WPR24FA053

The separation of a propeller blade during the initial climb due to insufficient blade retention forces. Contributing to the accident was the propeller blade retention design, manufacturing defects that prevented the propeller from achieving adequate grip forces, the pilot’s decision to use the propeller with an untested engine combination, and the lack of initial and ongoing maintenance documentation provided to the pilot by the manufacturer.

Shortly after takeoff, during the airplane’s initial climb, a witness just below the airplane’s flight path heard the airplane “go quiet” before he saw white pieces fall from the airplane. The airplane continued to maintain a level flight attitude and he looked away. When he looked up again, he saw the airplane rapidly descending in a nose-down attitude before it struck the ground in the field across from his house and burst into flames. The airplane was not trailing smoke or vapors at any time before impact. Postaccident examination revealed that one of the airplane’s propeller blades had released from the hub in flight but its collar detached from the root and remained within the hub. The second blade root remained in the hub and its blade had broken off at impact. Examination of that remaining blade root revealed wear and fracture features consistent with the blade sliding outboard in the hub grip as the initiating event. This outboard movement likely caused an imbalance that led to vibration, wear, and release of the opposing blade followed by airframe vibrations so extreme that the airplane became uncontrollable before it rapidly descended to the ground. The initial movement of the blade was due to insufficient retention force between the blade root and the hub. Despite extensive thermal damage, no additional issues with the engine or airframe were found that would have precluded normal operation. The pilot had upgraded the experimental amateur-built airplane with a Lycoming four-cylinder, 160-horsepower, direct-drive aircraft engine and, about 8 flight hours before the accident, he installed a two-blade ground-adjustable uncertified composite propeller. The design of the propeller was such that the blade was retained in the hub by frictional clamping forces. The propeller blade root incorporated a collar, comprised of non-structural epoxy and a fiber layup orientation that did not provide for adequate strength in the spanwise plane. The collar was therefore only a positional reference locating the blade within the hub rather than a mechanical interlocking feature to help retain the blade if the frictional clamping forces of the hub were exceeded. Further examination revealed multiple voids and resin-starved regions within the retained blade root. These anomalies would have lowered the stiffness of the blade root, requiring more blade root deformation to achieve the required hub clamping pressure. The risk of cracking and crushing of the fiber plies was therefore increased, as was the subsequent loss of hub clamping pressure during operation. The liberated blade also exhibited a large, elongated void near its leading edge, which could also compromise blade root stiffness, frictional retention, and reduce the pullout force. The manufacturer of the propeller advised the pilot that although the propeller had been used with engines up to 170 horsepower, these were automotive engines and no data was available for the Lycoming engine. The automotive engine would have been fitted with a propeller speed reduction unit (PSRU). Although the accident engine’s power rating was comparable, it was direct drive and as such the propeller likely exhibited torsional excitation forces more extreme than if it had been installed on an engine with a PSRU. The propeller manufacturer did not make installation and proper bolt torque information available on its website, nor did it readily publish inspection and maintenance information. Evidence suggests that, if available, this information was not provided to the pilot with the propeller assembly. It is possible that with proper maintenance guidance, the pilot would have detected the impending failure before the accident. Several factors, therefore, could have contributed to the insufficient retention, including blade root design, blade manufacturing, engine application, and propeller installation and maintenance. Because of the multiple factors at play and their potential interaction, it was not possible to rule any of these out as contributing to the blade’s separation.

