Northwest Airlines reached a tentative agreement on a new five-year labor contract with its flight attendants yesterday, the latest in a series of recent contract settlements that have largely removed the threat of major airline disruptions during the heavy summer travel season.

The agreement between Northwest and Teamsters Local 200, which represents the airline's 11,000 flight attendants, must still be ratified, a mail ballot process that is expected to take at least a month. Details of the agreement were not immediately available.

Last month, flight attendants at American Airlines and mechanics at Alaska Airlines reached a contract agreement. Flight attendants at Alaska Airlines have agreed to submit their contract dispute to mediation if an agreement isn't reached by the end of June. Last week, Arlington-based US Airways reached an agreement on a new contract with the International Association of Machinists covering 7,000 mechanics at the airline. Baggage handlers and fleet service employees, who also are members of the IAM, ratified a new contract with US Airways in May.

The one major exception to the possibility of a sudden outbreak of labor peace is Trans World Airlines, where members of the IAM are free to strike on the eve of the July 4 holiday weekend. But even there, an immediate strike is less than certain. IAM members are expected to overwhelmingly reject TWA's last contract offer, but union sources yesterday indicated a willingness to go back to the bargaining table and try to reach a settlement.

The Northwest settlement, which came less than a week after the union membership voted overwhelmingly to strike, was hailed by both the company and the union.

Billie Davenport, president of Local 2000, issued a statement saying: "This is the agreement our members sent us to negotiate. We are proud to recommend it to our brothers and sisters for ratification."

Doug Steenland, Northwest's executive vice president, said: "We are pleased to have reached a tentative agreement on a new contract that reflects their [the flight attendants'] importance to Northwest."

The agreement comes less than a year after Northwest's pilots struck the airline for 15 days, crippling the airline's operations and costing the company millions of dollars in losses. The stock of NWA Inc., Northwest's parent, fell 18 3/4 cents a share yesterday, to close at $29.87 1/2 in trading on the Nasdaq Stock Market.

Under the National Railway Labor Act, the 1926 federal law governing labor relations in the railroad and airline industries, the flight attendants would not have been free to strike or take any other kind of job action if agreement had not been reached. Unions are not free to take what is known as "self help" until they are "released" by the National Mediation Board, which oversees disputes in the two industries.

At TWA, the situation is different. The IAM, which represents 16,000 mechanics, ramp workers, ticket agents and flight attendants at the airline, have been released by the mediation board and are legally free to strike if the company's latest contract offer is rejected.

TWA is considered to be the weakest financially of the major airlines and has argued that the offer now being voted on by the union membership is the best it can do.

Even the threat of a strike, however, could damage the airline's balance sheet, however, if potential passengers turn to other airlines to avoid possible travel disruptions.