For example if we should hover at 50 feet above ground level and experience an engine failure by the time we've lowered the collective pitch and moved the cyclic forward to gain speed we would run out of height before we attain the desired speed rotor rpm combination for a stabilized autorotation as per the manufacturer's recommendation. By the way a lot of books and pilots refer to this graph as the "dead man's curve".
"So what now?" a lot of students ask me. I believe that in order to utilize a helicopter for a certain mission one will and have to spent time in the "dead man's curve". Let's look at a couple of examples: power line maintenance; game capturing; logging; law enforcement and even general flying when one does any form of confined area landing. Obviously there are many more examples. My answer to students is normally plane and simple: "don't spent time in the dead man's curve if it's not necessary". If your work or mission dictates that you fly there by all means but don't make it a habit for your every day flying.
Why then does the manufacturer put the graph in the Pilot's Operating Handbook, normally with the words: "avoid flight in the shaded area of the height velocity diagram"? One reason might be to "cover" themselves, because invariably any engine failure that occurs while the helicopter is in the shaded area of the height velocity diagram results in serious injury or even fatality. I am by no means insinuating that's the only reason.
A last thought on this subject, if a person is VERY concerned about flying in the height velocity diagram I think one should consider in these modern times how many engine failures occur in a helicopter. I personally believe that most helicopter accidents are due to pilot error and not to engine or mechanical failure. I can't emphasize the fact more that if you don't need to spent time in the dead man's curve then don't and if you do try and minimize your time spent there.
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