Construction of the airplane was completed in 2004. At that time, it was equipped with a six-cylinder Corvair automobile engine and a composite propeller manufactured by Warp Drive Propellers. In 2015, the engine was replaced with a four-cylinder 125 horsepower Lycoming O-290-G engine; at that time, according to the airplane’s maintenance logbooks, it underwent 5 hours of Phase II flight testing. The pilot, who was also a licensed FAA Airframe and Powerplant mechanic, purchased the airplane in December 2019; by that time it had accrued about 200 hours of flight time. Over the next two years, the airplane accumulated a further 200 hours and underwent a series of engine changes. By September 28, 2022, the pilot had reinstalled the original O-290-G engine, which had now been overhauled and upgraded to 160 horsepower through the installation of cylinder assemblies designed for the Lycoming O-320 engine. A month before the accident, during the airplane’s most recent condition inspection, the pilot replaced the propeller (a Catto Propellers fixed-pitch 76-inch two-blade model), with a three-blade, composite, ground-adjustable propeller manufactured by NR Prop. There was no evidence in the logbooks to show that the airplane had gone through a flight test after the installation of the upgraded engine and new propeller. Correspondence between the pilot and representatives from NR Prop indicated that the two-blade propeller configuration had been tested with Yamaha engines, which require a PSRU, up to 170 horsepower; however, NR Prop had no data for installation on a Lycoming engine. The pilot purchased the three-blade adjustable pitch SR-118 assembly, with a diameter of 2,024 mm, in addition to a two-blade hub that was also compatible with the blades. The SR-118-2000 is advertised on the NR Prop website for installation on engines between 90 and 150 horsepower. According to online forum correspondence, the pilot stated that when the propellor arrived it did not include installation instructions and he did not know the mounting hardware torque specifications. Review of subsequent correspondence between the pilot and the manufacturer indicated that he then received torque values for the propeller hub, but there was no evidence he received formal installation or ongoing maintenance instructions. A friend of the pilot stated that after installation, the engine would not reach its rated rpm. The pilot therefore decided to switch to the two-blade configuration, using the two-blade hub and two of the three blades. On December 8, 2023, about 1329 Pacific standard time, an experimental amateur-built Series 5 Kitfox, N66180, was destroyed when it was involved in an accident near Eloy, Arizona. The pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot had flown from his home base of Ryan Field (RYN), Tucson, Arizona, earlier in the day to have lunch at the Eloy Municipal Airport (E60) restaurant. The pilot told a friend, who flew alongside him on the inbound flight in another airplane, that the flight was uneventful. The accident flight was to be the return leg to RYN. ADS-B data revealed that after takeoff from runway 20, the airplane climbed to about 2,450 ft mean sea level (900 ft above ground level) before making a left turn to the south. About that time, a witness just below the airplane’s flight path heard an engine “sputter” and then go quiet. He was not concerned with the lack of engine noise, because he assumed they were practicing engine-out procedures into the local field across from his house. He looked up and watched as white pieces fell from the airplane. The airplane continued to maintain a level flight attitude and he looked away. When he looked up again, he could see the airplane was rapidly descending in a nose-down attitude. It then struck the ground in the field across from his house and burst into flames. The airplane was not trailing smoke or vapors at any time before impact. The airplane came to rest in a dirt field 1.5 miles southwest of the departure end of runway 20. The entire structure was consumed by fire with only steel airframe and burnt aluminum, composite, and flight instrument remnants remaining. The first identified point of impact was an almost-complete impression of the airplane’s forward profile in the dirt, which included the main landing gear strut, wheels, wing leading edges, and lift struts. The engine had separated from the firewall and was in the center of the impression. About 20 feet beyond the engine, both wings had come to rest in line with the impact point. The cabin and tail structure were crushed aft, such that the rudder pedals were comingled with the remnants of the empennage. A single intact propeller blade (figure 1) was located 1 mile southwest of the departure runway, in the general vicinity of the flight path. A section of exhaust pipe, along with various items of cabin contents, an iPad, and clear plexiglass fragments were distributed another 1/2 mile closer to the main wreckage site. All remaining primary airframe structure, flight control surfaces, and engine components, along with the thermally damaged second propeller blade, which had fractured from the propeller hub on impact, were accounted for in the main wreckage. The propeller hub remained attached to the engine crankshaft, and the root of the ground-impacted blade remained clamped within the hub. Examination of the engine did not reveal any catastrophic failures or anomalies that would have precluded normal operation. Figure 1- ADS-B flight path with debris field, and propeller blade as-found. An accredited representative from the National Transportation Investigation Bureau of Ukraine, which was the state of the propeller manufacturer, was assigned to assist with the investigation. The propeller blade and hub assembly, along with the unused third blade, were sent to the NTSB Materials Laboratory for examination. The manufacturer did not have construction or layup documents available, so the third blade was used as a reference. Propeller Design The assembly consisted of two composite propeller blades and an aluminum two-piece hub, clamped together by bolts. Blade retention was achieved through friction between the outer cylindrical surface of the blade and an inner clamping surface on the hub through inboard and outboard grip areas. A collar in the middle of the blade retention area slotted into a groove on the hub to locate the blade spanwise within the hub (figure 2). Figure 2 - Root of the exemplar propeller blade Exemplar Blade The root sections of the exemplar blade were examined by X-ray computed tomography (X-ray CT), revealing that it was constructed of composite fiber polymer matrix layers built around an aluminum tube that was flush against the root of the blade. The innermost layer consisted of unidirectional glass fiber wound circumferentially around the tube. The next layer consisted of unidirectional glass fibers oriented along the axial direction of the blade. The next layer comprised four plies of plain-woven glass fiber fabric with warp and weft at +/- 45°. The next layer was comprised of multiple plies of carbon fiber fabric, and the final outermost layer consisted of a +/- 45° glass fiber fabric ply. Within the collar feature, the fiber layers bowed radially outward, partially filling and forming the collar. The rest of the collar consisted of resin-rich wedge-shaped regions of epoxy along the upper and lower edges. In some regions, the excess volume between the bowed layers was filled by resin pockets or voids, and in other regions it was filled with unidirectional circumferential glass fiber (figure 3). The outermost fiber ply in this collar was severed by the collar machining process. Figure 3 - X-ray CT of the exemplar blade root and collar. Separated Blade The collar of the sep