With less than a month to go before its announced start-up date, New York Air today begins another round in its fight to run low-fare air service between Washington and New York.

Today's Civil Aeronatics Board hearing on the new airline's fittness to operate is one of three remaining obstacles the firm must clear before it begins service on Dec. 14. Besides the need for a fitness finding by the CAB -- something the board is expected to grant in time -- two pending lawsuits also stand in the way.

One of the suits, filed by Northwest Airlines, challenges the right of the Department of Transportation to implement a plan for allocating access to Washington's National Airport -- a plan that cleared the way for New York Air to begin its service to New York's LaGuardia Airport in competition with Eastern Airline's Shuttle. An oral argument on the suit will be heard Friday by the Court of Appeals in St. Louis.

The other filed by the Airline Pilots Association, the union that represents pilots of Texas International Airlines, an affiliate of New York Air. Both are subsidiaries of the holding company, Texas Air Corp.ALPA sued Texas International but seeks an injunction against the operations of New York Air, arguing that TI formed the holding company so that New York Air could hire new employes and not be bound to TI's higher-wage labor contracts. Such an action violates federal collective bargaining laws, the suit alleges. A hearing on the company's motion to dismiss the suit is scheduled for next week in a New York court.

If New York survives all three rounds without suffering a knock-out blow, it will begin service between Washington and New York with fares 20 percent to 50 percent below the Eastern Airlines' current shuttle price of $60 each way. New York plans to charge $49 each way for weekday flights until 7 p.m., and $29 each way for flights after 7 p.m. and all day weekends.

While the Eastern Shuttle schedules flights at every hour and guarantees a seat to everyone who is at the gate by the scheduled departure time -- sometimes running as many as 30 extra sections a day to accomodate pasengers -- New York Air will operate about nine flights in each direction each day using 115-seat DC9 aircraft. Reservations will be required. New York Air plans to offer free drinks during business hours and snacks at meal times; the shuttle offers no food or drink service.

Last week, the CAB acted to make sure that the beginning of New York Air's proposed service on Dec. 14 wouldn't be delayed by processing of its certificate applications. In a 4-1 vote, the board decided to direct the administrative law judge who hears the application this week to certify the hearing record directly to the board instead of taking the time to write his own recommended decision. The board will schedule a hearing and then make its decision. Without such action, the board's process might have run through the end of the year; New York Air filed its application in September.