Posted 6 years 7 days ago ago by Randy Rowles
Last week, I was speaking at a helicopter safety event when a young flight instructor asked me a question. His question began with a statement…”I was told that it was far safer to conduct operations requiring a hover in the Height/Velocity Diagram (HV Diagram) in a turbine engine helicopter instead of a piston engine helicopter”. The HV Diagram is a graph that provides the ability for a pilot to conduct pre-flight planning of a helicopter so if the engine fails in flight, the ability of the helicopter to complete an autorotation is known. The follow-up question was directed at me. “Which type of helicopter engine do you believe is safer and why”? he asked.
For a moment…a very short moment, I tried to think of an answer that connected the dots. Then it hit me…”does it really matter”.
Research has been done on the reliability variance between turbine and piston helicopter engines. I’m confident that a manufacturer from either type of engine would have the ability to debate this topic and provide poignant details supporting their perspective. But in the case of the HV Diagram, it is a moot point. A flight operation within the “shaded area” of the HV Diagram for any helicopter is a risk and should be avoided.
The HV Diagram is agnostic to the type of engine associated with a helicopter. Specific to a make and model of helicopter, the HV diagram is only a piece of information available to the pilot. When used properly, the pilot will know when the auto-rotational performance of a helicopter, during an engine failure, is exposed. Operations within the HV Diagram is a risk, thus the pilot must weigh the benefit of conducting such operations carefully.
In single engine helicopters, the HV Diagram is often located in the Performance Section of the Rotorcraft Flight Manual (RFM) or Pilots Operating Handbook (POH), as applicable. This would place the operation at the discretion of the pilot; a heavy burden of responsibility for sure.
Many segments of our industry operate in the HV Diagram for a majority of the flight. External load, utility, and power line construction are only a few examples. Are these mission segments considered unsafe? No. They do however require the pilot to have certain skill sets and experience to minimize the risk.
So, my answer to his question…”It Doesn’t Matter! I prefer to be in the helicopter being operated outside of the HV Diagram when the engine quits…no matter what type of engine is installed”.
About Randy: Randy Rowles has been a FAA pilot examiner for 20 years for all helicopter certificates and ratings. He holds a FAA Gold Seal Flight Instructor Certificate, NAFI Master Flight Instructor designation, and was the 2013 recipient of the HAI Flight Instructor of the Year Award. Randy is currently Director of Training at Epic Helicopters in Ft. Worth, Texas.