"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.
Last November one safety team became concerned that Boeing was relying too heavily on tight, precise installation of the fasteners. It worried that a loose fastener could not be detected after construction.
"The latent failure of any one fastener leaves the airplane one event away from a catastrophic incident" caused by a spark, the team's safety review stated.
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.
"We don't have to make it flight critical," Gillette said.
Gillette said this kind of debate is common among engineering teams.
"These are really strongly held opinions by really bright people," Gillette said. "It's almost like politics — once you believe in a solution, you really believe in it."
Gillette said that back in November the fasteners were not working as required — some were pulling right through the skin.
But Boeing adjusted the fastener design and installation process. And to test for loosening of the fasteners, Mitsubishi Heavy Industries, which is making the wings, has shaken skin panels through the equivalent of one and a half airplane operating lifetimes.
Extensive tests on fastener installation will be completed within weeks, Gillette said.
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.
"It is not a good idea to put all your eggs in one basket," he said."
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.
"It is not a good idea to put all your eggs in one basket," he said.
note: the Safety Team's view did NOT prevail???
Last edited by FlightPathOBN; 20th May 2013 at 21:22.