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Helicopter Safety - Weather Conditions

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There are many examples of weather related accidents. Although helicopters can fly when there is a relatively high surface wind (because of the high tip path speed compared to the surface wind speed), the influence of the weather certainly accounts for a large proportion of all helicopter accidents. The sort of weather that can have an influence is (changing) wind, rain, cold, snow, fog, clouds, or lightning. Do not underestimate weather influences’. There is a tremendous amount of energy stored in the weather system, which can be terrifying when (partially) released over a short period of time.

An important type of cloud in aviation is the Comulonimbus cloud (Cb, as pilots call them). The Cb is an important source of local turbulence, heavy rain, and lightning. The Cb is fed by thermal energy at the earth’s surface, or from any other thermal source (e.g. bush fires or warm air blowing upwards). Cbs can often be found in a line (for example, near a cold front). If the Cb develops, they store vast amounts of water vapour. During this development stage, the wind direction is up towards the Cb. A Cb can easily extend to 30 thousand feet in height. Ultimately the Cb will become unstable, and will release its energy as wind (and rain). The wind direction is now downwards (and spreading out in all directions over the earth’s surface). The vapour in the Cb is released in the form of water: it starts raining. The rain and wind activities of the Cb can be very severe and local. Watch out for Cbs!

Lightning travels between a Cb and the earth’s surface; between Cbs, or even within a Cb. Aircraft are rarely struck by lightning, and when a helicopter is, two things happen. At first, the lightning enters the aircraft and, secondly, it leaves the aircraft from a different place to the one through which it entered. Accordingly, the helicopter becomes part of the current’s conducting patch. When this happens with helicopters, it does not, usually, cause significant problems. However, modern composite rotor blades are more sensitive to lightning strikes than their metal counterparts. The composite is a poor conductor of electrical current, which leads to it being dissipated throughout the composite rotor blade, with (excessive) heat development being the result.

The cold can also cause problems. Firstly, cold engines, fluids (oil), bearings, and everything else that should move, are not protected to the degree that a warm mechanical system is. Ice is another phenomenon to consider carefully, since ice on rotor blades develops much faster than on the fuselage. When trying to assess the level of icing on the rotorblades, the best option is judge the engine power required, since the ice on the rotorblades cannot be directly observed, for obvious reasons. Smaller helicopters are not certified to fly in icy conditions. When there is ice on the rotors, the rotor system can become unbalanced, with possibly (severe) vibration being the result.

Don't Get Caught in a Fatal Trap!

A vital resource for pilots, technicians, and helicopter enthusiasts, this book analyses every possible helicopter accident in detail. It looks at accidents that have been caused by a broad range of factors, such as technical problems, weather influences, mechanical failures, human factors, and many more. The treatment and analysis of each cause is dealt with in depth. What makes this text invaluable is that throughout this work an attempt is made to analyze accidents with a view to finding common causes, solutions, and enabling preventive measures to be defined. This makes 'Fatal Traps for Helicopter Pilots' a potential life saver.

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HeliStart is authored by Peter Goossens.

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