New York Air said yesterday it will begin low-fare service between Washington's National Airport and Newark International Airport on July 15.

The announcement came a week before People Express, a recently formed Newark-based airline, is scheduled to announce the same starting date for low-fare service between Newark and Baltimore/Washington International Airport.

Service on the two airlines will almost double the current number of daily flights between the Washington area and Newark.

New York Air, which already operates 14 daily round trips between National Airport and New York's LaGuardia Airport, said it would operate six round-trip flights between Washington and Newark on weekends and five flights daily on weekends. As on its other routes, the airline will use a two-tier system, offering lower fares on weekends and at off-peak times during the week.

New York Air, a subsidiary of Texas Air Corp., which also owns Texas International Airlines, started up six months ago with its Washington-New York service; it now also serves Washington-Boston, New York-Boston, and New York-Cleveland.

On July 1, the airline begins service between New York and Louisville with an unusual $500,000 community pledge to support the new service.Under a program organized by the Louisville Chamber of Commerce, Louisville's travel agents are committed to purchasing and selling $500,000 worth of New York Air tickets between now and the end of the year. The money was guaranteed by the United Kentuckey Bank. "The money is nice, but it's the community effort which got our attention," a New York Air official said yesterday.

In other aviation news yesterday:

Texas International announced another "seat sale" offering discounts ranging from 50 percent to 76 percent off regular coach fares from 20 cities to travelers willing to accept certain restrictions. The special fares are available on TI's flights from BWI.

Mississippi Valley Airlines became the latest customer to order the new Shorts 360, a 36-seat airliner being produced by Short Brothers Ltd., the aircraft manufacturer based in Belfast, Northern Ireland.