A federal safety investigator at Dulles International Airport is trying to figure out why one of the landing gears on a United Airlines Express commuter plane collapsed Wednesday night, sending the plane skidding off the runway. None of the 31 people aboard was injured.

Minutes before landing, the pilot of the Saab 340 turboprop dropped the landing gear and noticed that the cockpit display showed that only two of the three gears were locked, a United spokesman said.

The pilot received permission from air traffic control to fly by the airport instead of land. As the plane flew past, air traffic controllers could see that the gear was down. "But they had no idea if it could hold," said United spokesman Jeff Green. The plane then flew for several minutes as the flight crew tried to troubleshoot in the air, he said.

When the plane landed at 9:45 p.m., the right main landing gear failed to lock in place and the plane skid down runway 19L and came to a stop in a grassy area off to the side. Because air traffic controllers were aware of the emergency, they notified responders at the airport, who met the plane.

Passengers were escorted out of the plane through the main cabin door and taken to the terminal. The plane had a lap child and 27 other passengers on board, as well as one flight attendant and two flight crew members.

United and Shuttle America, the airline that operated the flight, said the incident was an isolated one. Shuttle America began operating United Express flights a year ago, when United ended its agreement with Atlantic Coast Airways, which now operates as Independence Air.

"We're very confident in the safety of our program," said Warren Wilkinson, spokesman for Republic Airways, the holding company for Shuttle America and Chautauqua Airlines. "Shuttle America has a perfect passenger safety record."

United Express Flight 7564 departed at 7:14 p.m. from White Plains, N.Y., and was scheduled to land at 8:45 pm, the carrier said. It was the aircraft's eighth flight that day, having flown between Dulles and Allentown, Pa., Roanoke, State College, Pa., and White Plains without incident, Wilkinson said.

Shuttle America uses a combination of Saab 340 twin turboprops and larger Embraer jets. The Saab 340 has two main landing gears under the wings and a landing gear below the nose. Wilkinson said the airline plans to phase out its Saab 340s by the end of the year and replace them with 70-seat jets.

The National Transportation Safety Board sent an investigator to Dulles yesterday. Records show that the twin turboprop was built in 1985.