I read about a lighning strike on a recent thread and it got me thinking about how the 787 will deal with lighning strikes since it is going to be a mostly composite airplane. As I understand it, normal airliners withstand lightning strikes because they are made of very conductive materials -- mainly metal. If lightning strikes a composite airliner, it won't conduct the energy through the plane; instead, the lightning will essentially get "trapped" in the non-conductive material and explode.

does anyone have any insights on what boeing is doing to deal with this problem?

Hmmm... Good thought. I witnessed (and heard) a carbon-fibre pole get struck by lightning. It got blown into millions of carbon "needles." The carbon was glowing a bright white because it was so hot.

Mark

I understand that there are military aircraft in service that use this construction. Thus surely the issue of lightening strikes has been solved. Thus it is a non-issue.

Here is my theory.
Aircraft electrical equipment is generally wired much like your car- one wire to the device, and the device grounded through its frame and the aircraft skin. The negative lead on the generator is also connected to the skin (believe it or not, folks, our cars are wired the same way).
Composite general aviation aircraft (the Beech Starship I am discussing here) have a "ground plane" of metal mesh between layers of the composite material. When you go to add a new electronic device, you sand the composite down to the wire mesh, and ground your electronics into the mesh.

SO- my theory is that the lightning will go into the wire mesh, and dissipate out the back.

ALSO- another idea... How much of the 787 fuselage will actually be composite? I think it's just the skin.. which would mean that longerons and whatever else might still be metal.. maybe they could link these metal components together and create a path for lightning to exit the plane???

If someone else has the answer, feel free to shoot my ideas down.
Have a good weekend,
R

not sure about the % of composite in the wing, but from the looks of the photos of the construction methods, almost the entire fuselage seems to be "wrapped" in composites. my guess is that lightning would tend to strike wings (tips, especially) and not the fuselage. not that it couldn't hit the fuselage, just that it seems less likely.

will the wing be metal? if so, maybe this explains their plan to ground the airplane.

There are several ways to dissipate an electrical charge in composite structures.

1)An aluminum screen can be laminated under the top layer of fabric. This is typically used on a carbon/graphite component.

2)You can flame spray aluminum on a composite component.

3)A thin aluminum foil can be bonded to the outer layer.

4)Aluminum wires can be woven into the top layer of the composite fabric. This is found in fiberglass and Kevlar components.

If I remember right, the Lear Fan had a layer of aluminum plasma sprayed on the outside of the airframe. Again, trusting my memory, the Starship had a copper mesh included in the layups. Including a mesh in the layups is the most common protection used.

To clear up a couple of points. First, according to the Boeing 787 website, the entire fuselage including the skin, formers, and stringers, and the wing of the 787 will be made of composites. Second, for some reason, I'm not sure why, but lightning seems to strike the fuselage much more frequently than the wings. I've repaired damage on literally thousands of fuselage lightning strikes but could probably count on both hands the number of wing strikes that I've seen. Also when lightning strikes it usually emerges out the bottom of the fuselage on it's way to the ground. Lightning pulses very rapidly and often times leaves a track of strikes as if machine gunned down the fuselage.

The grounding and bonding of the composite structure on the 787 will be a non-issue as Boeing has I'm sure already taken care of it. The general public will not be informed of the methods used just as they probably haven't been informed of the exact grounding and bonding requirements and methods of any other of Boeings products. When the planes enter service the technicians that work them will be trained on the methods employed and how to maintain them, perhaps then one of us will be able to share that info. Until them maybe a Boeing employee can share the info we're looking for.

Dl757Md

As stated, a wire mesh is laminated into the composite parts to provide conductivity. I imagine on the 787, with most of the plane being composite, they will have to take specific care in "grounding" the wire meshes of the different components to each other and to the metal structure. This is unusual for structural components but is already a standard procedure for all equipment on the aircraft.

I imagine that special "grounding points" will exist on the main structural components both to interconnect them and to allow equipment to be grounded.

