What happened
On 3 January 2019, a Pipistrel Alpha Electro 167, registration HB-SAA, was performing a private flight from Ecuvillens aerodrome. Shortly after takeoff, the pilot noticed abnormal readings on the engine parameters. Within seconds, the aircraft's motor power dropped precipitously from 75 kW to approximately 10 kW, accompanied by "ERROR" and "DRIT OVERTEMP" warnings on the cockpit display.
As the power controller began overheating, the pilot attempted to manage the descent and increase airflow to cool the propulsion unit. Despite attempts to reset the power controller by adjusting the power lever, the motor power remained critically low. Realizing that level flight was impossible, the pilot executed an emergency landing in a field near Corpataux-Magnedens. The aircraft struck the ground hard on its nosewheel, causing the nose gear to buckle. Although the pilot deployed the ballistic parachute system just before impact, the canopy did not fully deploy, and the aircraft flipped over, resulting in severe damage to the airframe. The pilot sustained minor injuries and was able to exit the wreckage alone.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the aircraft's flight data, the pilot's statements, and the technical components of the electric propulsion system. The investigation focused on why the motor power dropped so significantly and why the cooling system failed to maintain operational temperatures. Investigators also reviewed the structural integrity of the seat belt attachment points and the potential hazards posed by the high-voltage batteries during the emergency response.
Findings
- The primary cause of the power loss was a failure of the cooling system's circulating pump due to a faulty electrical connection.
- The lack of redundancy in the cooling system meant that a single component failure could lead to the overheating of the power controller.
- There were no cockpit warnings available to alert the pilot to the pump failure during the pre-flight inspection.
- The investigation identified that the left lap belt attachment point failed to withstand the forces of the impact, tearing away from the airframe.
- The high voltage and energy density of the aircraft's main batteries present specific safety challenges for emergency services during accident response.
Safety action
Following the investigation, the STSB issued several safety recommendations to EASA and FOCA, including the need for redundant cooling systems in similar propulsion units and improved structural standards for seat belt attachments. Additionally, the manufacturer released a service bulletin instructing pilots to verify coolant temperature increases during pre-flight checks to confirm pump functionality. The FOCA has also undertaken measures to increase awareness among aerodrome fire brigades regarding the specific hazards associated with electric aircraft accidents.