What happened
On February 7, 2015, a Robinson R66 helicopter, registration HB-ZOK, was conducting a flight training mission originating from Bern Airport. After performing several external landings, the flight instructor directed the aircraft toward the Düdingen region to practice autorotation maneuvers.
Following an initial successful practice from an altitude of 1,000 feet, the crew initiated a second autorotation attempt. During the flare phase of this second maneuver, the flight instructor adjusted the collective pitch to prevent the rotor RPM from overspeeding. During this intervention, the instructor noticed that the turbine engine was no longer producing power. Despite the loss of power, the crew managed to perform a controlled landing in an open field, resulting in no damage to the aircraft and no injuries to the two crew members.
The investigation
Following the incident, the flight instructor inspected the fuel supply and its regulation mechanisms after consulting with the maintenance provider; no mechanical defects were found. The crew subsequently restarted the engine on the ground, verified all parameters were correct, and returned to Bern Airport safely.
An investigation by the SUST examined the aircraft's technical condition, the fuel quality, and the environmental conditions at the time of the event. The investigation confirmed that the aircraft had no technical malfunctions and that the fuel quality was within standard specifications. However, investigators noted that the ambient ground temperature was approximately -4°C and that no fuel anti-icing additive had been used in the fuel supply.
Findings
- The engine failure was highly likely caused by the lack of fuel anti-icing additives during operation in cold temperatures.
- The engine manufacturer warns that at temperatures below 4.44°C (40°F), the use of anti-icing additives or other ice elimination methods is required to prevent engine flameouts.
- The engine manufacturer specifically recommended that the operator implement the use of fuel additives to mitigate the risk of flameouts in cold weather.