Background
The multi-engine airplane was being repositioned to its base airport when the pilot requested a change in destination, but no reason was given for the alteration. Radar data indicated that the airplane entered the left downwind leg of the traffic pattern, maintained pattern altitude, then executed a right turn of approximately 250 degrees to align with the final approach leg.
The Accident
During the final approach, the airplane descended and struck a house in a residential neighborhood short of the runway. According to the airplane's pilot operating handbook (POH), the minimum multi-engine approach speed was 95 knots indicated airspeed (KIAS), and the minimum controllable airspeed was 82 KIAS. Radar data showed the airplane's ground speed was about 69 knots, with a probability of a direct crosswind.
Post-Accident Examination
Post-accident examination of the propellers indicated that both were turning at a low power setting at impact. A controlled test run of the right engine revealed a partial power loss. Inspection of the throttle and control assembly found two damaged o-rings. After replacing the o-rings with comparable ones and reinstalling the assembly, the engine operated smoothly without anomalies. The damage to the o-rings was consistent with being pinched between the corner of the top o-ring groove and the fuel inlet surface during installation.
It is probable that the right engine suffered a partial loss of power while on final approach due to the damaged o-rings, and that the pilot retarded engine power to prevent the airplane from rolling to the right. Investigators found no mechanical malfunction of the left engine that would have prevented it from maintaining published airspeed.
Probable Cause
The pilot did not maintain minimum controllable airspeed while on final approach with a partial loss of power in the right engine, which resulted in a loss of control. Contributing to the accident was the partial loss of engine power in the right engine due to the improperly installed o-rings in the engine's throttle and control assembly.