A twin-engine Army airplane crashed into woods in a dense blanket of fog near Davison Airfield at Fort Belvoir yesterday morning, killing all four servicemen aboard.

The plane, a twin turboprop from Fort Bragg, N.C., slammed into a wooded area about a quarter-mile from the field's runway just north of U.S. Rte. 1 at 6:47 a.m., according to Army officials. Aboard were the pilot, copilot and two passengers, one of whom was an officer.

Witnesses reported hearing a loud explosion when the plane went down. "There was one great big kaboom," said Maj. Joe Bowab, who said he was outdoors at the base when the crash occurred.

Authorities at the airfield said the plane, an Army U-21 Alpha, an executive airplane that is the military version of the civilian Beech King Air, apparently was on an instrument approach.

They said visibility at the time -- one mile, with an estimated 100-foot fog ceiling -- was below minimum standards for a safe landing, but that the pilot would have had the option of descending to 200 feet to survey runway conditions. The field, located southeast of Shirley Highway , was closed immediately after the crash, officials said.

Four safety experts from the Army Aviation Center at Fort Rucker, Ala., went to Fort Belvoir to determine the cause of the accident. Officials said the charred and scattered debris will be examined and a recording of communication between the pilot and the tower at the airfield will be played. The recording was made in the tower; the plane was not carrying a cockpit voice recorder.

At the crash scene yesterday, officials said they did not know what caused the plane to go down. "At this point, there is no indication of any trouble before" the crash, said Lt. Col. Miguel Monteverde, an Army spokesman.

The plane, which could carry four passengers in addition to a crew of two, left Fort Bragg at 5:34 a.m. and was due at Davison about 7 a.m., officials said. A spokesman for Walter Reed Army Medical Center in Washington said it was a regular Wednesday flight to shuttle doctors from Walter Reed to the North Carolina post.

The plane crashed into the woods from the southeast, shearing off tops of trees in its path and scattering debris over an area of more than 1,000 square yards. There were unconfirmed reports that witnesses saw a fireball above the treeline at the time of the crash.

Four Walter Reed physicians who were scheduled to take the plane's return flight to Fort Bragg were at the airstrip when the crash occurred and rushed to the scene to see if they could help.

One of them, Lt. Col. Allan Glass, said he was sitting in the terminal reading a newspaper when "something shook." When he ran to the scene, he said, he found smoldering fires around the wreckage of the plane and smoke. He said three of the bodies were in a pile, and the fourth was off to the side.

Reporters led to the scene yesterday afternoon saw a charred heap of twisted metal in a thick forest of oaks and poplars. The wreckage was not distinguishable as an aircraft. A bright yellow hunk of the fuselage, evidently torn off as the plane crashed into the woods, was perched about 30 feet up in one of the trees. Debris was strewn all around.

The plane is commonly used in the Army to ferry people and small cargo, officials said. A spokesman for Beech Aircraft Corp. said planes like the one in yesterday's crash have a "very reliable" safety record. Beech started producing the plane in 1967, a company spokesman said.

Davison Airfield, located about 15 miles south of Washington, generally is used as a base from which to ferry military officers from out of town around the metropolitan area.

The four men killed in the crash were identified as Chief Warrant Officer Richard N. Blunt, 44, the pilot, of Richmond; Chief Warrant Officer 3 John D. Hensley, 39, the copilot, of Cameron, N.C.; Lt. Col. James E. Palmer, 44, of Franklin, N.C., and Master Sgt. William M. Reeves, 46, of Palatka, Fla.