CEN25LA117 · Vans RV-7A

During the initial climb when the airplane reached 800 ft above ground level (agl), the airplane shuddered and shook violently. The pilot reduced the throttle, which reduced the shaking, then completed a forced landing in a field. After landing, he noticed that one of the three propeller blades had separated from the hub and was found nearby in the field. The airplane sustained substantial damage to the engine mount. Examination of the propeller assembly revealed that the No. 2 propeller blade likely failed due to fitment issues within the propeller hub, leading to operational vibrations to be transferred directly and unevenly into the blade root, which resulted in fatigue cracking and catastrophic failure. The primary feature that initiated the failure was a likely improperly machined bearing seat in the No. 2 blade slot. The initiation point of the primary fatigue crack of the blade root correlated with the missing region of the bearing seat. The missing bearing seat allowed the blade root fillet to contact the hub. The bearing seat inside the blade slot was the surface that the cartridge bearing radially interfaces with, permitting the centripetal load of the propeller blade to evenly transfer into the hub. In the slots for blade Nos. 1 and 3, this surface was present and appeared mostly consistent around the circumference of the lip. However, the No. 2 blade seat vanished at the 12 and 6 o’clock positions. This would concentrate the centripetal forces generated by the propeller blade to the corners of the housing shells, where cracking was evident in three of the four locations. It is unknown why the No. 2 blade slot would be machined differently than the Nos. 1 and 3 slots. Additionally, the fact that this feature was observed on both the forward and rear shells, which are different parts and would be run on different processes, suggested issues may be related to programming or fixturing of the parts. The fact that two occurrences were not detected before release of the product raises questions about the presence of a quality control system issue for the manufacturer. The propeller hub exhibited several additional features that exhibited undesirable machining characteristics. The internal threaded section of the blade root exhibited chatter marks running along its length. Chatter marks are typically undesirable machining marks that form when running a given cutting tool through a material faster or with more force than its ability to evenly cut through the material. This can lead to resonant vibrations in the tool piece and cause a wavy or uneven pattern in the work piece. In high performance components, these may create unaccounted for stress concentrations leading to fracture initiation and growth. While there was no evidence that these were a primary driver of the failure, they appeared to contribute to the fracture propagation. Additionally, horizontal machined features were observed on several of the slots, consistent with tooling damage or features occurring during the roughing process of the machining the shell. Similar features were present on both rear and forward shells to varying degrees of severity. It was unclear whether there was an intended purpose for them in the design, but their inconsistency suggested a machining disconformity. While there was no evidence that any of them directly contributed to the failure of the propeller blade, their presence raised questions about the manufacturers machining quality control processes. Machining surface quality control of both the components of the blade roots and the hub assembly was also a contributing factor. Initiation of the primary crack along a single machining mark suggested its coarseness created stress raisers leading to fatigue crack initiation. While no information was provided about what design requirements exist for these specific components, certificated aircraft propellers have strict machining and finishing requirements due to their high vibration environments and critical risks.

Verbatim NTSB analysis and probable cause from the NTSB dataset