The union representing D.C. firefighters has challenged the promotion provisions of the Fire Department's new affirmative action plan in a lawsuit filed in U.S. District Court.

The suit was filed Friday, just hours before the department promoted five black firefighters to sergeant -- an action that union leaders say was "an end run" to circumvent their lawsuit.

Thomas Tippett, president of Local 36 of the International Association of Firefighters, charged yesterday that the department ordered the promotion of five black firefighters at 5 p.m. Friday, skirting the usual procedures in several ways.

He said that the department skipped over white firefighters who were ahead of the five black men on the 1982 promotion list, which he also contended should not have been used because it had expired and been replaced by a 1984 list.

Tippett charged that the promotions were ordered at the "unprecedented" hour of 5 p.m. Friday, to become effective at6 p.m. Tippett said that promotions normally become effective Sunday, the beginning of the department's two-week pay periods.

Battalion Chief Ray Alfred, the department spokesman, confirmed that the five black firefighters had been promoted Friday, but he said the timing was not unprecedented.

"We have had promotions other than on Sundays," he said, adding, "I'm not going to comment beyond that."

The department's new affirmative action plan, which calls for strict numerical goals in hiring and promotion, is similar to many plans around the nation that have been attacked in lawsuits over whether such goals are a legal means to increase minority representation.

Alfred said yesterday, "There is a tremendous underrepresentation of minorities throughout the ranks" in the department. The chief, he said, submitted the affirmative action plan "to correct that," adding: "D.C. law requires him to do so."

The union's lawsuit, filed as a class action on behalf of members on the 1984 promotion lists, challenges only the promotion provisions of the plan, which call for specific numbers of minority members to be promoted to the ranks of sergeant, lieutenant and captain.

The suit maintains that the selection process, prior to the issuance of the new plan, had been ruled "nondiscriminatory in every respect" in a 1982 D.C. Office of Human Rights case.

According to the lawsuit, the new provisions require that the department select black candidates for promotion instead of white candidates who rank higher on the 1984 lists, "solely because of the race of the candidates . . . "

This, the suit charges, is unconstitutional and violates several civil rights laws.

George H. Cohen, the union's attorney, said that he will seek an injunction in court this week to put a hold on the five promotions announced last Friday, at least until a portion of the case has been decided.