One issue that, in my opinion, is more relevant than how this will be done is the extent of damage in case of a lightning strike. In metal planes lightning usually does no structural damage whatsoever, except to antennae, composite control surfaces or poorly grounded equipment. On the 787 any lightning strike would most likely prompt a careful inspection of the whole airplane, as conductivity will be lower and the potential damage much more serious. Delamination can be notoriously hard to detect.

mrocktor

mrocktor

In metal planes lightning usually does no structural damage whatsoever

I couldn't disagree more. While catastrophic structural damage is never a consequence, minor structural damage usually occurs to the skin and sometimes stringers and formers. The skin suffers pinholes or pock marks at the point of the strike. Since cracks can propagate from these defects they have to be evaluated and either burnished out or, if the depth of the damage is too great, drilled out and filled with a rivet. I've seen aircraft with over 100 rivets in a meandering path down the side of a fuselage from a severe strike.

On the 787 any lightning strike would most likely prompt a careful inspection of the whole airplane

True. This is however no different from any other aircraft. All AMMs have a procedure for lightning strike inspections. In a nutshell it's a careful inspection of the whole airplane.

The big problem I see with the composites on the 787 are that for the most part composites are hygroscopic. This means that they absorb moisture from the ambient air. Carbon is not nearly a susceptible to this as fiberglass or kevlar, but even small amounts of moisture are instantly turned to steam by the heat of a lightning strike. This is very explosive. The damage that occurs form this is usually quite obvious as panels are literally blown apart. Bonding can help but not completely eliminate it as an area about 1/4 to 1/2 inch around the strike point is heated by the plasma of the lightning bolt itself. The damage caused by this would still be abundantly evident with just a visual inspection. As I see it, bonding is important, but moisture protection via surface finishing(paint) and it's subsequent mtc. will be at least as important although it's mtc. has historically been far more neglected.

Dl757Md

Like lightening rods, there are screws installed through composites to provide safe paths for lightening, there are "lightening strips" on radomes and other places to control the attachment and direction of lightening as it passes through structures. Composite aircraft requirements for lightening strike inspections will be more rigorous because there are fewer and harder to detect paths for lightening.

Dl757Md,

I see where you are coming from. Let me rephrase:

In metal planes lightning usually does no structural damage whatsoever outside the entry/exit points.

I wanted to setress that the post lightning incident inspection for a "full composite" aircraft must be extremely careful, not downplay the procedures for current planes.

mrocktor

mrocktor

I agree that the the lightning strike inspections will need to very comprehensive and detailed. One thing that I think will aid in the detection of strike points is that the entire structure will be required to be painted. The burn marks that the lightning leaves behind in the paint will be obvious and will be a clue in detecting possible structural defects caused by the strikes.

Dl757Md

Tockeyhockey,

Welcome to the fourm! You have a valid question and that's one of the neat things about this particular fourm on Anet. You'll find many professionals here who will provide valuable insight.

Another nice feature here at tech/ops that separates this particular fourm from the rest of the message board is the respect displayed by everybody here. There are mechanics, pilots, ramp agents, fuelers, industry professionals and enthusiasts from every part of the globe flying and working on everything from piper cubs to 747s. This unique blend offers insight into how differently a question can be approached. As a result of this, disagreements do come up and you may have a few people that add their opinion or an answer that you don't like but again, respect reigns. Please refrain from making comments like the ones you made in response to SunriseValley in the future.

One thing that I think will aid in the detection of strike points is that the entire structure will be required to be painted.

Yet another reason why American is going to have to come up with something new if it buys 787s, even though the polished fuselage is a very strong part of brand awareness (I'm discounting the ugly A300 paint).



If the comparatively tiny companies of Stoddard-Hamilton and Lancair were able to solve this problem for their Glasair III and Lancair IV(P) respectively, then I'm sure Boeing will do so as well.

