Amateur-built Series V N53562 accident near La Grande, Oregon

Probable cause

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

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for WPR24LA260 for the full record.