Enter Air North.

The airline, new to the Washington area, begins twice-daily service Monday between Washington's National Airport and Rochester with an intermediate stop in Binghamton, N.Y.

Air North was one of two new entrants whose service to National this month was cleared by the Department of Transportation after an airline scheduling committee failed to come up with its own formula for allocating flying hours at National after Dec. 1. The other new entrant, New York Air, plans to begin low-fare service between Washington and New York's LaGuardia Airport on Dec. 14.

Air North, based in Burlington, Vt., will use Gulfstream I-C aircraft from Gulfstream American on its Washington service. The I-C is a modification of the Gulfstream I, originally designed and built as a corporate executive transport, whose fuselage has been extended almost 11 feet to increase its seating capacity to 37 passengers for commuter-type service. It is powered by two Rolls-Royce Dart turboprop engines, is fully pressurized, and can cruise at a speed of about 350 miles an hour. The plane was certificated by the Federal Aviation Administration in October.

Although new to Washington, Air North is a regional airline that has been in business since 1967, and last year served 130,000 passengers. Air North operated as a commuter service for Mohawk Airlines beginning in 1968 and then carried on as a commuter carrier for Allegheny Airlines after 1973 when Mohawk was merged into Allegheny. Air North was part of the Allegheny Commuter system until it received certification from the Civil Aeronautics Board this year as an independent carrier. One of the smaller airlines that has been filling the void left by larger airlines dropping short-haul routes, Air North receives about $750,000 a year in subsidy to serve five small northern New York communities. $7

Predominantly serving New York State now -- 13 cities -- plus Burlington, Air North is adding Boston in addition to Washington this month. In addition to the Gulfstream aircraft, its fleet consists of four 30-passenger Shorts Brothers aircraft and two 16-passenger de Havilland Twin Otters. Its work force numbers 225 employes.