What happened
On July 11, 2015, at 13:04 UTC, an L 13 SE Vivat motor glider, registration OK-0101, was performing a climb from runway 27 at Klatovy Airport (LKKT). At an altitude of approximately 100 meters, the pilot experienced a sudden increase in engine RPM and realized the propeller had detached from the engine. The pilot, noting no prior vibrations or warnings, executed a 180-degree turn to return to the airport and attempted to land on runway 09.
During the descent, the engine stopped. The pilot was unable to extend the landing gear in time, resulting in the aircraft landing on its belly. The impact caused the rear part of the fuselage to strike the runway, leading to a bounce and a subsequent nose-over. The aircraft came to rest approximately 100 meters from the runway threshold. There were no injuries to the pilot.
The investigation
The ÚZPLN investigation focused on the mechanical failure of the propeller assembly. Examination of the propeller hub revealed that all six M8 bolts (part number 662 030) used to secure the propeller to the hub had been sheared off. The fracture surfaces showed clear evidence of metal fatigue caused by fluctuating loads.
Further inspection of the engine area showed that the starter ring gear had also been damaged, with all six M6 bolts sheared and the retaining pins displaced. The investigation also identified discrepancies in technical documentation regarding the required tightening torque for the propeller bolts between the propeller manual and the aircraft maintenance manual. Additionally, a strength test of a sample bolt from the incident revealed that it failed to meet the required minimum tensile strength of 900 MPa.
Findings
- The primary cause of the incident was the separation of the propeller from the hub during flight.
- The failure was driven by a progressive loss of clamping pressure in the propeller assembly, which led to increased bending stresses on the bolts.
- Fatigue failure occurred at the internal thread interface of the bolts due to combined tension and bending loads.
- The pilot's decision to return to the airport resulted in a landing with the gear retracted, which caused secondary damage to the fuselage and landing gear due to a pilot error during the final phase of the landing.
- Discrepancies existed between the propeller manufacturer's instructions and the aircraft maintenance manual regarding bolt torque and securing methods.