SEATTLE — Boeing Co. machinists approved a new contract yesterday, ending a four-week strike that shut down the company's commercial airplane assembly plants.

About 80 percent of those voting accepted the three-year pact, said Mark Blondin, president of Machinists District Lodge 751. Approval cleared the way for workers to return to their jobs as early as last night.

A simple majority was needed to accept the offer. Union leaders had recommended that membership vote for the deal, saying it addressed key issues — pensions and healthcare benefits.

The average machinist represented by the contract is 49 years old and makes $59,000 a year.

The strike by about 18,400 machinists began Sept. 2.

Under the new contract, workers will see no changes to healthcare premiums. Pension payouts will rise nearly 17 percent, to $70 per month for every year worked.

Instead of any general wage increase, the revised offer gives machinists in the Seattle area, Gresham, Ore., and Wichita, Kan., cash payouts of about $11,000 over the three years. Boeing dropped a plan to offer incentive payments based on whether the company meets or exceeds financial targets.

"To me, this is a win," said Kent Sprague, 51, a Boeing machine repairman who voted for the contract. "I'm a little disappointed that Boeing didn't offer this initially."

Boeing said the total cost of the new offer was similar to the proposal rejected by machinists at the start of the strike.

"We're very pleased with today's contract ratification by our machinists and look forward to their return to work," Alan Mulally, president and CEO of Boeing Commercial Airplanes, said.