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Helicopter Safety - Dangerous Flying Manoeuvres

Settling with power

Settling with power (or vortex ring state) describes the situation in which a helicopter is descending rapidly without much lateral (e.g. forward) speed, but with substantial engine power. Adding power will make things worse and won't stop the descent. This settling with power condition occurs when the helicopter is descending in the vortex that is created by its own rotorblades. In normal circumstances, these vortices are of no influence as the helicopter leaves them behind (normal flight). However, when the helicopter is hovering and descending, it will stay in its own vortices, and the addition of power will stall the blades, making the speed of descent even worse. The settling with power condition occurs as a result of two phenomena; firstly there is the vortex itself, which leads to a large amount of induced flow. And secondly, the angle of attack is such that blade stall is reached even with modest collective settings. The blade stall is concentrated towards the centre of the rotor disc. The angle of attack at the blade tips is very low in these circumstances, and so no thrust is generated there either. During settling with power conditions, large and sudden variations in the angle of attack take place, and the helicopter will pitch and roll.

Ground resonance

A situation exists in which a helicopter can suffer from very severe resonances. The severity of these is such that the helicopter can be destroyed. The resonance is powered by the rotor system, and only develops to complete amplitude when the helicopter makes ground contact. Hence, the name ground resonance. The rotor system generates the resonance when the centre of gravity of the rotor system is not aligned with the centre of rotation. As this can happen more easily with fully articulated rotor systems, ground resonance mainly occurs with these systems. When detecting ground resonance, you can basically do two things.

Dynamic rollover

When a helicopter comes into contact with the ground on one side of the machine (e.g. the left or right skid), there is a possibility that it will roll over onto that side. Dynamic rollover happens when the rotor disc is applying a sideways force (thrust vector) while the helicopter is on the ground on one side. The only movement the machine can make in this situation is to roll over. Dynamic rollover can easily happen when a skid is accidently stuck or attached to the ground. When dynamic rollover starts it will be over in a few seconds, and so you must either react very quickly, or prevent it from happening.

Wire stikes

It is difficult to imagine helicopters striking a wire when you have never flown one. Wires are easily seen from the ground when set against a clear blue sky. However, when in the air, power lines have the earth’s surface as their background, which makes them much more difficult to detect. Moreover, wires are thin and therefore not easily noticeable. As helicopters regularly fly into wires, a strategy against doing so is needed. It is based on a simple idea. Avoid them.

Mast bumping

When a teetering rotorhead is 'flapping' beyond its mechanical limits, it will bump against the mechanical stops on the rotorshaft. This mast bumping will, inevitably, lead to disaster (broken mast, separation of rotor blades). This condition can occur when the rotor disc is no longer loaded. An unloaded rotor disc cannot produce any thrust vector, so it is no longer under control. In an attempt to regain control, the necessary extreme control input will also lead to an extreme tip path, causing mast bumping. Mast bumping can also occur on fully articulated rotor heads, although, it is then called 'droop stop pounding' and does not generally result in a disastrous ending.

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