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Subject:
Civil a/c lightning protection
lightningtest
3/10/2006 4:52:42 AM
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I long ago promised to offer up something about what we do at the lab in which I work.
This is a really interesting article with some excellent links.
My take:
Military men & women are perhaps as well trained as any to judge how improbable but very significant risks stack up, lightning protection of a/c is a game of such risks. What do you think?
article source:
link
references quoted:
link
link
link
start of quote from article:
By Dominic Gates
Seattle Times aerospace reporter
"Thanks to well-developed protection systems on traditional commercial jets, no airliner crash in the United States has been caused by lightning in more than 40 years. It's a remarkable record, since Boeing estimates that every commercial airplane is hit by lightning on average about twice a year."
"But Boeing engineers designing the 787 face new challenges, since they are building the first commercial airframe made entirely from carbon fiber-based plastic. The composite airframe will not readily conduct lightning away, as traditional metal ones do."
[....]
"The main danger airplane designers must guard against is sparking inside the wings, which serve as the jet's main fuel tanks."
[...]
"In November, one top safety-engineering team expressed serious concern. That team's internal review, obtained by The Seattle Times, concluded: "It cannot be shown that the current wing-lightning-protection approach will preclude ignition sources in the fuel tank.""
[...]
"...review was part of a healthy internal debate that ultimately assures the best engineering solution."
[...]
"Boeing is taking a multilayered approach to lightning protection of the 787 fuel tank:
? The initial lightning strike must be dispersed quickly around the airframe to prevent concentrated damage. Also, the airplane's electronic flight instruments must be shielded from disruption by the intense electromagnetic field. To accomplish this, Boeing will embed a thin metal mesh or foil in the outer layers of the composite fuselage and wings.
? A slight gap between a wing-skin fastener and the hole it goes into could be a source of sparking as current jumps the gap. Boeing will install each fastener precisely and seal it on the inside to ensure a snug, spark-free fit.
? Inside the wings, any gap along the edges where wing skin meets internal structural spars could cause a spraying out of electrons in a lightning strike ? a phenomenon called "edge glow." Boeing will seal the edges with nonconducting goop or glass fiber.
? And, in case the efforts to shut out ignition sources fail, Boeing will install a nitrogen-generating system (NGS) that reduces flammable vapor in the wing tanks by filling the space above the fuel with inert gas."
[...]
"...The team recommended making the NGS system "dispatch critical," meaning the airplane is not allowed to take off if the nitrogen system isn't functioning."
"The team was praised for "unwavering determination" in pursuing its solutions to the lightning-safety issues "despite the unpopularity of this position with others" ? but its view did not prevail."
[...]
"FAA regulations demand a cold statistical outcome: The 787 design has to ensure that the chance of lightning sparking a fuel-tank explosion in flight is less than one in a billion."
"Gillette said the NGS system is expected to operate at least 97 percent of the time, but the safety systems combined will assure the 787 exceeds the one-in-a-billion probability target."
end
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