WPR24LA260

Separation of the propeller blades from progressive damage at the blade roots due to overtightening of the hub bolts.

The pilot reported that shortly after takeoff he heard a loud, violent bang that shook the airplane. He could see that the propeller was not moving and subsequently made a forced landing to a nearby beet field. After the airplane came to rest, the pilot observed that two of the three propeller blades had separated from the propeller hub. He later found one of the missing propeller blades about 3,000 ft from the main wreckage; the remaining propeller blade was not located. Examination of the propeller hub, the two recovered blades, and the blade roots revealed that all three blades had fractured at the retention grooves near the blade roots. Examination of the fracture surfaces revealed thumbnail cracks extending from the blades’ surfaces inward, consistent with fatigue failure. The examination evaluated whether the blades were installed in accordance with the propeller manufacturer’s instructions. The instructions called for the hub bolts to be torqued to between 75 and 100 in-lbs. The airplane’s logbook did not have records of the tightening torque used on the hub assembly. Although the hub bolt torque value was measured at 32 in-lbs., the measured torque was likely affected by the absence of two of the three propeller blade root cores. The gap between the two halves of the hub was measured between 0.016 and 0.022 inches, which is less than the manufacturer’s design specification of 0.030 inches, and consistent with excessive clamping force applied by overtightening of the hub bolts. All three blade retention bores in the hub showed a black residue, especially at the outboard end of each bore near the hub split line. This appeared to be material transfer from the blades’ polymer caps to the hub. The measured widths of the impressions on the caps left by the hub split line were also less than 0.030 inches, again consistent with overtightening of the hub bolts. The overtightened bolts would have applied excess compressive stress to the blade roots, resulting in damage that progressed to fatigue failure. Review of available maintenance records showed no evidence of propeller overhaul consistent with the manufacturer’s recommended time between overhauls for their propellers.

On July 23, 2024, about 0845 Pacific daylight time, an experimental amateur-built Kitfox Series V airplane, N53562, was substantially damaged when it was involved in an accident near La Grande, Oregon. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, the airplane departed runway 35 at the La Grande/Union County Airport (LGD), La Grande, Oregon. During the left turn to crosswind, a loud, violent bang occurred that shook the airplane. The pilot noted that the propeller had stopped, and he identified a nearby dirt road to land on. As the airplane approached the dirt road, he realized the airplane’s airspeed was too fast for the road and turned left to land in a beet field. During the landing roll in the field, the airplane nosed over and came to rest inverted. Postaccident examination at the accident site revealed that 2 of the 3 propeller blades had separated from the propeller hub and were not immediately located. The pilot reported that he located one of the two separated propeller blades about 3,000 ft from the accident site a few days after the accident. The ground-adjustable-pitch propeller consisted of a metal hub that held together three laminated wood blades. The root of each propeller blade was covered by an approximately 0.05-inch-thick polymer cap. According to assembly instructions provided by the propeller manufacturer, the blade roots are placed into the hub, and the hub halves are bolted together. After the blade pitch angle is set, the installer must torque the hub bolts to between 75 and100 in-lbs, leaving a gap of 0.030 inches between the hub halves. The instructions stated the bolt torque must never exceed 100 in-lbs, and warneds, “If overtightening occurs, possible blade damage to the root end of the blade may result.” The hub and the two recovered blades from the propeller assembly were shipped to the NTSB Materials Laboratory for further examination. The blade roots and polymer caps of the two separated blades remained within the blade bores in the hub. The spacing of the hub halves measured between 0.016 and 0.022 inches. A torque transducer applied to one of the hub bolts indicated the bolt was torqued to 32.4 in-lbs. Examination of the separated blades revealed they had fractured at the retention grooves near the roots. Additionally, the propeller blade that remained attached to the hub had begun to separate near the root. The fractures had features consistent with progressive cracking, such as thumbnail cracks with progressive beach marks. The polymer caps on all three propeller blade roots showed an impression left by the split line of the hub. These impressions were measured, and the average width of the impressions ranged between 0.024 and 0.028 inches. The polymer caps showed rubbing wear at the surfaces that contacted the hub. All three blade retention bores in the hub showed a black residue, especially at the outboard end of each bore near the hub split line. Manufacture date stamps on the blades revealed that the blades were manufactured in June of 2006. According to themanufacturer, their propellers should be overhauled every 500 flight hours or 5 years. Review of maintenance records showed no evidence of propeller blade overhaul since 2006. The most recent propeller blade installation was recorded in 2003, and the most recent annual condition inspection occurred on October 1, 2023. The airplane’s logbook did not have records of the tightening torque used on the hub assembly.