Shortly after departure, when the airplane was approximately 250-feet above ground level, oil began to spray onto the windshield. The pilot checked the engine gauges and noticed a drop in oil pressure. As the pilot turned onto the crosswind leg of the traffic pattern, the engine started to operate abnormally and he performed a 180-degree turn back to the runway. The airplane touched down on the last 250 feet of the runway and proceeded through a fence. The airplane sustained substantial damage during the impact with the fence and came to rest upright, among several parked buses. A postaccident examination of the airplane by a Federal Aviation Administration inspector revealed that the engine’s oil cap was hanging by its chain not secured on the engine case. The cap’s gasket appeared worn; however, the inspector could not determine when the cap came off. The pilot reported that the engine was serviced with oil prior to the flight and that he thought that the oil cap was secured afterward.