Building the 787 | When Lightning Strikes

 

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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.

That means Boeing will have to do more to prevent lightning from damaging the planes, said Ed Rupke, senior engineer with respected consulting firm Lightning Technologies of Pittsfield, Mass.

An airplane often actually triggers a lightning bolt to the nose, the leading edges, the tail or the wings as it flies through an electrically charged cloud. The main danger airplane designers must guard against is sparking inside the wings, which serve as the jet's main fuel tanks.

Most of the time, after a flash and a bang, lightning damage is minimal, and airplanes fly on to their destinations.

Engineers in Everett are debating the best way to achieve that outcome for a largely plastic airframe. 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."

Walt Gillette, who leads the 787 engineering team as Boeing's vice president of airplane development, said the review was part of a healthy internal debate that ultimately assures the best engineering solution.

Composites are not new in commercial aviation, he said. And although the safety team's conclusion was "absolutely true at the time" it was written, he said, by the end of the testing and analysis now in progress, the 787 will meet strict Federal Aviation Authority (FAA) requirements.

"It's part of the art of creating safe airplanes," he said. "No new design can step backward in safety."

One chance in a billion

Lightning strikes are an unavoidable part of the airline business. "Getting in and out of airports like Minneapolis or (others in) the Midwest, (commercial jets) are hit a lot," said Jack Schroeder, president of Lightning Diversion Systems, which sells devices for protecting airplanes.

Typically, a bolt moves backward across a wing or fuselage before the charge exits to the ground milliseconds later. At the point of entry on a metal skin, the aluminum can melt, leaving a pitted surface or a small hole.

"You can't hit aluminum with 200,000 amps and expect nothing to happen," Gillette said. "But it's not a safety-of-flight issue."

On the composite fuselage of the 787 a strike is unlikely to penetrate more than the outer layers of carbon fiber.

Such damage needs repair but is not a big deal. More serious is the possibility that the electric charge passing through the airplane will create a spark inside the wing, potentially causing a fuel-tank explosion and destroying the aircraft.

In 1963, over Elkton, Md., 81 people died when the fuel tank of a Pan Am 707 exploded in flight, apparently after being struck by lightning. In 1976 near Madrid, Spain, all 17 people aboard an Iranian Air Force 747 jumbo jet died when a lightning strike to the wingtip ignited the jet fuel and blew the wing apart.

The 1963 crash led directly to tighter design regulations. Then in July 1996, TWA flight 800 exploded over New York, killing 230 people. Although that explosion was not due to lightning but was blamed on a wiring short circuit in the center fuel tank, it brought further tightening of fuel-tank safety that applied to all new commercial airplanes developed after 2001, starting with the 787.

"As an industry, we all have reacted with intensity to the TWA 800 situation," Gillette said, "we want to ensure that doesn't happen again."

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.

Billy Martin, who chairs an industry committee that provides guidance on lightning-protection standards, said preventing electrical sparks inside the fuel tank is the essential lightning-protection element.

The NGS, he said, is "an additional warm-and-fuzzy" added hastily after the TWA 800 crash.

"I don't believe that's necessary to fly the airplane safely," said Martin, who is a principal engineer with business-jet manufacturer Cessna.

Industry experience

Engineers at Boeing and elsewhere note that while the 787 is the first all-composite airframe, the industry has experience with the material.

Gillette pointed to the Airbus A340, which carries fuel in its composite horizontal tail — a structure as big as the wing on a narrow-body jet. More than 300 of those operate worldwide, with no reports of lightning problems since first flight in 1991.

"The technology has been safe for the past 25 years or so in using composites and lightning protection," said Rupke of Lightning Technologies. "I think the confidence is there to use that technology."

Boeing "will have to do a lot of testing, a lot of analysis and provide the FAA a lot of data" to show the 787 meets the same protection standard as an aluminum airplane, said Dave Walen, the agency's chief scientific and technical adviser on lightning.

Gillette said his team is perhaps only months away from agreeing with the FAA on an overall 787 certification plan, which will include proving that the risk of a lightning-induced fuel-tank explosion is less than one in a billion.

"When it's all done, the end of a five-year process ... the FAA will evaluate all that we have done," Gillette said, "and they will find that we have met the rule."

Provided By:  Dominic Gates

Seattle Times aerospace reporter