PPRuNe Forums - View Single Post - 787 Lightning strike issues v FAA
View Single Post
Old 14th Dec 2019, 22:59
  #32 (permalink)  
Join Date: Sep 2019
Location: leftcoast
Posts: 2
Originally Posted by tdracer View Post
OK, let's do some math. 787 production rate is 14 aircraft per month - split evenly between Everett and Charleston. At any particular time, there are 7-8 aircraft in various stages of final assembly - at each line. So between Everett and Charleston that's 14 - 16 wing sets. Once they push a new 787 out the door, it's far from ready for delivery - it has to be painted, go through various functional tests and fueling/defueling checks, production acceptance flight tests, customer acceptance flight tests, and the actual ticketing and delivery. While I've heard of this taking as little as two week, four to six weeks from rollout to delivery is more typical. Last time I overlooked the Everett flight line, there were at least 12 787s parked there. I've not seen the Charleston flightline, but I have no reason to expect it would be much different, so that's around another 24 aircraft. Add it up and we're looking at 38-40 wing sets - without accounting for any inventory, wings currently being produced, or units in transit. In fact, 40 shipsets is right about where I'd expect it to become an delivery issue if the change wasn't certified. Which, interestingly, is pretty much what's described in the linked article.

Dave, sorry if referring to Tom as "reasonably sharp" came across as condescending or insulting - that certainly was not my intent and I apologize if it came across that way. Tom is very competent (unlike a few other FAA types I dealt with) - my point being he's a Propulsion specialist, which in itself covers a wide range of issues. You can't expect him to be an expert at lightning as well. That's why Boeing has an EMI/Lightning group made up of experts in the field that support the various other disciplines so that they don't need to be EMI/Lightning experts.

Are you sure about the lack of copper in the 787 carbon fiber matrix? Because that's certainly contrary to what I was told by my co-workers who were on the 787 program. It also creates lightning concerns unrelated to fuel tank ignition. While carbon structure is a reasonably good conductor, it's electrical resistance is still an order of magnitude worse than aluminum structure - which greatly increases the induced voltages/currents on the aircraft wiring when a lightning strike passes through the surrounding structure - so copper was added to lower the electrical resistance and keep the induced levels reasonable. I know the the systems on the 787 were certified for higher lightning transients relative to primarily aluminum aircraft (such as 777 and 747-8), but those levels were still lower than what would have been required if it was pure carbon without any additional conductive material added to the matrix.
TDracer- perhaps part of the confusion comes from dates of changes in the production process such that both answers regarding copper mesh and conduction and lightning strikes are ' correct'

The Seattle Times: Business & Technology: Building the 787 | When lightning strikes

Is a great explanation as to what WAS about 12 years ago.
this partial quote from the article re Walt Gillette is a great summary as it applied THEN.

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. . . . , , , 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.

. . . 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."
Of course that was THEN - Walt has since retired.

There were similar problems on the B2- as to level of protection- and Walt was there. And from memory and not classified, ALL of the B2 was rated at the highest level of protection.

Grebe is offline