NTSB Factual Report

On November 15, 2015, about 1445 eastern standard time, an experimental, amateur-built Sonex, N7035Z, operated by a private individual, was substantially damaged during a forced landing into trees, while on approach to Foothills Regional Airport (MRN), Morganton, North Carolina. The private pilot and passenger were not injured. The personal flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed for the local flight that departed Lower Creek Airport (NC27), Lenoir, North Carolina, about 1440.

The pilot reported that during initial climb from NC27, about 3,000 feet mean sea level, the propeller separated from the airplane and fell to the ground. The pilot turned the airplane toward MRN and attempted to glide to runway 21, but did not have sufficient altitude to glide to the runway and impacted trees about 600 feet short of the runway.

The pilot owned the airplane and performed the maintenance on it himself. He completed construction of the airplane in 2005 and it was issued a Federal Aviation Administration (FAA) experimental airworthiness certificate. The airplane was equipped with an Aerovee 80-horsepower engine. At the time of the accident, the airframe and engine had accumulated 278.5 hours since new and the airplane's most recent annual condition inspection was completed on April 14, 2015.

Examination of the airplane by an FAA inspector revealed that it came to rest nose-down in trees about 600 feet prior to the approach end of runway 21. During the impact, both wings and the fuselage sustained damage. The inspector added that review of the maintenance records revealed that the airplane had experienced a propeller strike on August 28, 2006, at 21.9 total hours since new. Following the propeller strike, the pilot replaced the propeller, but did not inspect the engine crankshaft. Additionally, the crankshaft was not required to be inspected, through such means as ultrasonic or magnetic particle inspection, on an experimental airplane.

NTSB metallurgical examination of photographic evidence revealed that the crankshaft fractured just aft of the propeller mounting hub/flange, allowing the propeller assembly to separate from the airplane. Examination of images of the fracture surfaces on the hub attached to the propeller revealed fracture surface features consistent with fatigue crack propagation through the wall thickness of the crankshaft and subsequent overstress fracture of the remaining portion of crankshaft.

According to a representative of the engine manufacturer, they did not publish guidance regarding the maintenance actions necessary following a propeller strike. When contacted by customers following such events, they routinely recommend some form of inspection relative to the severity of the event. For minor cases, a measurement of crankshaft run-out was recommended, with procedures regarding how to conduct the measurement published in a service bulletin. For more serious cases, the manufacturer typically recommended an engine disassembly, magnaflux inspection, and possibly a precautionary replacement of the crankshaft. The engine manufacturer could not locate any records documenting contact with the accident owner/pilot following the airplane's 2006 propeller strike.

According to FAA Advisory Circular 43.13-1B - Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair, "Every engine that suffers a sudden stoppage must be inspected in accordance with the manufacturer's maintenance instructions before being returned to service. If the engine manufacturer does not provide the required information, then the engine case must be opened and every major component part must be inspected using visual and/or nondestructive inspection procedures as applicable."

NTSB Probable Cause

The pilot/owner’s inadequate maintenance inspection following a propeller strike, which resulted in the subsequent in-flight propeller separation due to propagation of fatigue cracks in the crankshaft.