Re: Thunderstorms

From:         rdl1@ukc.ac.uk
Organization: University of Kent at Canterbury, UK.
Date:         26 Jun 94 16:18:25 
References:   1 2
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In article <airliners.1994.1351@ohare.Chicago.COM> ehahn@fairlite.mitre.org (Ed Hahn) writes:
>While commercial airliners do not get hit often by lightning, it is
>not uncommon.  Airliners have static discharge wicks, which look like
>black "antennas" sticking out the trailing edge of wings and tail
>surfaces.  Lightning tends to hit these rather than the fuselage.
>
>These static wicks do another duty by dissapating static charge which
>accumulates on the fuselage into the air stream by a mechanism I don't
>know too much about, quite frankly.  The main problem with static
>buildup is that it can affect the comm radios and other RF devices.
>Airlines go through much effort to insure that static wicks and
>antennas are securely bonded to the airframe.

The other mechanism is triboelectric charging. Essentially droplets can be
charged, by the same charge separation mechanisms that lead to lightning
discharges between clouds. When an aircraft flies through a cloud of
charged droplets, it can acquire their charge and build up a high potential
- the capacitance of the aircraft is much less than that of millions of 
little drops.

If this potential exceeds a breakdown threshold, a lightning-like discharge
can occur from the aircraft. Some early aviators with rubber wheels used to
get a jolt when deplaning, until metal skids were used.

As Ed says, the main problem is RF interference these days.

Some space probes (Galileo, Huygens) also carry static discharge wicks
to protect from lightning or triboelectric charging.

Ralph Lorenz
Unit for Space Sciences
University of Kent
UK