Accident Sequence
On April 8, 2023, at approximately 1937 mountain daylight time, an experimental amateur-built Van’s RV-7, registration N17NK, sustained substantial damage during a forced landing near Morgan, Utah. The pilot, who was also the builder, was not injured. The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91.
The pilot reported that the airplane departed for a local flight around 1916. About ten minutes into the flight, he detected an odor of burning oil when the cabin heater was activated. He decided to return to the departure airport. Approximately one minute later, he felt a light but abnormal vibration in the airframe. Seconds after that, the airplane shuddered, oil covered the windscreen, and the engine lost all power. The pilot determined that the nearest airport was beyond gliding range and chose to land in an open field. During the landing roll, the main wheels sank into deep snow, causing the airplane to nose over. The fuselage and empennage sustained substantial damage.
Propeller and Engine Data
Photographs of the accident site showed that the propeller assembly had separated from the engine. The assembly was later recovered approximately two miles from the accident location. Data from two panel-mounted flight displays indicated normal engine parameters—cylinder head temperature, exhaust gas temperature, oil temperature, oil pressure, fuel pressure, and rpm—until 19:28:15. Between 19:28:15 and 19:28:18, engine rpm increased from about 2,300 to 2,841, then decreased to zero within two seconds.
Examination of Propeller Bolts and Maintenance History
A National Transportation Safety Board materials laboratory examination of the propeller attachment bolts revealed that five of the six bolts remained connected with safety cable within the hub bolt holes; the sixth bolt was missing. Fracture surfaces of all five remaining bolts showed features consistent with progressive cracking, including crack arrest marks, ratchet marks, and multiple thumbnail-shaped cracks. Two bolts exhibited multiple fatigue crack initiation sites. Scanning electron microscope analysis found no corrosion pits or voids, and although dissimilar materials were observed, they were inconsistent with manufacturing inclusions. The fractures were consistent with fatigue that initiated at multiple sites on each bolt and propagated across the cross sections until the remaining material failed by overstress. Two of the five bolts displayed more extensive fatigue than the others.
Review of the airplane’s maintenance records indicated that it had flown three hours since its most recent condition inspection. During that inspection, performed by the pilot/builder, brass flakes were found in the oil sump and filter. After consulting with a maintenance consultant, the pilot planned to fly an additional ten hours to monitor engine performance.
The pilot stated that during the inspection, he removed the propeller to clean sludge from the crankshaft end. He reported no visible defects when inspecting the propeller and attachment bolts. He re-installed the propeller, torqued the attachment bolts with a torque wrench, and used a ratcheting safety cable tool to install .32-inch safety cable. The pilot referenced the MT Propellers technical data supplied with the propeller but could not recall the torque value used. According to the MT Propellers Operation and Installation Manual E-124, the specified torque for propeller attachment bolts is 63-66 ft/lbs.