Two Montgomery County men were killed and a boy was critically injured when their twin-engine private plane crashed in a field yesterday evening as it was attempting to land at the Montgomery County Airpark in Gaithersburg, authorities reported.

Investigators said the plane crashed about 6:45 p.m. several hundred yards from the airport runway after one engine appeared to malfunction while the plane was approaching the airport at an altitude of about 800 feet.

One of the victims was identified as Henry J. Zuranski, 62, of the 7200 block of Connecticut Avenue, Chevy Chase. Police said he was the pilot of the plane, a Piper Apache.

The other victim was identified as his nephew, Christopher A. Zuranski, 19, of 8737 Sleepy Hollow La., Potomac. A neighbor said he was a student at Towson State University.

The boy, identified as Mark D. Zuranski, 8, also of the Sleepy Hollow Lane address, was taken by Maryland State Police helicopter to Children's Hospital in Washington. He was listed in critical condition last night.

County police said Mark and Christopher Zuranski were brothers.

All three occupants of the plane were pinned inside after it landed in a grassy field on the west side of the airport runway, near the 8600 block of Snouffers School Road. The airport is located in the northern part of the county, about 20 miles from downtown Washington. County police said that after the reported malfunction, the pilot of the plane tried to correct his course by bearing to the left. However, police said, the plane turned instead to the right and came down in the field.

The nose and right side of the plane apparently struck the ground first. When rescue workers arrived at the scene of the crash, 200 to 300 yards from the road, they found the nose of the plane crumpled. The instrument panel had buckled and part of the right wing appeared torn from the fuselage and bent.

The two men were reportedly in the front seats of the white-painted plane and the boy was strapped in behind them.

No fire was reported.

The cause of the engine malfunction was not immediately known last night.

The crash was under investigation by both the Federal Aviation Administration and the National Transportation Safety Board.

It was not immediately known where the plane was coming from or how long it had been airborne. The FAA said that the plane was flying under visual flight rules, which do not require the filing of a flight plan.