N17NK

On April 8, 2023, at about 1937 mountain daylight time, an experimental amateur-built Van’s RV-7, N17NK, was substantially damaged when it was involved in an accident near Morgan, Utah. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

According to the pilot/builder, the airplane departed for a local flight about 1916. About 10 minutes into the flight, the pilot noticed the odor of burning oil when the cabin heater was turned on, so he turned back to the departure airport. About one minute later, the pilot noted “some light but not normal vibration in the airframe.” Seconds later, the airplane shuddered, oil covered the windscreen, and the engine lost all power. The pilot estimated the nearest airport was beyond the glide range of the airplane, so he chose to land in an open field. During the landing roll, the airplane’s main wheels sunk into deep snow, and the airplane nosed over. As a result, the airplane sustained substantial damage to the fuselage and empennage.

Photos of the airplane at the accident site revealed the propeller assembly had separated from the engine. The propeller assembly was later recovered from a field about two miles from the accident site.

Data retrieved from two panel mounted flight displays revealed the cylinder head temperature, exhaust gas temperature, oil temperature, oil pressure, fuel pressure, and rpm were consistent with normal engine operation until 19:28:15. Between 19:28:15 and 19:28:18, engine rpm increased from about 2,300 rpm to 2,841 rpm before decreasing to 0 rpm 2 seconds later, as seen in Figure 1.

Figure 1: A portion of recorded engine data.

A National Transportation Safety Board materials laboratory examination showed that five of the six propeller attachment bolts remained connected with safety cable within the bolt holes in the hub. The sixth bolt was missing. The fracture surfaces of all five remaining bolts exhibited features consistent with progressive cracking, crack arrest marks, ratchet marks, and multiple thumbnail shaped cracks. Two bolts displayed multiple fatigue crack initiation sites. Further examination with a scanning electron microscope did not reveal any corrosion pits or voids, and although there were some dissimilar materials observed, they were inconsistent with inclusions during the manufacturing process. Overall, the fracture surfaces were consistent with fatigue that had initiated in multiple sites on each bolt and propagated across the bolt cross sections until the remaining material succumbed to overstress fracture. Two of the five examined bolts displayed more extensive fatigue than observed on the other bolts.

Review of the airplane’s maintenance records revealed it had flown three hours since its most recent condition inspection. During the condition inspection, which was performed by the pilot/builder, brass flakes were found in the oil sump and filter. After correspondence with a maintenance consultant, the pilot intended to fly the airplane an additional 10 hours to monitor the engine’s performance.

The pilot stated that during the condition inspection, he removed the propeller to clean sludge from the end of the crankshaft. He stated that he did not see any defects when he visually inspected 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 in the bolts.

The pilot stated he referenced the MT Propellers technical data included with the purchase of the propeller. He could not recall the torque value he used during the installation of the propeller. According to MT Propellers Operation and Installation Manual E-124, torque values for propeller attachment bolts are 63-66 ft/lbs.