several concessions, which would increase workloads, reduce health benefits and make drastic changes to the pension plan, the union says.

The CAW also obtained the services of a conciliator.

Three unions representing Air Canada flight attendants, ground crews, counter personnel and other workers have united to fight the airline's plan to introduce a defined-contribution pension plan for new workers.

The unions say they want Air Canada (TSX:AC.B) to stop considering proposed changes that would force new hires onto defined-contribution pension plans.

Defined-benefit pension plans, which Air Canada employees currently earn, are designed to provide retirees with a predictable income, but they expose the airline to additional costs if the pension fund's assets aren't able to pay for the benefits.

With defined-contribution plans, the airline's contribution is limited to a set negotiated amount and payouts to retirees depend on the performance of the underlying investments.

Air Canada was forced into creditor protection from April 2003 to September 2004, due in part to the cost of dealing with the company's pension deficit.

Air Canada's unions agreed to accept numerous concessions worth billions to help the company to survive, but they insisted their defined-benefit pension plans be saved.

During the restructuring, the airline saw a major potential backer walk away from a plan to invest in Air Canada after the unions refused to allow new employees be given defined-contribution pensions.

On the Toronto Stock Exchange, Air Canada's shares fell three cents at C$2.07 in morning trading.