---From the accident report:
"The process that culminates in a lightning flash begins with the formation
of an ionised column, called a 'leader', which travels out from a region
where the electric field is sufficiently high in field values that it
initiates progressive breakdown. This critical field is thought to be about
900 kV/m for water droplets, or 500 kV/m for ice crystals. For a negative
discharge to earth the column advances in 'zigzag steps' (a 'stepped
leader'), each about 50 m in length and separated by pauses of 40 to 100 s.
The diameter of a stepped leader is between 1 m and 10 m although the
current involved, which is low (approximately 100A), is probably
concentrated in a small ionised core of about 1 cm diameter. The average
velocity of propagation is 1.5 x 105 m/s and this leader may form branches
on its downward path to the ground.
When it is near the ground it causes high electric fields to form at
projections such as trees and buildings and these send up leaders, one of
which will make contact with the tip of the downward propagating leader.
This has the effect of 'closing a switch' and the position in the lightning
conduction channel where this occurs is known as the 'switching point'.
When this occurs, a return stroke is initiated which retraces and discharges
the leader channel at a velocity of some 5 x 107 m/s and this is
characterised by a current pulse of great amplitude and high luminosity.
After the first return stroke, further strokes may occur as higher areas of
negative charge regions are discharged; the leaders for these usually follow
the same path as the first, but in one continuous sweep at a velocity of
some 2 x 106 m/s."
Hi,
A couple of us had a discussion about the principle of joining
all metal portions (or lots) together to form a basic 'ground' path of
mutual quality throughout. [see middle paragraph] My objection was that,
having experienced this 'leader' personally, I was not about to create a
large ground structure which might then give rise to the production of a
larger leader than otherwise, but rather to have a number of smaller
sections unconnected - so as not to produce larger, more attractive leaders.
I felt that a smaller ground mass perhaps created a smaller 'leader' and
therefore was less attractive to the potential main strike.
The analogy to a barn 'lightning arrestor' is not applicable
because its purpose is to attract the strike and lead it to ground (around
the barn), whereas I presume our task to make the aircraft less attractive
in an active field.
I propose we should spend more energy in constantly dispelling
static buildup (static 'wicks' or whatever) than in creating a metal
backbone. Just a thought.
Cheers, Ferg
The strength of the strike is not a factor because strong as the incident
strike was, I see that 7 out of 10 nearby were as strong, so presume that
any strike is destructively large.
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