A polite-seeming, casually dressed 26-year-old man plunged yesterday from the emergency door of a commuter airplane flying toward Washington from Harrisburg, Pa., at an altitude of 3,500 feet.

Pennsylvania State Police identified the man last night as Wilmer W. Stillman of Middletown, Pa.

A search for his body was expected to resume today in a wooded area near York, Pa., after being called off shortly after dark last night.

Authorities said they knew of no reason for the incident, which occurred "in a flash" about 1:30 p.m., 10 minutes after Pennsylvania Airlines flight 1231 took off with 27 passengers and a crew of three, according to airline officials.

As the twin turboprop plane took off from Harrisburg International Airport and headed south, the man, described as "very polite," sat quietly in the rear of the 30-seat aircraft, making small talk with his seatmate, according to Bill Clark, executive vice president of the airline.

Then, while the flight attendant was in the front of the cabin, the man unbuckled his seat belt, walked a few steps to the starboard emergency exit, unlatched it, and "pushed the door open and dove out," the airline official said.

The door is made to be easily unlatched, but at least 200 or 300 pounds of force are required to push it open while the plane is in flight, according to Clark.

With the plane climbing at about 140 mph, the 4 1/2-foot-high emergency door quickly slammed shut, Clark said.

Clark said Stillman, who wore a shirt and slacks, carried no luggage aboard.

After returning to Harrisburg, where passengers were interviewed by authorities, the plane resumed its trip here and arrived about 3:30 p.m., about 45 minutes late.

Pennsylvania Airlines operates as Allegheny Commuter service in association with USAir, Clark said.