CEN24LA038

About 25 minutes after takeoff, the engine started to run rough and shake violently. The pilot performed an off-airport landing in a soybean field. Upon touchdown, the airplane flipped inverted, which resulted in substantial damage to the fuselage. When the pilot exited the airplane, he discovered one of the propeller blades was missing. The pilot had installed the propeller about 18 months before the accident but had to replace the blades after 12 hours of service because the propeller blades were backing out of the hub. The manufacturer sent the pilot three new blades, which contained retention pins. Metallurgical examination of the blade root of the missing propeller blade revealed multiple fatigue cracks that initiated at the inner surface of the blade root where thread roots intersected a radial hole for the retention pin; the radial hole for the retention pin was located where the composite section of the propeller blade integrated with the metallic root. The darker appearance of the thumbnail-shaped region and curved bands of progression marks emanating from the cracks suggest they had initiated and had been growing before the accident flight. Chatter and ratchet marks, most likely a result of machining, were seen on the threads of the retention pin holes. A cross-section examination of the remaining two propeller blade roots revealed similar fatigue cracks emanating from the retention pin holes. The cracks were not visible from the surface and therefore would not be seen during routine propeller inspections. It is likely that the combination of the chatter/ratchet marks and operational vibrations resulted in fatigue cracking and ultimately overstress failure, which caused one of the propeller blades to separate at the blade root.

On November 14, 2023, about 1353 central standard time, a Bede Corp BD-4 C, N931AS, was substantially damaged when it was involved in an accident near Lowell, Indiana. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. About 25 minutes after departure, the engine began to run rough and shake violently. The pilot performed an off-airport landing in a soybean field. Upon touchdown, the airplane flipped over, resulting in structural damage to the fuselage. After exiting the airplane, the pilot discovered that one of the propeller blades was missing. The root of the missing propeller blade was still inside the hub. The missing blade was never found. The airplane was equipped with an AXSport Aviation AX3-70 propeller. The propeller was advertised as an experimental 3-blade electric constant speed/variable pitch propeller designed for non-type-certified aircraft. The blades were carbon fiber reinforced epoxy composite structures integrated with an aluminum alloy root section that were assembled into an aluminum alloy hub. The root of the missing propeller blade, propeller hub, and remaining 2 propeller blades were examined postaccident. The metallurgical examination of the propeller blade root from the missing propeller revealed dark gray thumbnail-shaped regions and cracks that originated at the holes for the retention pins—specifically, the inner surface where the thread roots intersected the retention pin holes. Chatter marks, wavy surface imperfections usually caused by vibrations transferred from the tool-work interface during the machining process, were observed on the flanks (straight side) of the threads. Ratchet marks, slight vertical steps within a fracture, correlated with the presence of chatter marks. One of the remaining intact propeller blades was not damaged on the surface, and the other propeller blade exhibited partial chordwise cracks and spanwise separation consistent with ground impact. Visual inspection of both blade roots did not reveal any visible cracks. However, both blade roots were cross-sectioned during the metallurgical examination and several flat crack profiles were observed, extending toward the outer surface from the threads of the retention pin holes. Maintenance logbook records revealed that the propeller was installed in May of 2023. In August of 2023, all three propeller blades were replaced after only 12 hours of service. The owner stated that the original blades did not have retention pins, and the blades were backing out of the hub. The manufacturer sent the owner three new blades that contained retention pins. The new blades were installed, and the propeller was balanced, in accordance with the operator’s manual, to a value of 0.02 in per second. The blade failure occurred after about 33 hours of service.