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In short, test pilots, using a fully loaded helicopter over level, prepared surfaces, try to enter auto-rotation from different combinations of height and speed. At some point, they look at each other and say "a normal, well trained pilot, could only just auto-rotate safely from that combination of height and speed. They plot these points on a graph, join the dots and say "see those shaded areas, don't go there".

In short, test pilots, using a fully loaded helicopter over level, prepared surfaces, try to enter auto-rotation from different combinations of height and speed. At some point, they look at each other and say "a normal, well trained pilot, could only just auto-rotate safely from that combination of height and speed. They plot these points on a graph, join the dots and say "see those shaded areas, don't go there". It represents the areas where you will not have enough of the combination of potential and kinetic energy to make a safe landing.

The third safety concern is the need for good skills and training. When performing towering take-offs in typical sport and private helicopters, the pilot must be gentle on the controls. Relatively large or rapid control inputs cause large spikes in the power required. When you have plenty of speed, this doesn't really matter but when climbing vertically, you might be calling for more power than is available. You must climb OGE slowly with gentle collective increases and, when clear of the top of the trees or other obstacle, ease the cyclic forward gently. It is this last input which is especially risky. If you push the cyclic forward as you would in a normal take off, you could find yourself running out of power and settling into the top of the tree with nothing left to give. The safe way to do it is a slow climb but obviously, this leaves you in the shaded area for longer. I was always relieved when I cleared the trees and got back into normal flight and the temptation, which must be resisted, is to increase your climb rate to reduce the time on the curve but of course, this reduces your power margins. The general principle is to use the power required and no more. Gentle climb, gentle movements, be ready to pick up wind as you clear the obstacle.

The third safety concern is the need for good skills and training. When performing towering take-offs in typical sport and private helicopters, the pilot must be gentle on the controls. Relatively large or rapid control inputs cause large spikes in the power required. When you have plenty of speed, this doesn't really matter but when climbing vertically, you might be calling for more power than is available. You must climb OGE slowly with gentle collective increases and, when clear of the top of the trees or other obstacle, ease the cyclic forward gently. It is this last input which is especially risky. If you push the cyclic forward as you would in a normal take off, you could find yourself running out of power and settling into the top of the tree with nothing left to give. The safe way to do it is a slow climb but obviously, this leaves you in the shaded area for longer. I was always relieved when I cleared the trees and got back into normal flight and the temptation, which must be resisted, is to increase your climb rate to reduce the time on the curve but of course, this reduces your power margins. The general principle is to use the power required and no more. Gentle climb, gentle movements, be ready to pick up wind as you clear the obstacle, and you'd better have made sure before you get there that it's a head wind.

For example, your operational need is to land in a clearing in a forest or perhaps a field surrounded by tall trees. In order to take off again, you need to climb vertically until you've cleared the tops of the trees so that you can transition forward to gain speed.

The third safety concern (and the last one to my knowledge) is the need for good skills and training. When performing towering take-offs in typical sport and private helicopters, the pilot must be gentle on the controls. Relatively large or rapid control inputs cause large spikes in the power required. When you have plenty of speed, this doesn't really matter but when climbing vertically, you might be calling for more power than is available. You must climb OGE slowly with gentle collective increases and, when clear of the top of the trees or other obstacle, ease the cyclic forward gently. It is this last input which is especially risky. If you push the cyclic forward as you would in a normal take off, you could find yourself running out of power and settling into the top of the tree with nothing left to give. The safe way to do it is a slow climb but obviously, this leaves you in the shaded area for longer. I was always relieved when I cleared the trees and got back into normal flight and the temptation, which must be resisted, is to increase your climb rate to reduce the time on the curve but of course, this reduces your power margins.

For example, your operational need is to land in a clearing in a forest or perhaps a field surrounded by tall trees. In order to take off again, you need to climb vertically until you've cleared the tops of the trees so that you can transition forward to gain speed. You might need to use on at night (because you cannot see if there are any obstacles ahead) or when taking off from snow or sand when your vision will be obscured, making a vertical take-off safer.

The third safety concern is the need for good skills and training. When performing towering take-offs in typical sport and private helicopters, the pilot must be gentle on the controls. Relatively large or rapid control inputs cause large spikes in the power required. When you have plenty of speed, this doesn't really matter but when climbing vertically, you might be calling for more power than is available. You must climb OGE slowly with gentle collective increases and, when clear of the top of the trees or other obstacle, ease the cyclic forward gently. It is this last input which is especially risky. If you push the cyclic forward as you would in a normal take off, you could find yourself running out of power and settling into the top of the tree with nothing left to give. The safe way to do it is a slow climb but obviously, this leaves you in the shaded area for longer. I was always relieved when I cleared the trees and got back into normal flight and the temptation, which must be resisted, is to increase your climb rate to reduce the time on the curve but of course, this reduces your power margins. The general principle is to use the power required and no more. Gentle climb, gentle movements, be ready to pick up wind as you clear the obstacle.

The fourth safety concern (and the last one to my knowledge) is not so immediately dangerous but can lead to trouble. In a helicopter without a large power reserve, it is possible to reach a point where the power required exceeds the power available, especially if you catch a gust of wind. In this situation, the rotor RPMs will start to reduce or "droop". The only possible response is to lower the collective. This normally means you have to give up and simply land and try again as above but any situation which leads to the rotor RPM dropping outside your control is to me, a safety concern.