Incident Overview
On July 11, 2015, at 13:04 UTC, an Aerotechnik-Kunovice L 13 SE Vivat motor glider with registration OK-0101 experienced a critical incident at Klatovy Airport (LKKT). The aircraft took off from runway 27. Approximately 100 meters above ground level, the pilot heard a sudden increase in engine RPM and realized the propeller had detached. The pilot shut down the engine and executed a 180-degree turn to land on runway 09.
Damage and Pilot Status
The glider touched down with the landing gear retracted, resulting in contact between the rear fuselage and the runway surface. The aircraft subsequently nose-overed and came to rest approximately 100 meters from the runway threshold. The pilot, a 71-year-old male with valid qualifications, was not injured. Damage was sustained to the front and lower fuselage, the nose gear, and the engine's starter ring gear.
Investigation Findings
The propeller was found intact about 100 meters from the landing site. All six M8 bolts securing the propeller to the hub had fractured. Macroscopic analysis indicated fatigue failure with crack initiation in the same cross-sectional area for all bolts. The fracture surfaces showed no signs of corrosion. Additionally, the starter ring gear screws were sheared, and the locating pins were dislodged due to relative movement between the rotating components.
Technical Analysis
The investigation noted discrepancies in torque specifications between the propeller manual and the glider's technical description. While the maintenance organization claimed proper torque application using a calibrated wrench, the original approval documentation lacked specific load calculations for these bolts. Subsequent strength checks confirmed that the joint could withstand operational loads within specified torque ranges (12.5–23.5 Nm), but established a technical service life of 400 flight hours for the bolts.