Atlantic Coast Airlines of Dulles yesterday reached an agreement to acquire up to 110 regional jets in a move that would allow the carrier to bring flights to communities that up until now had been serviced by slower turboprop planes.

Under the agreement, Atlantic Coast has signed a firm order to purchase 55 jets at a cost of $733 million from Fairchild Aerospace Corp. Atlantic Coast operates flights in the eastern and midwestern United States as United Express. Therefore, United Airlines must approve the contract.

"The under-50-seat planes . . . can serve communities that don't have excellent or competitive air service," said Rick DeLisi, a spokesman for Atlantic Coast. "This would be the largest order ever in our history."

Atlantic Coast plans to phase out its 19-seat turboprop airplanes and replace them with the "feeder" jets, which have fewer than 50 seats. The initial order consists of 25 328-JETs and 30 428-JETs, which can accommodate 32 and 44 passengers, respectively.

"Passengers have been very receptive to . . . regional jets," said Tara Hamilton, spokeswoman for the Metropolitan Washington Airports Authority. Atlantic Coast's agreement with Fairchild, she added, "is a good sign that Atlantic is going to be able to continue to grow."

Smaller regional jets have been a hot trend in the airline industry. However, for Atlantic Coast to operate the new craft as United Express flights, it must first clear one major hurdle: approval from United pilots.

In the past, pilot unions from many major airlines have resisted smaller feeder airlines and their pilots for fear that the growth of smaller carriers such as Atlantic Coast would infringe on their routes.

United currently has a clause in its contract with its pilots that allows its Express carriers (there are six) to operate only 65 regional jets with a maximum of 50 seats each.

Since those terms were negotiated in 1997, "the industry has seen the development of the smaller feeder jets," said Madison Walton, pilot spokesman for the ALPA (Air Line Pilots Association) unit at United Airlines. United approached the union in May with a proposal to include up to 284 regional jets of 44 seats or less.

Since then the two sides have been in negotiations to discuss the issue. Atlantic Coast is the first carrier to raise its number of orders past the 65 crafts stipulated by the agreement. If the pilots refuse to change the contract, Atlantic Coast could only operate the jets under its own name and not United Express.

It could be weeks or months before the pilots and United reach a decision. "This issue is big enough and important enough that it won't happen quickly," Walton said.