Helicopter Flight Information
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Helicopter Flight Information |
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Loss of Tail Rotor EffectivenessMany students, and private pilots will say that they had LTE when they experience a difficult hover. That is incorrect, what they experienced was a high pedal work load brought on by the conditions that can lead to LTE. There is yet another issue which involves the critical wind azimuth which is addressed at the bottom of this article. There are four conditions that lead to LTE. Those four conditions are the loss of translational lift in conjunction with - a left quartering headwind (main rotor vortex interference), a left crosswind (tail rotor vortex ring state), or a tailwind (weather cock stability), or any combination of those conditions. Note that the loss of translational lift is one of the conditions conducive to LTE. Any combination of these wind conditions can cause even greater difficulty, and as a result LTE can occur very swiftly. Understand that LTE is just what it says, you don't have it until you have lost control of the helicopter about its yaw axis; that is that the helicopter enters an uncommanded turn to the right (in American made helicopters, European helicopters will be opposite). Understand that the condition of LTE is not one which demands that the helicopter will crash. A well trained pilot who executes proper procedures can come out of LTE without crashing the helicopter. If the pilot was not at least verbally trained in the proper procedures for getting out of LTE, the results may be ugly. The chance of full recovery before ground contact is slim without excellent piloting skills. The only hope for a good outcome is that you maintain a focus on the horizon just as you were taught in your training, and that you follow the recovery procedures. Many students and some low time pilots will state that they can control the helicopter better if they look down - this is a common error accompanied with a foolish statement made by a pilot who does not have the experience to make that judgment. If you make this mistake while experiencing LTE, you will crash. Awareness - Knowing the conditions conducive to LTE; when the helicopter is actually being flown in those conditions, or when it will be flying in the conditions conducive to LTE. Most importantly, the loss of airspeed (translational lift) compounded by the wind conditions conducive to LTE. Recognition - An uncommanded right yawing tendency which must be corrected within 30 degrees for best results if possible. Of important note however is the fact that LTE can be flown out of even after multiple spins. Interestingly enough, the helicopter does not continue to accelerate in a right spin. As the wind strikes the tail rotor, the helicopter actually speeds up and slows down twice each spin revolution; the speed of the spin is therefore limited. The speed of the spin will be directly affected by the amount of power (fuselage torque) being applied. Avoidance - Maintain airspeed above 30 knots (45-knots is better) until the helicopter is turned into the wind. Never commence a landing without knowing the winds! Recovery procedures - Ultimately the procedures you will use depends upon the aircraft you are flying and numerous conditions that may occur at the time. The simple and not always correct textbook answer is: Apply full left pedal, forward cyclic, and reduce collective if terrain clearance permits. Some of the conditions which may not permit the textbook recovery are: High gross weight and/or operation at high density altitude where if you apply full left pedal, rotor rpm decay occurs. You must never give up your rotor rpm! Rotor RPM is lift. Terrain clearance may not permit the lowering of collective which would reduce the torque spin you are experiencing. What is terrain? many simply think of terrain as the ground, but it is anything that the helicopter could strike such as trees, buildings or any other possible item. Do not crash the helicopter; there is a way out! It is possible to fly out of LTE without crashing! Never give up and stop flying the helicopter, that will hurt. Full left pedal should be applied provided you have the power to do so without the loss of rotor rpm. This may mean that you must over-torque the aircraft if you have the power to do so, but an over-torque is way better than a crash. If you do not have the power to maintain the altitude necessary to clear the obstacles and apply full left pedal, then you must maintain power (collective) sufficient to prevent terrain strike while applying only as much left pedal as you can to minimize the spin and maintain rotor rpm at 100 percent. It is imperative that your focus is out in front of the helicopter where you can maintain the aircraft level as it spins. While doing this, you must then apply forward cyclic and fly out of the condition. The aircraft will straighten up and resume normal flight as soon as you gain translational lift sufficient to overcome the condition. The direction in which you fly makes no difference however the aircraft will resume flight quicker if you fly into the wind. Interestingly enough, if you knew which direction the wind was from and you had originally departed into the wind, the condition would not have existed in the first place, therefore it is not likely that you will know which direction the wind is from. Just ask any student pilot or instructor where on which side of the helicopter he will choose to take the wind if he can not take it on the nose and they will tell you off the left side. They say this because they have been pounded with LTE knowledge since they began training. Every one of them is well aware that LTE is onset by a wind condition beginning in a region from about 330 degrees off the left side of the nose of the helicopter and around the tail in a counterclockwise direction to about 120 degrees off the tail on the right side. Having been taught this, they will all choose to take the wind off the right side and this can be a costly mistake. This is a hazardous area of operation that most pilots will not be aware of until they begin to do commercial operations and then usually they encounter the conditions but are unaware of why this occurred when they were not operating in the zones described in LTE documentation which they have been taught throughout their training. Most pilots will not be aware of the figure until they begin to fly larger helicopters. The figure included in this article is from a Bell 206B or L POH, but is applicable to all helicopters with rotors that turn counterclockwise (European manufactured helicopters will be opposite). This figure is an indication of where you want to keep the wind during an OGE hover or any time your airspeed is lower than 30-knots while OGE. Do not underestimate the danger, if you fail to keep the wind in the clear zone of this figure you will notice that the helicopter becomes very hard to control, and will require extreme pedal work. This can be a very scary situation and may require significant altitude to recover. If an OGE hover is necessary it is critical that you know the wind direction and point the nose of the helicopter in such a way that the wind is from the clear zone in the figure. Obviously it is important that you know the direction of the wind. Often a slow orbit is necessary to avoid control issues. You may orbit in either direction, however it is much safer to orbit to the left. If you choose to orbit to the right, and your airspeed drops below 30-knots use extreme caution because as you rotate into the critical zone the helicopter will try to spin to the right. The intensity of this spinning tendency will be relative to the velocity of the wind. If you make this orbit to the left you will find that you have much better control as you maintain the orbit with left pedal pressure rather than right.
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