What happened
During a climb to cruise altitude, the pilot observed that the left engine was overheating. Upon investigation, the pilot determined that the cowl plugs had been left in the engine's cowl opening. In response to the overheating, the pilot shut down the left engine and initiated a return to the airport.
While performing an instrument approach, the pilot attempted to extend both the flaps and the landing gear. While on a 1-mile final approach, air traffic control advised the pilot to perform a go-around because the landing gear was not in the fully extended position. The pilot reported that the aircraft had already passed mid-field before power was applied for the go-around maneuver. Consequently, the airplane continued to descend until it entered a stall/mush condition. The aircraft then collided with a taxiway and slid into a nearby grass area. There were no injuries reported.
The investigation
An on-scene investigation of the aircraft's mechanical systems revealed that the normal and emergency hydraulic systems showed no anomalies. The CO2-operated emergency landing gear extension system was found to be functional, though it had not been utilized during the event. Additionally, the flaps were found in the full 'DOWN' position.