The International Association of Machinists union yesterday revealed that its new settlement with Eastern Airlines provides an immediate 21 percent pay increase. And in a related development the flight attendants' union raised the prospect of further labor trouble for the carrier.

Under the settlement reached Wednesday, hours before a strike deadline, machinists members would receive the 21 percent raise retroactive to Jan. 1, officials said, and a total increase of 32 percent over the life of the contract, which would expire Dec. 31, 1984. Eastern confirmed the union's estimate that the contract will cost $170 million, which is $80 million more than the carrier had first offered.

Union officials said they expect that the pact will be ratified by its 13,500 mechanics, baggage handlers, aircraft cleaners and other ground workers within two weeks.

The terms of the settlement provide the same overall percentage increase in pay as the proposal rejected earlier by the union members, but the raises will occur earlier, officials said.

A top-rated Eastern mechanic under the new pact would be paid a base rate of $15.91 an hour, or $2.76 above current salary, officials said.

The union now will have access to two profit-sharing plans, according to machinists' official Wallace Haber. One, due to expire June 30, 1984, continues the program under which workers contribute 3.5 percent of their salary to the company. Union members are paid interest on their contributions and a share in any profits of more than 2 percent of Eastern's annual revenue.

The second is a five-year, profit-sharing plan offered by the company in lieu of making wage increases retroactive to January, 1982, when the previous contract expired, Haber said. Under this plan, employes contribute nothing and receive one-third of any profits of more than 2 percent of annual revenue.

Meanwhile, Patricia Fink, an official of Eastern's 6,300 flight attendants' union, the Transport Workers' Union, said she would ask for an official declaration of an impasse in her union's 19-month-old negotiations with the carrier. That would trigger a 30-day, cooling-off period, after which the union could strike.