Saturday, September 1, 2007
Common Mistakes: Autorotation--Tradable Goods in the Marketplace of Physics
Wait a minute! What does all this mean to our pilot who has lost power and hears a low rpm horn? In general, very little. What is important are rotor rpm, airspeed, and altitude — all highly tradable goods in the marketplace of physics and essential ingredients for a successful autorotation. All too often I hear and see pilots focusing solely on "making the spot." While it is important to find somewhere appropriate to land, one must remember that a good autorotation must precede the landing!
Here on the sun-soaked prairies of Texas, we flight instructors at American Eurocopter perform hundreds of touchdown autorotations every year, training all and sundry, from law enforcement pilots to successful businessmen and -women. This provides the opportunity to observe many mistakes made during autorotation training. My fellow knights of the Grand Prairie traffic pattern and I have formulated a list of the most common mistakes we see, and some tips on how to correct them during this very important maneuver.
Let’s start from the top — 600 ft agl and 80 kt. Our goal: a successful practice autorotation to the ground. I emphasize the word "practice" as a reminder to the overzealous certificated flight instructor (CFI). We’re practicing autorotations, not engine failures over densely populated city streets. Being a conscientious CFI, I have already spent time with the aircraft sitting on the ground explaining what it is I expect from my student during the autorotation segment of the training period: who has the controls and when, what to do if an emergency power recovery becomes necessary, what to expect the aircraft to do upon the entry, and so forth. This, of course, should be followed by a properly executed demonstration.
We’re lined up and approaching the entry point. I tell my student to relax and remind him or her that helicopters like to be in autorotation. It is a natural condition for them. The moment has come: "3-2-1, enter."
The throttle is "chopped" and so begins common error No. 1: failure to properly lower the collective. Smooth is good, but all the way down is better! Remember, the purpose of lowering the collective is to remove drag. I find it helpful to aggressively finish lowering the collective for the student with a verbal and commanding, "Collective down!" This is probably the single most important mistake that must be corrected early. (CFIs should remind the student of this during the post-maneuver critique and again when about to enter the next autorotation. Don’t forget to cover the collective and pedals with your hands and feet carefully during the entry phase.)
A word about aft cyclic upon entry. My personal technique for the entry involves some cyclic flare. This lets me achieve my autorotational airspeed at the top, thus allowing me to keep my eyes outside the aircraft on the way down. An actual engine failure may require you to flare on entry to regain rotor rpm. However, the student should be reminded that aft cyclic upon entry could prove fatal in other autorotational situations.
Imagine climbing out on takeoff and passing through 200 ft at 60 kt when the need to autorotate arises. A cyclic flare here leaves you at 200 ft with no engine and no airspeed. In other situations, maintaining higher airspeeds should enable the pilot to stretch the glide.
There are always variables when it comes to autorotations. All this leads me to common error No. 2: forward cyclic. This error leads to a chain of events that could have been avoided with a discreet block of the cyclic with the instructor’s thumb or hand.
We begin to descend and the student recognizes the increase in airspeed and compensates with aft cyclic. This leads straight into common error No. 3: poor rotor rpm management. This can be a particular problem in the 180-deg autorotation, when rotor rpm builds rapidly in the turn and again when the student raises the nose coming out of the turn. The rotor rpm begins to climb and the student raises collective in an attempt to control it — usually too much. Now the rpm drops, down goes the collective and, of course, the nose. Airspeed builds and again, the student adjusts with aft cyclic, up goes the rpm again, and so on. The entire descent is spent with eyes inside and busy arms and legs.
Attitude is important, and the student can progress quite quickly with practice, with the instructor covering gauges (since rotor rpm can be heard and felt as well as seen) and blocking overly large control inputs. Remember, CFIs, there is a difference between blocking and taking control.) The student should be reminded to look at high rotor rpm not as a problem but more like an opportunity to reduce the rate of descent by using that extra energy. A gentle and slight raise of collective is enough.
The next area to discuss is the flare — how much, when to start it, and why? The FAA’s "Rotorcraft Flying Handbook" states a flare’s purpose to arrest descent rate and reduce airspeed. You can autorotate successfully without a flare, but only if you favor lengthy runs!
Common error No. 4: Not enough flare. This has two causes — not being aggressive enough or getting too low. Knowing when to flare is about the sight picture. If the ground is rushing up at you, it’s time to flare. Using benchmarks like tree or telephone pole height leaves too many variables, especially at night.
The flare’s depth depends mainly on meteorological conditions, namely wind velocity and density altitude. Your mindset should be: introduce the flare. If nothing happens, flare more. Make sure the collective stays down.
I have yet to hear or read any official statement that dictates that a flare is used to create a zero-ground run autorotation. For training purposes, a little ground run is advisable. The alternative is hard on the airframe.
If your aircraft requires an "initial pull," the CFI should take care to remind the student that this is a separate maneuver from the touchdown cushion. The instructor should be covering the collective carefully at this stage of the autorotation. Initial pull helps the aircraft level itself for touchdown. Most center-of-gravity locations end up forward of the mast. As the aircraft decelerates the c.g. is carried forward by momentum and rotates about the mast, pulling the nose down with it. See your applicable flight manual for landing attitude.
Finally, most common error No. 5: bad timing of the cushion. All too often, the student will try to time the cushion with the actual touchdown. That’s too early. We fall through and hit hard. If the cushion is too late, we simply hit hard. Neither is acceptable.
Ideally, the aircraft should touchdown at the top of our pull using all available energy. But experience has shown me that as long as we touchdown during a long, smooth pull, the desired outcome can be achieved regardless of whether we’re at the limit of collective travel or only half way up. Touchdown during the cushion rather than at it. If you’re leveling off too early, it’s vital to let the aircraft to settle before starting the cushion. Correct pedal input is also vital at touchdown, especially if practicing to grass.
In summary, an autorotation is undoubtedly a difficult, intricate maneuver involving a great deal of physics, aerodynamics, and limitless other variables. For the sake of creating a training environment that won’t overwhelm the student, the CFI should keep it simple. Advanced autorotation training comes later. Enter the autorotation at the same place each time. Achieve the immediate goals: airspeed and rotor rpm. These change little if attitude is maintained. Relax on the way down and look for flare height. When approaching that, look outside the aircraft. Introduce the flare and see how much you need. On level off, touchdown during the cushion rather than either side of it. Lastly, CFIs should remember that they have the opportunity to make a positive mark on the industry and lead by example.