US Airways reached tentative agreement last night on a new contract with the Machinists union, ending the threat of a weekend strike that could have caused air-traffic chaos throughout the eastern half of the nation.

The pact with the International Association of Machinists must be ratified by the airline's 7,500 mechanics and related workers. Details of the agreement were not immediately available.

Rakesh Gangwal, president of US Airways Group Inc., the airline's parent company, issued a brief statement praising government mediators for their role in reaching a settlement. "Our thanks go to the National Mediation Board for its constructive role in helping us to reach this important agreement," Gangwal said.

Gangwal had taken a personal role in the talks Monday in an effort to head off a strike threatened for 12:01 a.m. Sunday. The negotiations were crucial to the future of US Airways, which is struggling to overcome a variety of labor, cultural and financial problems that have threatened its plan to become a major global airline.

Union officials were not immediately available for comment.

The union was seeking a three-year contract with 4 percent annual pay increases, another 4 percent on the date the contract is signed and an additional 4 percent lump-sum payment to make up for the fact that union members have not had a pay raise since October 1995. The union also wanted protection for aircraft cleaners from any company effort to make their jobs part time.

The union membership overwhelmingly rejected a settlement offer in June, forcing negotiators back to the bargaining table for another try. The union would have been free to strike this weekend.

The Arlington-based airline, which has some of the highest labor costs in the industry, has insisted it would pay no more than 1 percent above what other major airlines pay.

Top-scale mechanics currently earn $23 an hour, while cleaners have a top pay of $16 an hour.