CEN18FA074 · MD Helicopters Inc 369HM

The commercial pilot and the crewmember departed on a power line aerial observation flight. Security video showed the helicopter sat on the ramp, in falling snow, for over an hour before departing for the accident flight. While two people were seen in the video walking around the helicopter before the flight, the distance was too great to ascertain how much snow or ice may have accumulated in the engine inlet area or what, if any, preflight snow or ice removal actions were taken by either the pilot or the crewmember.

One witness in the area observed the helicopter descend at an angle before it impacted the ground. Recovered GPS data indicated that the helicopter was operating in right turns consistent with the slow, hover flight (line inspection operations) with at an altitude between 180 ft and 220 ft agl and an airspeed of less than 10 knots.

Satellite, radar, and weather observations near the accident site supported low clouds and visibility with light to moderate snow fall at the time of the accident. Weather models and advisories support the probability of structural icing in the area at the time of the accident. In addition, witnesses described the snowfall as moderate at the time of departure from the airport and at the time of the accident. Photographs taken by law enforcement following the accident further illustrate falling snow and flat light and or white out conditions at the time of the accident.

Ground scars and damage to the helicopter were consistent with the helicopter impacting the ground in a vertical and level pitch attitude. Further, the damage to the main rotor hub and main rotor blades was consistent with excessive coning and downward flapping of the main rotor blades. These characteristics are consistent with a loss of engine power or thrust on the main rotor system.

The RE-IGN bulb stretch signatures are consistent with activation of the engine ignition system, indicating that power turbine speed was low. The lack of stretch signatures on the flashing engine out caution panel light is likely due to the light's cycle stage at the time of the impact.

There were no mechanical anomalies with the helicopter, engine, or fuel system that would have precluded normal operations. There was no damage to the compressor, compressor front support vanes, or first-stages rotor blades that was consistent with hard body ingestion.

Investigators were not able to determine how much snow or ice accumulated on the helicopter while it sat on the ramp or how much snow or ice was removed before the helicopter departed. The helicopter was operating at a slow forward airspeed and in hover for most of the flight increasing the likelihood of snow and or ice accumulation in the engine inlet area during the hover inspection operations. According to the engine manufacturer, snow ingestion cannot be ruled out as it does not always produce visible damage to the compressor.

The helicopter was not equipped to prevent or mitigate the accumulation of snow in the inlet; only to prevent snow and ice from entering the engine through the fuel system. The manufacturer required the installation of the auto-reignition system in order to operate in snow and ice; however, the low altitude where the loss of engine power occurred was likely insufficient for a successful auto-relight and recovery of engine power.

Performance charts indicate the helicopter was operating within an airspeed and altitude combination which would make a successful autorotation difficult to perform. The flat light and/or white out conditions would increase the difficulty of judging altitude, depth, and distance during an autorotation.

Verbatim NTSB analysis and probable cause from the NTSB dataset