Tl Ultralight Sro STINGSPORT N898N accident near Longmont, Colorado

Probable cause

The failure of the wood propeller blades in flight. Contributing to the accident was the owner/operator’s failure to complete required inspections on the propeller within the specified interval, which reduced the likelihood of detecting the cracks and may have contributed to the crack growth.

NTSB analysis

While conducting maneuvers at 2,500 feet, the airplane began to vibrate violently. The pilot aborted the maneuver and retarded the throttle to idle; the engine experienced a total loss of power. The pilot executed a forced landing into a field. After the landing, the pilot and the designated pilot examiner noticed that two of the propeller blades had separated from the propeller hub. An examination of the fractured blades and blade sleeves revealed that blade A likely fractured due to progressive cracking at laminate bonds at the interface between the wood blade and the metal sleeve. The powdery wood observed in the slots and at the root end of the blade shank were indicative of rubbing wood as the cracks progressed. The blade B fracture was likely secondary and occurred due to the vibrations associated with the imbalance created by the blade A fracture and subsequent separation. The propeller log indicated that manufacturer-specified visual inspections and torque checks were not conducted at the specified interval of 150 hours but at intervals of about 203, 320, and 250 hours. The delayed visual inspections reduced the likelihood of detecting the cracks and the delayed torque checks might have contributed to crack growth in the blade.

Text from the NTSB. Open the NTSB docket for CEN12LA655 for the full record.