Fair-weather access to Washington's National Airport is so open that another 20 flights a day will make little difference, New York Air argued before the Department of Transportation in its attempt to win the right to begin new, low-fare service between Washington and New York.

The current DOT-adminstered airport policy obstensibly limits operational slots -- allowed takeoffs and landings -- to 60 an hour: 40 for the major commerical airlines, 8 for the commuter airplines and 12 for general avaiation (private planes).

But in its lengthy filing with the DOT, New York Air points out that the hourly operations at National routinely exceed the slot allocations through exceptions to the rules and the almost limitless flights permitted in and out in good weather by the commuter and general aviation classes of airport users:

Extra sections do not count in the slot allocations. Consequently, Eastern Airlines in 1979 flew 10,540 air shuttle scheduled flights which counted against its allocation of slots as well as 10,251 extra sections, which didn't count against its allocation.

Commuter airlines reguarly exceed what would be their hourly slot allocations if bad weather prevailed. In September, for instance, commuters exceeded the 128 permissible daily operations allocated by the policy on 21 days, often by 30 to 50 flights.

General aviation in September also went way over the 192 daily slots that sector would be permitted if the slot rule were enforced because of bad weather. On 21 days in September, the number was exceeded; on 16 of the days, between 300 and 393 planes took off and landed.

The new airline, an affiliate of Texas International Airlines, said that it would be indefensible for DOT to allow commuters and general avaiation to exceed DOT's hourly and overall limitations regularly and routinely under its high-density-airport rule or let extra sections be excepted, and at the same time deny travelers the benefits of the competitive service it plans "for the lack of just 20 slots."

New York Air's comments were being analyzed along with those of other airlines by the DOT staff yesterday in drawing up an options paper for DOT Secretary Neil Goldschmidt.