What happened
On June 12, 2017, at approximately 12:00 local time, a ground testing procedure was being conducted at Modlin Airport (EPMO). The testing, performed on the gyroplane SP-YLI, followed an approved program designed to verify the pre-rotation capabilities of the FC-4 rotor system. The objective was to ensure the rotor could reach speeds exceeding 250 rpm and achieve a target of 260 rpm.
During the process of increasing rotor speed, the flexible pre-rotation shaft failed while the rotor was between 230 and 250 rpm. The pilot immediately disconnected the pre-rotation system, shut down the engine, and brought the rotor speed down to approximately 120 rpm before bringing it to a complete stop. Following the incident, all further testing was suspended to allow for a technical inspection of the drivetrain.
The investigation
Upon disassembly of the drivetrain, investigators discovered significant damage to the flexible shaft core, which is constructed from spring wires. The investigation established that the wires within the core had snapped, and the resulting deformation of the core had also damaged the shaft's outer casing.
Findings
- The primary cause of the incident was exceeding the maximum torque limit designed for the flexible shaft.
- This torque overload was caused by accelerating the rotor to high speeds too rapidly during the pre-rotation process.