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AUTOROTATION By Jannie Matthysen Every helicopter pilot spends many hours during his PPL training trying to perfect autorotations. Many fail to comprehensively master the technique, and continue their flying activities convincing themselves that they'll be able to handle whatever technical emergency the Law of Murphy may throw at them. This trend is not limited only to low-time PPL's, but many experienced pilots fail to understand the logic and purpose of autorotations, or the correct techniques to successfully pull it off. Contrary to what many non-helicopter types think, the rotors of a helicopter continue to rotate following an engine failure. We don't fall from the sky in a blaze of fire when the engine produces a hiccup. During an autorotation, the engine is either not producing power, or is, during low power settings, automatically disengaged from the rotor system which leaves the rotor system to be propelled only by airflow passing through the main rotors from below. This airflow is merely a product of the helicopter descending. Note that the rotor system is propelled - we can still control the tail rotor which implies that we have full control in all axes: yaw, pitch, and roll. The autorotation can therefore be compared to a glide in a conventional aeroplane. The fact that we have full control in all axes means that we can turn, accelerate, decelerate, or, for a very limited period of time, even produce a climb! We will, however, be descending at a rapid rate without the luxury of engine power. This leads us to the area where most pilots get stuck. They believe that all autorotations should be plain vanilla, straight forward, missionary position. When asked what the autorotation parameters are, the standard answer is "wings" level, 60 knots, with rotor RPM in the green arc. This is correct, but it only paints half the picture. When asked what the minimum autorotation speed is, the uncertain guess is always: "around 50 knots". The truth of the matter is that we can autorotate at any speed from zero knots through to the maximum autorotational speed for which a specific type of helicopter is certified. Even an autorotation in rearward flight is possible! The maximum speed is normally limited due to the fact that insufficient vertical airflow is present at high speed in order to propel the rotor system. Speed can be a valuable tool in autorotation. A higher speed dramatically stretches the autorotational or gliding distance in most types of helicopter. The next interesting point is that we could use rotor RPM to our advantage during autorotation. Low rotor RPM (within limits) can be achieved by applying collective pitch, which in turn produces an increase in lift. This can reduce the rate of descent or extend the autorotational distance. A successful autorotation is characterized by a controlled landing in a specific target area. In order to get to that target area, we may be required to execute a combination of turns and vary the rotor RPM, or indicated airspeed. Why are we then taught to autorotate at 60 knots if helicopters can autorotate at virtually any speed? The recommended autorotational speed of most helicopters closely matches its minimum rate of descend speed. This speed gives us the maximum time in the air in order to maneuver for a landing. The most important reason to maintain airspeed is to produce an effective flare at the end of the autorotation. During the flare, airspeed is used to slow the helicopter before impact with the ground. This deceleration occurs both in terms of downward velocity (vertical speed), and forward velocity. A very welcome by-product of the flare is an increase in rotor RPM. This occurs, in simplified terms, due to the rotor disc being exposed to a greater mass of airflow. The increased rotor RPM may then used to apply a greater collective pitch setting which reduces the vertical speed of impact even further. Statistics prove that engine failures account for a very low number of helicopter accidents. Why do we spend so much time practicing autorotations then? Take for example a soldier training for combat. His Sarge trains him to use his pistol, his rifle, hand grenades, smoke grenades, stun grenades, GPS, two-way radio, machine gun, and to top it all, he even fires a few rounds on the cannon! Great... the first time this soldier engages in combat and faces the enemy at close range, he uses whatever is in his hands at the time and he keeps shooting! He would be better-off taking only one very big rifle that instills a sense of confidence and familiarity into combat thereby eliminating the confusion of choice under severe duress. The low-time helicopter pilot is very similar to this soldier. We teach him to identify engine failures, a whole array of warning lights, tailrotor pitch control failures, tailrotor drive failures, vortex ring, LTE, etcetera, and etcetera. The simple truth is that most pilots, when facing the stark reality of a real emergency, would have crashed before they have correctly identified the threat or taken appropriate action. In single engine helicopters an autorotation remedies almost all the catastrophic failures or emergencies that the pilot could encounter. A tailrotor control failure in a Robinson helicopter, for example, is not difficult to identify, but is almost impossible to effectively control by any pilot who has not extensively trained, and who is not extremely current for just such an emergency. The average PPL who flies once or twice a month has a very slim chance indeed of handling such an emergency. Just like the soldier, we hand this pilot only one weapon: the autorotation. The low-time pilot, who is competent and confident in handling autorotations, possesses a much better chance of surviving a critical system or component failure than a pilot who has only a vague idea or below-standard training in handling a number of specific emergencies. In an ideal world, all qualified pilots would be adequately trained to identify and correctly remedy a variety of specific emergencies. The sad reality is, however, that the average low-time helicopter pilot is no more than an occasional recreational aviator. In the moments of panic following a critical failure, the chances of correctly identifying the emergency and taking the appropriate action, are very slim indeed. Previous incidents prove that a well executed autorotation presents the pilot's best chance of survival. Stop questioning the value of autorotations and don't dread the prospect of practicing them. Ask your friendly instructor to take some time and teach you the correct autorotation techniques and the many variations thereof. It is one of the most valuable weapons in the arsenal of a competent helicopter pilot |
