Accident Details
On December 9, 2010, at 1645 eastern standard time, a Flight Design GMBH CTLS, registration N30LK, operated by Zone Aviation, sustained substantial damage when it impacted terrain during a forced landing near Willard, Ohio. The flight originated from Union County Airport in Marysville, Ohio, at 1620 and was en route to Lorain County Regional Airport in Lorain/Elyria, Ohio, as a 14 CFR Part 91 personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The private pilot was uninjured, and the passenger sustained minor injuries.
Pilot Report and Engine History
The pilot, who had rented the airplane to accumulate cross-country flight time for an instrument rating, preflighted the aircraft and noted the oil was at the dipstick's upper quantity line with no mechanical anomalies. After takeoff, he climbed to 10,000 feet mean sea level (MSL) and then descended to 9,500 feet MSL, where he remained for about 10 minutes. A "snap or pop" sound occurred, followed by engine vibration. Carburetor heat application had no effect. Engine speed decreased from 5,150 rpm to 4,000 rpm with the throttle unchanged at approximately 85 percent. The engine then experienced a total loss of power, and the propeller stopped rotating during the descent and forced landing.
The airplane was powered by a Rotax 912 ULS2 engine, serial number 5651168. Its last 100-hour/annual inspection was at 503.7 hours on October 20, 2010. The last oil and oil filter change used Aeroshell Sports Plus 4 oil at 558.1 hours on November 30, 2010. Engine logbook records showed multiple oil and filter changes with various brands (Aeroshell Sports Plus 4, Penzoil 20W50 Motorcycle, and unspecified Penzoil). No engine component repairs or replacements were recorded prior to the accident, and the operator stated the engine had never experienced a propeller strike.
Examination Findings
Post-accident examination at the operator's base, supervised by the FAA, revealed the engine and firewall were intact and attached to the fuselage, though the fuselage sustained substantial damage. The crankshaft could not be rotated by hand with spark plugs removed. Spark plugs appeared consistent with normal operation, except for a damaged number 3 plug. Engine oil level in the dipstick and oil tank was within normal range. The oil filter contained no metallic debris. The cooling system was intact without leaks. The valve train and cylinder heads (except number 3) showed no signs of hammering.
The number 3 piston was fragmented. The exhaust valve was fractured at the head/stem area, and the exhaust spring was fractured in half. The cylinder wall was scored, and the piston skirt broken. The intake valve was intact except for a triangular piece broken off at the valve seat. The connecting rod was twisted, with the piston pin offset 3/4 inch from the rod. The crankshaft was twisted between the main webs. No blue discoloration from oil starvation was observed. The crankcase, main bearings, camshaft, and lifters appeared normal. Metal debris from the number 3 cylinder was found throughout the crankcase and gearbox.
Metallurgical Analysis
Metallurgical examination of the number 3 cylinder exhaust valve spring (part number 838155) identified a fatigue fracture caused by a non-metallic inclusion. The fatigue originated at the coil side face and covered about 25 percent of the material cross-section, initiating approximately 210 microns below the spring surface. The inclusion exceeded the metallographic wire rod inspection limit of 15 microns. Rotax cited the International Valve Spring Wire Manufacturers Association, noting that inclusions are unavoidable in steel and that modern high-quality steels may contain inclusions larger than 15 microns.