NTSB Factual Report

On April 25, 2014, about 1742 central daylight time, a Flight Design GMBH CTLS light sport airplane, N106SF, was substantially damaged during a forced landing following a loss of engine power near Greenville, Texas. The pilot and passenger sustained minor injuries. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight originated from the Fox Stephens Field – Gilmer Municipal Airport (JXI), Gilmer, Texas, about 1635. The intended destination was the Denton Enterprise Airport (DTO), Denton, Texas.

The pilot reported that a routine maintenance inspection was completed earlier on the day of the accident. During the initial flight after the inspection, the engine exhaust gas temperatures differed by up to 150 degrees, although they remained within the specified operating limitations. He elected to divert to the Gilmer Municipal Airport (JXI), Gilmer, Texas. A mechanic reportedly informed the pilot that the EGT difference was not unusual for that particular engine. However, the spark plugs were removed and examined. Minor debris was removed from one of the upper, right spark plugs. Otherwise, the spark plugs exhibited normal wear signatures. The engine oil did not appear to contain any contamination. The spark plugs were re-installed and two subsequent engine run-ups were normal.

A third run-up was completed before takeoff from JXI; both the run-up and the takeoff were normal. As the flight progressed, the EGT indications were within limitations and, after 25 minutes, had equalized. About 5 minutes later, the engine and propeller "stopped abruptly." The pilot's attempts to restore engine power were unsuccessful. He initially planned to divert to the Majors Airport (GVT), Greenville, Texas; however, the airplane did not have sufficient altitude to reach the runway. He subsequently elected to execute a forced landing to an athletic field located about 3 miles north of the airport. During the landing, the nose landing gear separated and the airplane nosed over.

A postaccident engine examination was conducted under the supervision of a Federal Aviation Administration airworthiness inspector. The examination revealed that the no. 4 cylinder exhaust valve had failed near the valve head. Subsequent metallurgical examination determined that the failure resulted from bending fatigue. The no. 4 cylinder, piston, and exhaust valve head exhibited extensive gouging, scraping, and deformation consistent with secondary damage due to the failed exhaust valve. The damage also included a hole in the top of the piston with extensive secondary impact damage as noted. The tappets exhibited light to moderate spalling, with the exception of the no. 4 cylinder exhaust tappet, which exhibited severe wear. The wear had reduced the height of the exhaust tappet by more than 5 mm as compared to the intake tappet. In addition, the no. 4 cylinder exhaust valve cam lobe was severely damaged with large spalls visible on both the opening and closing sides.

At the time of the accident, the airplane, a Flight Design CTLS (s/n F-09-10-13), had accumulated 1,613 hours total time in service. It was powered by a 100-horsepower Rotax 912 ULS 2, 4-cylinder, reciprocating engine (s/n 6775304), with an identical service time. Maintenance records indicated that the engine complied with the requirements for an extended time-between-overhaul (TBO) of 2,000 hours. The most recent inspection was completed on the day of the accident at 1,608 hours time in service. The maintenance records noted that engine oil was changed at that time. The magnetic plug was noted as "dirty, but OK." In addition, the entry noted that an exhaust pipe leak on the no.2 cylinder was repaired and that two exhaust springs were replaced. There was no specific mention of any maintenance work to the no. 4 cylinder.

NTSB Probable Cause

The fatigue failure of the No. 4 cylinder exhaust valve due to excessive wear of the corresponding train tappet, which resulted in a total loss of engine power.