During arrival, the flight was issued the current altimeter setting. The pilot requested vectors for an approach to runway 18, but the ARTCC controller could not accommodate the request. The pilot was cleared for a VOR/DME-A approach. MDA for the approach was 1080 feet msl (600 feet agl). FBO personnel observed the aircraft fly over the airport and advised that it was too low. One of the pilots replied that they would be ‘staying under the clouds.’ The pilots had planned to land on runway 36, but reported they lost sight of the runway and would change to runway 18. Subsequently, the aircraft collided with a tree about 1 mile NNE of the airport, while on a westerly heading (approx position of base leg for a VFR pattern). The aircraft came to rest about 700 feet west of the tree and was destroyed by impact and fire. The weather was in part: scattered clouds at 200 feet, overcast at 900 feet, visibility 3 miles with rain and fog, wind from 360 deg at 7 kts. The airport manager estimated there was a 200 feet ceiling at the airport. PROBABLE CAUSE: “Failure of the pilot to follow IFR procedures and maintain the minimum descent altitude (MDA). Factors related to the accident were: darkness, the adverse weather conditions, and the tree.”