What happened
On August 24, 2016, a private VFR flight involving an amateur-built HB-Flugtechnik HB 207 Alfa, registration HB-YLO, experienced a sudden loss of engine power. While cruising near Schiers at approximately 6,250 ft AMSL, the pilot heard a loud, metallic noise from the engine compartment, immediately followed by a sharp increase in engine RPM to roughly 5,000.
Realizing the engine was no longer providing thrust through the propeller, the pilot initiated a glide toward Bad Ragaz airfield. During the descent, the pilot deployed the landing gear and flaps to increase drag, but later retracted them when the aircraft appeared too high for a direct approach. Ultimately, the aircraft touched down in a field approximately 250 meters short of the runway threshold at Bad Ragaz. The impact caused significant damage to the landing gear, propeller, engine cowling, and both wings, though the pilot escaped without injury.
The investigation
SUST examined the mechanical components of the propulsion system, specifically the propeller reduction gear. The investigation focused on the drive belts and the mounting hardware of the propeller carrier. Investigators analyzed the maintenance history, noting that the pilot, who was also the owner, performed annual inspections. Metallurgical analysis revealed that a component of the propeller carrier and one of the mounting bolts had suffered fatigue fractures. Furthermore, the investigation looked into the alignment of the drive belts and the specifications of the hardware used to secure the propeller assembly.
Findings
Technical analysis determined that the failure of the drive belts was caused by a combination of several factors. The investigation identified improperly executed maintenance work, unclear instructions regarding the alignment and tensioning of the propeller reduction gear drive belts, and the use of unsuitable material specifications for the propeller carrier mounting elements.
Specifically, the fatigue of a mounting bolt likely caused the propeller carrier to shift, leading to misalignment between the upper and lower drive belt pulleys. This misalignment caused the belts to run laterally against the pulley edges, generating intense friction and heat, which ultimately led to the thermal degradation and complete failure of both drive belts. Additionally, the pilot's flight tactics—specifically the decision to commit to a direct approach by deploying gear and flaps prematurely—contributed to the aircraft landing in unfavorable terrain, increasing the extent of the damage.