American airlines and airplane-makers are fighting to keep the values of their plane purchases confidential, fearing a government plan to disclose the information may make it harder to negotiate for the lowest price.

United Parcel Service Inc., United Airlines, Boeing, Airbus, and General Electric oppose releasing the data after 10 years of confidentiality, according to government filings and spokesmen interviews. Appraisers who estimate plane values for banks say disclosure would make investments in the equipment less risky and help stabilize the airline industry.

“We do not want our competitors to have this information, nor do they want us to have theirs,” said a United Parcel spokesman in Washington, David Bolger.

Airlines and manufacturers are seeking to prevent rivals from learning details of their negotiations on almost 300 planes delivered in America each year.

The Transportation Department is taking comments on the disclosure issue until December 27 and will make a decision early next year, said a department spokesman, David Smallen.

At an October industry conference in Miami, five appraisals of a 1990 Boeing 747-400 passenger plane ranged from $38 million to $54 million, said John Keitz, president of BK Associates, an appraisal firm in Manhasset, N.Y.

“That’s ludicrous,” Mr. Keitz said of the range, which was broad because “we don’t have any reliable source of recent sales information.” The disclosure of decade-old data would cut the range 25%, he said.

The Transportation Department has collected information on aircraft and engine sales since before deregulation in 1978.The government used the data to set ticket prices and now collects the information in its role overseeing the airline industry.

The department published the price data until UAL Corporation’s United requested confidentiality for a decade in 1993. A dozen carriers including AMR Corporation’s American, Delta Air Lines, Northwest Airlines Corporation, and Southwest Airlines later made similar requests. The department is now reconsidering a plan to release the 10-year-old data after the airlines objected.

United Parcel, the world’s biggest express courier, told the department it wants the data to remain secret for at least another decade. Foreign airlines would gain an advantage in negotiations for new airplanes because they don’t have to submit the information, the Atlanta-based company told the department.

United, the world’s second-largest airline, backs a two-year extension of confidentiality. The Chicago-based airline considers decade-old plane and engine values “highly sensitive and confidential,” a United lawyer, David B. Olaussen, told the department. Disclosure would show rivals “how far they could likely push Boeing or Airbus in negotiations,” he wrote.

Toulouse, France-based Airbus, the largest plane-maker, is “absolutely in agreement with airlines,” a company spokesman, Clay McConnell, said. Boeing yesterday told the department that aircraft transactions “are among the most sensitive in the business world” and that disclosure would harm manufacturing interests. General Electric, the largest jet engine maker, views the data as proprietary, said a spokeswoman, Deb Case.

The secrecy makes it more difficult to assess market volatility and risk, which discourages some investors from financing aircraft, said John Vitale, president of Chantilly, Va.-based AVITAS Incorporated, the world’s largest aircraft appraisal firm.

Airlines “may be short sighted” in opposing the data release, Mr.Vitale said. Airlines’ reliance on public markets for financing “necessitates far more free flow of information, and I think releasing the data would facilitate that.”