US Airways Inc., taking advantage of its new, quieter aircraft, said yesterday that it will start 6 a.m. shuttle service between Reagan National Airport and New York's La Guardia early next year--giving it an edge in the battle for business travelers.

Federal noise regulations currently bar aircraft that exceed specified noise levels from taking off or landing before 7 a.m. or after 11 p.m. US Airways said its new Airbus A320 meets the noise standard and therefore can fly outside the time restrictions.

The new US Airways planes have what is known as a 75-decibel footprint on takeoff from Reagan National when the aircraft engines are at full power. According to the Labor Department's Occupational Safety and Health Administration, 70 decibels would be the equivalent of a noisy restaurant or highway traffic at close range. Because the new planes can gain altitude faster, the footprint of the engine noise is one-tenth that of the planes they are replacing.

Reagan National does not have a curfew on flights for aircraft that meet the noise standards. With a 6 a.m. departure, businesses might be able to save the cost of a hotel room for executives with early morning meetings.

The Arlington-based airline said it would begin 6 a.m. shuttle service at both Reagan National and La Guardia on Feb. 1. US Airways will also begin 6 a.m. shuttle service between La Guardia and Boston's Logan International Airport at the same time.

An airline spokesman said there were no immediate plans to extend nighttime service between Washington and New York beyond 9 p.m., currently the last shuttle flights offered from either city.

The early service was the latest move in an escalating shuttle war over Washington between US Airways, Delta Air Lines Inc. and United Airlines Inc.

Delta, the only other airline to offer direct hourly shuttle service to New York City, will not be able to compete for the 6 a.m. business until June at the earliest, when it is scheduled to take delivery of a new generation of quieter Boeing 737-800s which would allow it to meet the noise standards.

"With these new morning departure times between Reagan National and La Guardia airports, US Airways Shuttle passengers will be the first to arrive in New York and Washington each business day," said B. Ben Baldanza, US Airways senior vice president for marketing. He said the new service would give US Airways a "significant schedule advantage" for business travelers planning early morning meetings and same-day travel.

In May, US Airways announced that it was converting all nonstop flights from Washington to New York and Boston to hourly shuttle service and that it would replace its 30-year-old fleet of aircraft. In the preceding months, both United and Delta had said they planned service expansions from Washington to New York and Boston. The new United service to the two northern business hubs was part of its service expansion at Dulles International Airport and does not involve Reagan National.