Incident Overview
On February 7, 2015, at approximately 14:30 local time, a Robinson R66 helicopter (registration HB-ZOK) departed Bern Airport for a training flight heading west. After several external landings, the flight instructor decided to proceed to the Düdingen region to practice autorotations. Following a terrain reconnaissance, the first autorotation exercise was conducted from an altitude of 1,000 feet above ground level, after which a go-around was performed. A second autorotation was initiated from the same location. During the flare, the flight instructor intervened with the collective pitch control to prevent a rotor overspeed and noticed that the turbine engine had ceased producing power. He successfully performed a touchdown without damage in an open field.
After a telephone consultation with the maintenance organization, the flight instructor examined the fuel supply and its regulation mechanism, finding no anomalies. He then decided to restart the engine, conducting a ground check procedure. All parameters being normal, the crew flew back to Bern Airport without further incident.
Investigation Findings
The Swiss Safety Investigation Board (SUST) conducted a summary investigation. The helicopter exhibited no technical defects, and no irregularities were found in the fuel system. Analysis of the sampled kerosene confirmed proper fuel quality. At the time of the incident, the ambient ground temperature was approximately -4°C. No fuel anti-icing additive had been mixed into the fuel.
The engine manufacturer's warning states: "WARNING: AT TEMPERATURES BELOW +40°F (4.44°C) SOME TYPE OF FUEL ICING PROTECTION IS REQUIRED, SUCH AS AN ANTI-ICING ADDITIVE OR A MEANS OF AIRCRAFT FUEL ICE ELIMINATION. ENGINE FLAMEOUT COULD RESULT FROM FAILURE TO USE FUEL ICING CONDITIONS."
Rolls-Royce, the engine manufacturer, commented: "If fuel additives are not used when operating in cold temperatures there is a high probability of an engine flame-out. Rolls Royce would highly recommend that Mountainflyers change their practice and begin to use of fuel additives."
Conclusion
The SUST concluded that the engine stoppage was most likely attributable to the non-use of fuel anti-icing additive.
Probable Cause
The engine shutdown was most likely caused by the non-use of fuel anti-icing additive during cold temperature operations.