Pilotpip,

I think that whenever someone disrespects a question that is posed in a forum, the person who posed the question reserves the right to fire back. Basically, what sunrisevalley was saying was that "everybody knows how it's done, so don't bother to ask." well, i don't know how it's done and i wanted to know!

since i posted my retort, the comments on the board have started to get me closer to an understanding of the technical aspects of lightning strikes on a composite plane. sometimes, you have to get the idiots out of the conversation before people start making sense.

respectfully yours -- tockeyhockey

Tockeyhockey,
Disagreement is often a part of the discussion process here and isn't of itself a bad thing, but insult and escalation contribute nothing to the forum. Even if you formed the personal opinion, after reading one post, that SunriseValley is an "idiot", what's the purpose of extending your insult to all Canadians?

I'd urge you not to be so quick to disregard Pilotpip's gently worded advice.

sorry to offend the canadians out there. i actually love canada. but please, if anyone agrees with what surisevalley was saying, they wouldn't be on this board in the first place.

here's essentially what he said: there is no purpose for this message board. it's like saying "the wright brothers invented the airplane. the issue of lift has been solved, so it's a non-issue."

you have to admit that it was a really stupid comment.

let's get back to discussing the 787, which seems to be a very exciting plane. i want to figure out hoe boeing is going to deal with lightning strikes!

Tockeyhockey,

You'll find pretty often on these boards that those comments happen. You'll get one or two that throw their two cents in without any sense that they have any idea of how things actually work (a great thread in general topics about the A380 being able to glide demonstrates this perfectly). Blow it off, and move on. You'll more than likely get many good answers from people that do know what they're talking about. This is one of the challenges of being on a fourm that accepts anybody willing to pay the price of admission.

Back on topic, Check out Lancair and Sirrus. While building only small GA aircraft, they have lead the way for some years in composite development and as a result have had some interesting hurdles in certifying their aircraft for IFR flight mainly as a result of the problems that composites have when struck by lightning. I've read a couple stories on how they achived this but looking through my "archive" of magazines such as "Flying", "Plane and Pilot" and "AOPA Pilot" has not netted any results.

you have to admit that it was a really stupid comment.

What an insensitive thing to say. His comment was no more "stupid" than your question TockeyHockey.

You could have just ignored it if you felt it added nothing to the discussion.

the lightning will essentially get "trapped" in the non-conductive material and explode.

Another brilliant statement....but nobody is calling you stupid. I think most have a little more tact than that. And what exactly does SunriseValley's nationality have to do with anything?

Step back a bit and reconsider your comments....

actually, my language on lightning strikes is pretty accurate -- because composites are not conductive, the energy does not flow through them. instead, it expands outward (explodes) and splinters the carbon-based material. see post 3.

his nationality has nothing to do with his stupid comment -- you are correct. but why does everyone feel the need to be the feeling police on this board? sorry if i offended canadians, but i'm an american and i reserve my right to offend everyone!

anyway, my main point about sunrisevalley was that it is incredibly dumb to post something just to say "let's not talk about this." THIS IS A CHAT BOARD! that's what we do -- talk about things! who has the time to go on a chat board about airplanes to tell people that we shouldn't talk about airplanes?!

rather than attack me personally for what may be perceived as a nasty response, lets examine the post that i was responding to. if you're not going to do that, then stop talking about it.

anyway, this thread seems to be dead. thanks to everyone who took my question seriously and gave me some new information to think about.

Some clarifications:

"Composites" is a very unspecific word. But for non-technically minded journalists it is often as technical as they can go.

Composite means exactly as the word says - some things composed together:

For aircraft four different composite types are used, and they are very different:

1. Glass fibre
2. Aramid fibre (often called "Kevlar", which is the DuPont trademark name)
3. Carbon fibre (sometimes also named graphite fibre)
4. Boron fibre.

What will be used for the main structures of the 787 is carbon fibre. Unlike the other types of composites carbon fibre is a quite good electrical conductor. Not nearly as good as aluminium which is second to only copper. But still quite good. Therefore carbon fibre cannot be used for radomes and such.

At least here in the tech/ops forum it should be banned to use the word "composites" since it's a generic name of so many different materials with extremely different properties.

At least here in the tech/ops forum it should be banned to use the word "composites" since it's a generic name of so many different materials with extremely different properties.


