Amateur-built BD-4 N931AS accident near Lowell, Indiana

Probable cause

Fatigue cracking of the propeller blade root, which resulted in the in-flight separation of the propeller blade from its root. Contributing to the accident was the improper machining of the blade root threads, which resulted in chatter/ratchet marks that weakened the strength of the blade root.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for CEN24LA038 for the full record.