The Lockheed C5A, which already has flown though more political turbulence than any other military transport plane, is about to run into still more as a result of a Pentagon decision to buy an updated model for the Air Force, which would rather have something else.

Deputy Defense Secretary Frank C. Carlucci earlier this week told Missouri Sens. John C. Danforth (R) and Thomas F. Eagleton (D) that their home state contractor, McDonnell Douglas of St. Louis, will not get the job of building a new plane to fly military gear to distant trouble spots like the Middle East and Persian Gulf.

Instead, the senators said Carlucci told them, the Pentagon intends to buy a fleet of modernized C5As made in Georgia along with KC10 tanker-cargo planes made by the Douglas part of McDonnell Douglas in California.

Army, Air Force and Marine leaders, in what was called a "the 12-star letter," four for each of the three generals who signed it, had recommended buying instead the new McDonnell Douglas C17.

Danforth and Eagleton, in a joint statement, said they "strongly question" whether Pentagon civilians stayed within the law by running a transport competition, won by the McDonnell Douglas C17, "and then arbitrarily awarding the contract on a sole-source basis to a loser of the competition," Lockheed.

The senators said they would pursue that question within the Senate Governmental Affairs Committee.

This by itself should create considerable political headwinds for the C5A. While Danforth and Eagleton are senior members of the committee, Republican Sen. Mack Mattingly of Georgia, where Lockheed would build the new fleet of C5As, is on it as well.

Additional trouble for the C5A is likely to come as senators and representatives on other government procurement committees quiz Defense Secretary Caspar W. Weinberger about a Dec. 18 letter he received from W. H. Sheley Jr., a division chief with the General Accounting Office.

"The Air Force analyzed both new and existing military airlifter designs," Sheley wrote Weinberger, "and determined over an extended period of time that the CX C17 is the most cost-effective choice between new or existing military airlifter designs to meet most of the airlift shortfalls. If an aircraft other than the CX is to be selected, then industry should be given the opportunity to compete on aircraft of that size and capability.

"Failure to do so could constitute sole-source awards for multibillion-dollar systems without permitting industry to submit proposals for an aircraft with different size and capabilities than the CX."

William H. Taft IV, Pentagon general counsel, reportedly warned his superiors that competing contractors might be able to stop the C5A purchases on that ground unless the specifications were written to fit only the C5A.

Air Force sources said yesterday that their leaders wanted the C17 transport plus KC10 tankers but were told by Carlucci and Richard D. DeLauer, Pentagon research chief, to choose between the C5A plus KC10 tankers and money to militarize civilian airliners for emergency use, as well as a lowly funded C17 development program.

They chose the C5A-KC10 package, expected to include about 45 C5As and 40 KC10s for over $10 billion if Pentagon civilians prevail.

"This way," said one embittered general, "they can say it was our choice."