Hear, hear. I agree 100%, Preben. It's like saying "metal", instead of "aluminum", "titanium", etc.


2H4

"actually, my language on lightning strikes is pretty accurate -- because composites are not conductive, the energy does not flow through them. instead, it expands outward (explodes) and splinters the carbon-based material. see post 3."

No, actually it is not. The energy does flow through the composite material. In fact saying that "energy expands outward" or "energy explodes" makes no sense.

What happens is that while flowing through non conductive material, a large portion of energy is converted into heat. While electrical current does no damage to structures, heat does. The overheated material cracks and splinters - or explodes if you will.

Now, maybe that is what you meant, but your "language" was far off the mark.

mrocktor

How about gliders?
A lot of them are made by composites. And although they mostly fly in good weather (am I right or wrong?) some gliders must have been hit by lightning.

Yours in realtime
JHSfan

The airworthiness standards, even for gliders require that all external surfaces be electrically bonded. If not, can you imagine the huge static discharge you would get from a glider that just landed?

"No, actually it is not. The energy does flow through the composite material. In fact saying that "energy expands outward" or "energy explodes" makes no sense."

anyone who has taken a high school physics class knows that energy does not flow through composite materials. that is why composites are often used as insulators.

when a huge amount of energy hits a carbon-based material, it does not flow through it -- it turns into heat rapidly, expands, and explodes. my use of the word explode is accurate, and my use of the word "trap", while perhaps not correct technically, was perfectly descriptive of what happens. "trapped" just means that it was not allowed to flow through.

what exactly are you trying to nit-pick here? everyone else seemed to understand my post, except you and sunrise valley.

Tockeyhockey

You are correct that most composites are insulators. The problem is that with the voltages involved with lightning they can conduct electricity. The resistance of an insulator is infinite with respect to lower voltages. However there is a voltage threshold where an insulator becomes a resistor. This threshold varies from insulator to insulator. When the material becomes a resistor electricity flows through it and heat is created by the resistance to that flow.
Lightning itself is an example. Air, normally an insulator, becomes a resistor.
Another example of this is the use of a megohmmeter or megger to check for deteriorated insulation on wires. A megger applies about 1000vdc to the wire to check for a short to ground that would show infinite resistance on regular ohmmeter using 9vdc. The insulation still has the integrity to not transmit the lower voltage but it does transmit the higher voltages.

Cheers
Dl757Md

thanks DI757Md -- i guess my main point was that regardless of the technical aspects of voltage thresholds, it's clear that metal is better at dealing with lightning strikes than composites. i'm still not exactly sure what boeing's plan is to reduce the risk to composites by lightning strikes.

and by the way, the 757 is the most beautiful plane in the sky. long legs, a thin body, and big engines! sexy!

Once again, carbon fibre, as will be used for main structures on the 787, is a fine electrical conductor for any voltage.

Not as good as aluminium which is the second best conductor only next to cupper.

Put an Ohm meter on the ends of your carbon fibre fishing rod and you will see almost nill resistance.

Therefore the majority of the posts on this thread are plain wrong because they mix up carbon fibre with an entirely different composite such as glass fibre used for building hulls for pleasure boats and such. And for radomes on airliners where a non-conductive material is needed.

Comparing glass fibre to carbon fibre is like comparing pine wood to steel.

"Composite" is a real trash word in this respect since it only tells that the stuff was produced (composed) from two or more things, Hell, my birthday cake was made of composites - flour, sugar and cream.

For those interested in history - a lot of people think that carbon fibre is something new.

Not really so. It was invented already in 1962 by Royal Aircraft Establishment (RAE) in Farnborough, England. RAE can in many respects be called the "British NASA".

The first serious application of carbon fibre was fan blades on the prototypes of the Rolls Royce RB 211 engine.

But it never proved popular for fan blades since it is less FOD resistant than titanium. While titanium tends to bend when en engine swallows a goose or such, then the carbon fibre blades tend to break instead.

