LAX07LA218 · Clement Zenith Zodiac CH601

The accident flight was the airplane's maiden flight where the pilot was to complete a series of shallow banks. After doing the predetermined maneuvers, which included four simulated approaches to landing, he descended to traffic pattern altitude. As he maneuvered onto the base leg, he reduced the engine power from about 4,500 rpm to 2,000 rpm. The airplane began to lose a significant amount of altitude, and in response, the pilot moved the throttle control forward. The engine then had a total loss of power. The airplane descended and impacted the roof of a private residence. Examination of the airplane and engine following the accident revealed the upper spark plugs were all a similar black coloration, indicating that the fuel/air mixture was rich. Additional evidence of a rich mixture was found with dark black residue lining the tail pipe exhaust. After starting the engine during a postaccident examination, the throttle control was manipulated and the engine responded appropriately. The engine emitted a thick white smoke during operation. Investigators noted that when retarding the throttle toward the idle position, the throttle arms did not move against the idle adjustment screw stops. Manual manipulation of both throttle arms to the carburetors' adjustment screws resulted in a loss of power. The same action was repeated four times, producing the same results. The right carburetor bowl contained a brown colored liquid similar in color to a maple syrup; the odor was consistent with a varnished fuel. The left carburetor bowl contained a small amount of fluid that was similar to that of the fluid found in the right carburetor, though a shade lighter in color. The fuel found in the center fuel tank was also varnished. In the 7 months prior to the accident, the engine had only been run on 11 occasions, all on the ground, for a total of 2 hours of operational time.

Verbatim NTSB analysis and probable cause from the NTSB dataset