National and Dulles International airports are expected to see rising revenue almost consumed by increased costs during their first four months after passing from federal to local control, according to a proposed financial plan.

When the new Metropolitan Washington Airports Authority assumes control of the two facilities a week from today, it will start running an enterprise that has seen its profits soar with the rise in local air traffic, particularly at Dulles.

The airports' profits almost doubled to about $20 million in 1986, a year in which the number of flights at Dulles rose 99 percent.

Costs, however, will increase over the next year after the federal government stops paying the bills for items ranging from employment benefits to deer fences.

Costs also may rise when the transfer frees the airports to expand their work forces, which have been cut in recent years because of federal budget pressures.

The projected result over the four months ending Sept. 30 will be a profit of $6.5 million, according to a financial plan to be considered by the authority's board Wednesday.

If that profit rate were projected over 12 months, it would be a net income of $19.5 million, virtually level with the 1986 profit. Because the four-month figure reflects a one-time cost of $3 million to pay off certain federal contracts, the actual 12-month profit could reach as much as $25.5 million.

This financial plan deals with the day-to-day operations of the airports. It does not address the board's larger task of financing an estimated $700 million in improvements to the facilities.

However, the airports' profits over the next year could look very different depending on the growth of revenue and the spending decisions made by the airports board.

The airports can expect to take in $24.3 million in operating revenue in the next four months, according to an interim financial plan prepared by the board's finance committee to guide the authority until its first full fiscal year begins Oct. 1.

The committee proposed that the board spend $17.7 million in the next four months, a figure that would include a variety of new expenses. The proposal includes:

Spending $9.1 million, the largest single expense category, on employe pay and benefits. That includes health and life insurance, workers' compensation and retirement benefits that had been provided by the federal government at lower cost. Hiring 30 additional firefighters, 15 police officers and 17 other employes to perform new jobs such as clerical support to the board, at an annual salary cost of $1.7 million. Spending $121,000 for water, an increase of 195 percent. Under federal management, National received water free from the Army Corps of Engineers; Dulles had no such arrangement. Buying electricity for $1.8 million, an increase of 1 to 1.5 percent. Paying $1.8 million for legal, financial and administrative consultants to assist with the transfer. Insuring the airports at a cost of $1 million, because they no longer will be covered by the federal government's self-insurance. Buying $2,000 worth of identification cards for all new employes. Paying $42,000 for a deer fence at Dulles, where six deer have been struck by aircraft in the past year.