What happened
On April 5, 2023, at St. Gallen-Altenrhein Airport, a ground collision occurred between a Schempp-Hirth Ventus cM motor glider, registration HB-222 and HB-RCO, and a Pilatus P3-05 aircraft.
While preparing for flight, the pilot of the HB-2223 attempted to start the engine. Although the propeller rotated, the engine failed to ignite. During the attempt, the starter motor remained engaged even after the pilot released the starter button, necessitating the shutdown of the ignition to deactivate it. The pilot exited the cockpit to inspect an electrical relay located beneath the seat pan. After reassembling the seat and turning the ignition back on, the engine unexpectedly started at high RPM.
Unable to control the surging engine, the pilot was forced to jump clear of the aircraft. The motor glider rolled across the apron toward taxiway "November," where it struck a Pilatus P3-05 (registration HB-RCO) that was taxiing toward the runway. The Pilatus P3-05 was occupied by a pilot and one passenger. There were no injuries to any occupants, though both aircraft sustained heavy damage.
The investigation
SUST examined the mechanical and electrical systems of the HB-2223 following the collision. The investigation focused on the aftermarket electrical modifications and the engine's throttle linkage mechanism.
Findings
- The investigation identified a faulty electrical installation involving an aftermarket relay. This relay, installed to allow the starter to be operated via an additional button on the control stick, had a short circuit between two contact wires. This defect caused the starter to activate automatically whenever the ignition was switched on.
- A mechanical failure in the throttle system contributed to the uncontrolled engine speed. The throttle lever was connected to two carburetors via a Bowden cable that split into two separate lines. At the junction, the cable sleeve for one carburetor was so loose that pulling the throttle lever failed to return the throttle butterfly valve to the idle position, leaving the engine running at high RPM.
Safety action
- The investigation concluded that the accident was caused by improperly executed installations within the motor glider's propulsion and electrical systems.