Three beleaguered airlines -- United, Pan Am and Eastern -- have a potential $100 million or more riding on a seemingly minor amendment to an obscure tax bill.

The amendment, sponsored by Sen. Robert Dole (R-Kan.), chairman of the Senate Finance Committee, would benefit airlines taking advantage of the final year of corporate tax "leasing" in the event of their bankruptcy.

The amendment provides, in effect, a form of insurance for companies that purchase tax deductions from the airlines under the tax leasing program, and this protection, in turn, makes the transactions more valuable to the airlines. If the airlines went bankrupt and their planes were sold overseas -- a likely result -- the leasing companies could lose some of their valuable tax breaks under current law. The amendment is designed to prevent that loss.

For Eastern, the leading proponent of the Dole amendment, it would mean significantly reduced costs for the planned acquisition of 20 new $30 million aircraft.

Russ Upshaw, Eastern vice president for government affairs, said he had no estimate of the net potential savings, but congressional aides said the figure could range from $40 million to $80 million. "With the specter of bankruptcy, you pay a premium," Upshaw said.

Pan Am, according to Rosemary Murray, the airline's systems director for government affairs, is seeking to make the Dole amendment retroactive. That would allow Pan Am to regain control of $35 million it has been required to put into an escrow account for at least five years as a condition of one of its corporate tax leases.

United Airlines has also been seeking approval of the Dole amendment, but no one was available at the firm's office late yesterday afternoon.

Earlier this year, Dole led the fight to phase out corporate tax leasing, and a number of persons were surprised to see him emerging as a proponent of the pending amendment. An aide said he was persuaded that Eastern faces a serious threat of bankruptcy without the additional money.

If an airline goes bankrupt and aircraft originally purchased through corporate tax sales is sold abroad, the tax breaks under the leasing program are invalidated, and the company that bought them faces a sharply increased bill from the Internal Revenue Service.

To protect against this event, companies buying tax breaks have been requiring the financially troubled airlines to put up a separate escrow account, as in the case of Pan Am, or get less money for their tax breaks, as in the case of Eastern.

The Dole provision would eliminate this threat, allowing the tax breaks to remain "alive" even if the aircraft is sold abroad.

The Dole proposal faces a tough fight in the House, however. It is opposed by Texas Internationl Airlines, which does not plan to buy any more aircraft through corporate tax leases. Key Texas members of the House Ways and Means Committee, including Rep. J.J. Pickle (D-Tex.), are objecting to the legislation.