HILLSBORO, Ore. -- Icing in the carburetor likely caused a single-engine experimental plane to crash in a field south of Hillsboro last winter, the National Transportation Safety Board has concluded.

Pilot Dale D. Wilson, 54, of Aloha died in the crash. He was the only person aboard the plane that went down in heavy fog on Dec. 30. Wilson had radioed to the Hillsboro Airport control tower that he was having engine trouble, authorities said at the time.

Wilson, a groundskeeper at West Hills Christian School, was flying to Newberg to meet his wife and other family members for lunch.

The home-built plane flipped onto its top after striking the ground. Authorities relied on all-terrain vehicles and helicopters to reach the muddy, waterlogged field.

The NTSB said in a report formally issued May 31 that the plane, a Tessier Avid Flyer, lost power in weather conditions that commonly create the problem of carburetor icing.

An earlier report from the NTSB referred to documents from the Federal Aviation Administration that said Wilson's plane had a heating system designed to prevent carburetor icing, and that an examination of the plane's wreckage suggested the heating system was working fine.

The NTSB report added, however, that the FAA's figures showed that Nelson was operating the plane "in an ambient environment where serious carburetor icing could be expected to occur."

After the crash, Brent Wilson said his father had planned to sell the airplane because it was becoming too expensive to store.