But practically all propeller blades on large, modern turboprop planes are made of carbon fibre these days. Reasons: Strong as steel, lightweight as wood and stiff as concrete. (And less expensive than gold - slightly).

You'll find answers to many of the questions raised here in a Seattle Times article published Sunday Feb. 5, 2006, which provides inside scoop from 787 engineers.

"No airliner crash in the United States has been caused by lightning in more than 40 years … But Boeing engineers designing the 787 face new challenges, ... "

http://seattletimes.nwsource.com/htm...echnology/2002844619_boeing05.html

Quoting Prebennorholm (Reply 22): At least here in the tech/ops forum it should be banned to use the word "composites" since it's a generic name of so many different materials with extremely different properties.

COMPOSITES!
COMPOSITES!
COMPOSITES!
COMPOSITES!
COMPOSITES!
COMPOSITES!



I just got this image in my head from the A.nut version of Life of Brian:

MATTHIAS:
Making it worse?! How could it be worse?! Composites! Composites! composites!
CROWD:
Oooooh!...
OFFICIAL:
"I'm warning you. If you say 'Composites' once more--
[MRS. A. stones OFFICIAL]
Right. Who threw that?
[silence]
Come on. Who threw that?!"  


For those too young to remember: http://www.mwscomp.com/movies/brian/brian-04.htm

Quoting Mrocktor (Reply 24): No, actually it is not. The energy does flow through the composite material. In fact saying that "energy expands outward" or "energy explodes" makes no sense. What happens is that while flowing through non conductive material, a large portion of energy is converted into heat. While electrical current does no damage to structures, heat does. The overheated material cracks and splinters - or explodes if you will.

Concur. I've worked on a couple metal airplanes that got hit by lightning. The damage is found at the point of ingress and any place where the current jumps a gap-there you find arcing. In the case i was most familiar with the lightning strike hit a propeller and burned a few roller bearings inside the engine.

Quoting Tockeyhockey (Reply 27): anyone who has taken a high school physics class knows that energy does not flow through composite materials. that is why composites are often used as insulators.

An excellent example of why high school physics courses are routinely covered in a scant few weeks of college-level material: they don't actually teach much.

Electrical current (the form of energy in question here) flows through carbon better than money flows through Congress. It doesn't flow through glass, plastics, concrete, or other types of common materials, and you must make your distinction on a material-by-material basis. Any other approach is sloppy.

As noted in an earlier post, the majority of the "composite" in the 787 is carbon fiber, and it will be interesting to see what the conductivity of the final material actually is. In fact, carbon is a fantastic conductor of *two* forms of energy -- electrical current and heat --and is considered something of a holy grail in the semiconductor industry. If you could build a "chip" out of carbon instead of the present metal oxides, you might achieve an effective processor which can dissipate its own heat at several TIMES (or more) that experienced in current chips.

Quoting Tockeyhockey (Reply 27): when a huge amount of energy hits a carbon-based material, it does not flow through it -- it turns into heat rapidly, expands, and explodes. my use of the word explode is accurate, and my use of the word "trap", while perhaps not correct technically, was perfectly descriptive of what happens. "trapped" just means that it was not allowed to flow through.

No, your description is sloppy, vague, and much worse, it's incorrect. We're not nitpicking. We're trying to correct fundamental flaws in the way you think and your knowledge base. Nitpicking would be pointing out that you never seem to capitalize the beginning of your sentences -- though every second grader knows to do this, even in Canada.

First, you don't define "energy." Which form are do you mean? Heat? Electrical? Kinetic? Potential? Second, the flow of electrical current does not by itself create heat. Even high school physics gets this one completely correct, because heat is created only when the flow of electrical current is *resisted*. The higher the resistance, and/or the higher the current flow, the more heat is resisted.

Heat itself is not sufficient to cause anything to rip itself apart. There must be some kind of mechanical action for that to happen, like water turning to high pressure steam, or stresses induced by local expansion of the material itself.

But all of this would be moot if you would stop insisting that carbon is an insulator. It's not; it never was. If your high school teacher taught you this so that "everybody knows it" I'd go have a little chat with him. Then again, maybe he is (was) teaching human geography in Colorado these days.

