Northwest Airlines pilots agreed yesterday to end their 109-day strike against the carrier, a walkout that eventually involved 10,000 employes.

The airline, serving 39 cities in the United States and seven in the Far East, had cut back its service drastically after the pilots walked off the job.

The Airline Polots Association announced shortly after midnight Monday that a three-year agreement had been reached. A spokesman for the 1,500 pilots called the contract a "draw."

The airline said the contract will cost $51 million over three years in improved wages and benefits.

Before the strike, annual pilot salaries ranged from $8,700 for a first-year pilot to $84,888 for the captain of a Boeing 747 flying international routes. The average was about $49,000. In fullpage advertisements before the end of the strike, Northwest said pilots make up to $105,000 a year.

Roy Erickson, vice president of public relations for the airline, said it was inevitable air fares would go up because salaries make up a high percentage of flight costs. He also said the company would continue negotiations with unions representing cabin attendants and machinists.

The strike began in a dispute over pilots' salaries, pensions, lay-offs of pilots whose seniority protection was expiring, and minimum rest and duty periods.

Loss of business due to grounded airplanes eventually left 8,500 other Northwest workers without jobs.

The airline said it would begin to return its service to normal after requalification of pilots. The Federal Aviation Administration requires recertification of pilots who have not flown for 90 days.

Ken Waldrip, the pilots' spokesman, said he hoped the process could be completed in three weeks, "but it all depends on the company. If it wants to go full bore on it, they can do it in two weeks," the pilot said.

Once the strike began, Northwest was able to maintain only limited service. Before it started, the carrier each day was averaging 142 flights and flying an average of 4,800 passengers in and out of Minneapolis-St. Paul.

The strike had caused Northwest to delay initiation of its Scandinavian service and low-fare flights between Chicago's Midway Airport and St. Paul-Minneapolis, Cleveland and Detroit.

The pilots had sought to trim their maximum work day from 14 hours to 13 and one-half hours, a demand their spokesman said was dropped.

However, he said the new contract will give the pilots a minimum eight-and-one-half-hour rest period between flights. That is a 15-minute improvement over the previous contract, but 30 minutes less than the pilots had sought.

The airline declined to comment on terms of the settlement.

The 109 day strike is the longest pilots' strike in the airline's history. The pilots struck for 95 days in 1972 and for four days in 1975.The Brotherhood of Railway and Airline Clerks stuck the airline for 163 days in 1970.

Money Northwest received from the Mutual Aid Pact during the strike became an issue. The pact of 15 airlines helped support Northwest with payments of 35 to 50 percent of the company's operating costs.

In the second quarter, despite the strike. Northwest reported earnings of $20 million. It received $64 million from the pact, about one-third of its operating revenues.

The pilots, and many politicians, argued that the aid encouraged the airline to prolong the strike.

The striking pilots got $450 to $750 a month, averaging $700, from assessments of ALPA members flying for other airlines.

The bitterness of the strike is visible in the reaction to the 21 pilots who returned to work before it ended. ALPA distributed a "Know Your Scabs" booklet listing their names and addresses.

NWA, in its advertising during the strike, referred to the people in the cockpits as "senior pilots." Jim Halvorson, another spokesman for the pilots, said the 21 pilots are "extremely unpopular" and are likely to be ostracized.