What happened
Two distinct incidents occurred involving an L 13 SE Vivat, registration OK-6118, operated by Skutečský Aeroklub s.r.o., within a short period in 2014.
On 12 April 2014, the pilot performed a landing at Skuteč airport with the landing gear retracted. The pilot failed to extend and verify the gear, and because they were wearing communication headsets, they did not hear the audible warning triggered by the use of air brakes while the gear was up. The aircraft landed on its belly, causing damage to the propeller and the lower fuselage.
On 14 June 2014, during a recreational flight, the aircraft experienced a loss of the propeller in flight. The pilot noticed a sudden increase in engine RPM and subsequently realized the propeller had detached. The pilot responded correctly by shutting down the engine and performing an emergency landing in terrain. The aircraft and crew sustained no injuries during this second event.
The investigation
The ÚZPLN investigation focused on the mechanical failure of the propeller assembly and the maintenance history of the aircraft. Investigators recovered the lost propeller (referred to as propeller No. 1) approximately two and a half months later. Technical analysis of the propeller revealed it was an unapproved, substandard part; it was constructed with an improper axial wood grain, was undersized, and had an amateurish surface finish.
Further examination of the propeller hub bolts showed that all six M8 bolts had sheared due to fatigue. The investigation also identified that the propeller mounting assembly was improperly installed. Specifically, a repaired spacer (vložka) was not flush with the hub, leading to a loss of clamping pressure and the eventual loosening of the assembly. Additionally, the investigation uncovered evidence of fraudulent practices, including the use of a falsified identification label on a second propeller (propeller No. 2) and discrepancies in the aircraft's maintenance documentation.