The threat of a massive air travel tic-up during the holiday season was averted yesterday when air traffic controllers and the Federal Aviation Administration reached tentative agreement on a three-year contract.

"The American flying public can look forward to safe expiditious trips over the Christmas holidays," promised John F. Leyden, president of the Professional Air Traffic Controllers Organization, which bargains for the nation's 17,000 controllers.

PATCO, whose 14,500 members have been working without a contract since July, had threatened a slow-down during the holidays - the period of heaviest air traffic - if the impasse in negotiations continued.

In a slowdown, a pressure tactic the union has used in the past, the controllers increase the time allotted between take-offs and landings. The controllers have argued that they are doing so in the interest of safety. Slowdowns at key airports - New York, Chicago and San Francisco - can snarl traffic around the country.

Leyden, addressing 350 PATCO local representatives who had come to Washington to lend visible support to the union's bargaining team, said the contract was "the best ever in the federal sector."

FAA Administrator Langhorne T. Bond said through a spokesman that the agreement, reached after an 19-hour, all-night bargaining session Wednesday and yesterday, was "fair and equitable."

The FAA spokesman said "one of the pluses is that it will provide greater stability in FAA relations with PATCO." The new contract will extend three years, a year longer than previous ones.

Leyden said the contract, which must be ratified by PATCO's national membership, would:

Give controllers with seniority the right to transfer to equal-paying jobs at other airports and air traffic control centers that aren't as busy.

Continue the PATCO program under which controllers' are automatically released from their jobs when they represent the union during investigations of aviation accidents.

Guarantee controllers immunity from disciplinary action after they report any near-collisions between air-plains.

Give controllers the right to bid for similiar but higher-paying jobs at other airports.

The negotiations did not involve pay - only working conditions. The typical air traffic controller is paid $28.623 a year.

Talks between PATCO and FAA began last May but made no progress, the union's Leyden said, until FAA Administrator Bond and U.S. Transportation Secretary Srock Adams began to take a personal interest.

Bond met with Leyden a couple of times in recnt weeks, according to an FAA spokesman, and participated in the concluding all-night bargaining session until 3 a.m.

Leyden, who got a standing ovation when he announced the contract terms to his local representatives, also said PATCO's muscle flexing - the mass gathering of local representatives from around the country - got talks moving along.

"I was prepared to call for a job action the week prior to Christmas," he said. "I believe that threat had sufficient impact to give us some language that was reasonable."

The agreement has no effect on the dispute involving controllers at National Airport, who held a slowdown recently to underscore their efforts to upgrade jobs in the control tower from a GS 13 ranking to GS 14 - a change that would mean a $5,000 annual pay boost. The issues is before the Civil Service Commission.