What happened
Following a propeller strike incident, the Bantam light sport aircraft, registration ZU-CZU, underwent a mandatory engine shock load inspection. During the repair process, technicians at Jabiru SA addressed corrosion on the propeller flange by removing the damaged material and repainting the surface. The engine and propeller were subsequently reinstalled by Micro Aviation SA. After only 10.6 hours of flight time following this maintenance, the propeller assembly severed from the propeller flange during operation. The pilot was able to execute a successful emergency landing, and the aircraft sustained minimal damage.
The investigation
An investigation into the failed propeller assembly utilized scanning electron microscopy and visual analysis to determine the cause of the separation. The examination of the hardware revealed that five of the six bolts used to secure the propeller had failed. The investigation focused on the assembly method, the condition of the bolts, and the state of the propeller flange. Metallurgical analysis of the remaining bolt showed evidence of extensive bending and stretching, while the failed bolts exhibited fatigue-related features, specifically beach-marks and striations, indicating that the bolts had been loosening during service.
Findings
Several critical factors contributed to the failure of the propeller assembly:
- Incorrect assembly orientation: The propeller bolts were installed with the threaded ends facing aft, which contradicted the manufacturer's prescribed methodology. This orientation likely made it more difficult to apply proper torque to the nuts.
- Absence of required hardware: The assembly lacked the prescribed Belleville washers, which are essential for maintaining consistent torque on the bolts.
- Improper torque application: Evidence of rotational marks on the aluminum spinner flange suggested that torque was applied to the bolt heads rather than the nuts. This method, combined with the friction from the steel bolt heads against the soft aluminum, likely resulted in the bolts being significantly under-torqued.
- Paint degradation: A thick layer of protective paint on the flange had broken down at the bolt locations. This degradation likely contributed to the loss of clamping force as the bolts moved within the flange.
Safety action
To prevent future occurrences, the following recommendations were established:
- Strict adherence to Original Equipment Manufacturer (OEM) instruction manuals regarding assembly methodology and bolt orientation.
- Compliance with standard engineering practices for torque application, specifically applying torque to the nut end of the fastener.
- Re-evaluation of the paint application process on the propeller flange by the OEM and the SACAA to ensure it does not compromise structural integrity.
- Implementation of a policy to avoid the reuse of propeller bolts and nuts once they have been previously installed and removed.