Helicopter Safety - Weather Conditions
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.
- Take weather conditions into account, and change your plans as necessary.
- If necessary, land your helicopter and take shelter.
- Avoid Cbs.
- Know that under the influence of a Cb the helicopter can be forced ('blown') down.
- Always be prepared for sudden changes in wind direction.
- Always have your helicopter inspected by a qualified engineer after a lightning
strike.
- When it is cold, the pre-flight inspection is of even more importance, do not rush
it just because it is cold.
- Be aware that snow won't interfere with the engine (e.g. in the hover)
- De-ice the helicopter carefully before flight.
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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