I was only a few hundred hours into my life as a helicopter pilot when I received what was arguably some of the best training that any pilot could get. I was flying along in the right seat of an Airbus Helicopters AS350 when my partner, a veteran Vietnam U.S. Army instructor pilot, said, “Is that a deer down there?!” As I looked out the right window in the direction he pointed, he reached down and pulled the throttle to idle. The aircraft yawed, and very soon thereafter the low rotor horn blared throughout the cockpit as we began to lose altitude and rotor RPM.
My saucer-sized eyeballs snapped forward and made sense out of the decaying rotor and increasing airspeed. I instinctively lowered the collective, adjusted my airspeed to that recommended in the flight manual and regained RPM back into the high green range on the dial, silencing that obnoxious low rotor horn. All this happened very rapidly, with only a minimal amount of sheer panic. Now all I had to do for the next 10 seconds was wait for the ground to approach before I flared and cushioned.
With laser focus, I held airspeed and RPM rock-steady, proud of myself as I watched the big, beautiful sod farm ahead of us rise up higher and higher in my windscreen. Pride quickly turned back to panic, as my perfectly held autorotation was going to put us just short of a beautiful forced-landing area ahead. Doing my best, I flared at the top of the trees as my instructor re-introduced the throttle. We terminated in a power recovery and climbed back up to altitude.
“And you ain’t even getting shot at!” he exclaimed with a smirk.
Message received. Although I had made a good textbook recovery out of what was hopefully the closest I’ll ever come to an actual engine failure, there will most likely be other factors to continually deal with during your very short ride back to the ground (hopefully the least likely of which is being shot at). In fact, Murphy insists that your engine will fail at the least practical moment. For this reason, you need to be able to maximize command of your aircraft to the moment of touchdown.
While the textbook numbers are certainly a safe place to be for your autorotation, you do have some wiggle room that will allow you to choose and make it to your landing spot more precisely. Looking at a power-required-versus-airspeed chart for an average helicopter, you’ll see that airspeed for minimum sink rate occurs at the bottom, or “bucket,” of the curve, while the speed for maximizing glide distance occurs slightly faster, where a line drawn from the origin is tangent to the curve (max lift-to-drag ratio).
There is also an optimum rotor RPM that will help extend your glide distance. This optimum RPM is more difficult to calculate within the scope of this article. But generally, if you were to cut the rotor blade into lots of tiny slices along its span and maximize the lift of as many slices as possible, you’d get an additional decrease in descent rate and an increase in glide distance. Most light helicopters will benefit from slowing the rotor to a speed slower than normal power-on RPM to achieve this max lift condition (heavier aircraft may actually require faster RPM). But lower the RPM with caution. There is a point of diminishing returns, as descent rate will quickly increase if RPM is brought too low, and excessively low RPM can become unrecoverable. Designers do not provide this optimum glide RPM, but an instructor and student can enter autorotative descents at altitude and experiment with adjusting RPM in the green arc above and below that of normal power-on, noting its effect on descent rate.
While our options are often limited, we helicopter pilots do have a modicum of control over where we end up at the bottom of an auto. There is much more that can be discussed with respect to the aerodynamics of precision autorotations. But, of course, it starts with good decisions and can be expanded upon with thorough knowledge of the limits of your aircraft, and perfected with safe recurrent training. RWI