What happened
On March 18, 2004, a Stinson L-5, registration I-AEEI, was performing a glider tow flight near the Aosta Regional Airport in Italy. While climbing at approximately 1,800 feet AGL near the Quart Castle, the pilot experienced intense engine vibrations. Approximately 30 seconds after the vibrations began, the propeller completely detached from the engine.
In response to the rapid increase in engine RPM, the pilot reduced the throttle to minimum and released the glider tow cable. The glider continued its flight normally, while the pilot managed to glide the aircraft back toward the departure airfield. The aircraft landed on the grass adjacent to the runway in the opposite direction of the initial takeoff. There were no injuries to the pilot or third parties, though the aircraft's engine and propeller sustained damage.
The investigation
The ANSV investigation focused on the mechanical integrity of the propeller assembly and the engine flange. The propeller, a four-blade wooden Hoffmann model, was recovered from a nearby wooded area. Examination of the propeller blades revealed damage consistent with an impact with the ground.
Investigators analyzed the engine shaft flange and the aluminum spacer. Significant damage from hammering and fretting was discovered on the contact surface between the engine flange and the aluminum spacer. Furthermore, the investigation found that five of the six bolts used to secure the propeller assembly had been sheared off at their threaded ends. One bolt remained in place but had been pulled out of its socket, causing deformation to the threaded bushing.
Technical analysis of the bolts by the Italian Air Force Flight Test Center confirmed that the failures were caused by a fatigue mechanism induced by alternating bending stresses on the bolt shanks. Evidence of dark dust accumulation on the propeller hub and the propeller spinner plate indicated that the components had been moving relative to one another for an extended period.
Findings
- The primary cause of the propeller detachment was insufficient torque applied to the propeller mounting bolts.
- This insufficient tightening may have resulted from an error during the last periodic inspection, improper calibration of torque wrenches, calculation errors during unit conversion, or dimensional changes in the wooden propeller hub due to environmental factors (temperature and humidity).
- The lack of a secure connection between the engine flange and the propeller hub allowed for cyclic bending stresses, which eventually led to the fatigue failure of the bolts.
- The presence of dark dust deposits and the removal of protective paint on the hub surface confirmed that the components had been in a state of relative motion for a prolonged period prior to the incident.