According to the pilot during the turn from the base leg to the final approach the engine experienced a partial loss of power. The airplane impacted a tree causing substantial damage to the wing spars. A Federal Aviation Administration inspector examined the wreckage and noted that one spark plug was found backed out of the No. 1 cylinder and was suspended from the electrode wire. The pilot reported that he had changed the engine spark plugs the day before the accident and that he may have cross-threaded one of the spark plugs.