Amateur-built KITFOX accident near Eloy, Arizona, December 8, 2023

The NTSB's narrative final · quoted from the NTSB record

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.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.