Boeing Co. is in the process of buying a 25-acre site just north of Seattle- Tacoma International Airport, with tentative plans to set up an air freight center to expedite delivery of critically needed aircraft parts.

Boeing is buying the airport site from SeaTac Washington, a limited partnership affiliate of Trammell Crow, a major development firm, and will lease an adjoining property in a runway clear zone for parking and movement of trucks, a Port of Seattle memo disclosed.According to Bill Hughes, airport property manager, Boeing executives said the company would expect to use a major part of the site for warehousing of spare aircraft parts. Because airlines often need those parts on an emergency basis, having an air freight center near the airport may make it possible to deliver them more quickly, the port memo noted.

Mr. Hughes said Boeing has disclosed few details and regards its development plans for the site as not yet final. The port land originally was leased to SeaTac Washington under a plan that called for it to be a buffer along the edges of a business park.

Neither Boeing nor Trammell Crow would comment on the transaction. Paul Binder, a Boeing corporate spokesman, said the deal for the property is not yet complete.

The port memo, prepared for review at a Port of Seattle commission meeting scheduled Tuesday, says Boeing has concluded negotiations for sale of the entire 25 acres and is now asking the port to transfer the lease for the buffer property.

Under terms of a proposed agreement with Boeing, the giant aircraft manufacturer could use the port land for parking or for maneuvering of trucks.

Acquisition of the airport land is the latest of several major real estate purchases Boeing has carried out in the last few months. The company has bought several office buildings it previously leased in Kent, Wash. Analysts have suggested the expanding aircraft company wants to avoid building up too much cash and as real estate prices on Puget Sound have been rising rapidly, Boeing sees an opportunity to control long-term costs.

Disclosures of plans for an airport parts center came as Frank Shrontz, Boeing chairman, confirmed in comments made in Washington, D.C., that the company is also considering opening a new plant in the Tacoma area.

Boeing is aleady the largest private employer in Pierce County, the area surrounding Tacoma. Thousands of employees in Boeing's Kent and Renton plants, situated between Seattle and Tacoma, live near Tacoma, where housing prices are markedly lower than in Seattle.

Rep. Norm Dicks, a Democrat whose district includes Tacoma, has urged Boeing to build a plant in Pierce County, possibly in connection with the company's new 777 widebody aircraft.

Boeing last month filed environmental plans for expansion of its widebody assembly plant in Everett, Wash., north of Seattle, and specified that expansion there might involve up to 6,000 additional workers.

Congressional sources suggested, however, that some wing fabrication work might be done near Tacoma.

John McCarthy, a Tacoma port commissioner, said he also had urged Boeing executives to open a plant in Pierce County.

But Boeing has also modified some expansion plans. For example, the company last week disclosed it would not build a full new assembly line for jets at its plant in Renton, but would instead convert an area previously used to build military versions of the 727.

Commercial production of the 727 was discontinued after Boeing introduced the fuel-efficient 737, but production of military models based on the 727 design had continued through this year. Boeing said existing assembly areas can be used because military orders have been filled.

In press briefings, Boeing executives have noted growing concern about traffic at the Renton plant, which builds 737s and 757s. The company last week said it would give the city of Renton a large grant aimed at finding ways to lessen the traffic problems.