On the one hand, you want us to accept your scathing criticism of another poster's (unwarranted) response, and on the other, you wish to be completely free of the same criticism. You wanted to know how Boeing would handle this issue, but you completely ignore the corrections to your blatantly erroneous understanding of the physics and engineering involved in the solution. I recommend you read and learn; there are those in this thread who obviously know far, far more than you and could obviously teach you a thing or two about specific thought and research.

Quoting ContnlEliteCMH (Reply 35): An excellent example of why high school physics courses are routinely covered in a scant few weeks of college-level material: they don't actually teach much.

Same with any other subject. Math, economics... High school is just much less focused.

Quoting Starlionblue (Reply 36): Same with any other subject. Math, economics... High school is just much less focused.

It has to be because it's job is to impart a successful education to nearly everybody regardless of potential. College, well, they'll throw you out in a heartbeat and smile while they do it.

The math issue is particularly troublesome, because it is the language of science and engineering. Don't speak math? Then you don't understand science or engineering. Come to think of it, you don't understand accounting, economics, or investing. Then you say things like "The record profits that Exxon recorded are scandalous" depsite a drop in profit margin or any examination of how the oil companies actually make money. I mean, we wouldn't want real numbers and provable knowledge to get in the way of cool-sounding mumbo-jumbo designed to cover it up, now would we?

Example: the definition of "change" and "delta" are exactly the same when written mathematically. Oh, sorry... that's another thread.

I wonder what the voltage drop, errrr delta, errrr change is across the 787 fuselage at lightning currents?  

Quoting ContnlEliteCMH (Reply 37): The math issue is particularly troublesome, because it is the language of science and engineering. Don't speak math? Then you don't understand science or engineering. Come to think of it, you don't understand accounting, economics, or investing. Then you say things like "The record profits that Exxon recorded are scandalous" depsite a drop in profit margin or any examination of how the oil companies actually make money. I mean, we wouldn't want real numbers and provable knowledge to get in the way of cool-sounding mumbo-jumbo designed to cover it up, now would we?

I could rant at length about how much I agree with you. 

Fascinating technical info here - thanks all.

Quoting LMP737 (Reply 6): 3)A thin aluminum foil can be bonded to the outer layer.

Is this readily possible? If so, how much weight would it add (compared to, say, paint) and what would the foil look like? I'm thinking of what AA could do to keep its look on a composite aircraft should it order the 787.

Quoting Starlionblue (Reply 33): MATTHIAS: Making it worse?! How could it be worse?! Composites! Composites! composites! CROWD: Oooooh!... OFFICIAL: "I'm warning you. If you say 'Composites' once more-- [MRS. A. stones OFFICIAL] Right. Who threw that? [silence] Come on. Who threw that?!"

  

Sorry for the late reaction but I didn't see that at the time!

While I'm not disclosing anything new that hasn't been added, the fact that there is a wire mesh embedded in the composite material says that a high voltage discharge like a lightning strike has been very carefully considered in the design of the 787. And yes, damage is both encurred in a metal as well as a composite structure. Both the electrical energy and the point source heat energy has to be calculated and demonstrated to the FAA that the aircraft is capable of handling it in the structure before it ever gets built, much less flies. And detection techniques are in place for each material. And repair techniques are in place for each material. So to answer the basic question, yes, it has been designed for. Now as far as the specifics of the design, thats where it gets very interesting. If you wanted to get very technical, determine the electrical field patterns for a metal tube and then for a composite tube. And then consider whats inside both tubes, i.e. fbw controls, electronics, pace makers, etc. A metal fuselage operates in itself as sheilding. Not so in carbon fibre composites. Shielding those elements will be the responsibility of the mesh. Its like the mesh surrounding a basic signal cable that keeps extraneous electrical signals from entering the cable. So in short what I am saying is that the structure isn't really at risk, its all the things in the structure that have to be able to survive the strike. So considering that brings all kinds of surge protectors, circuit breakers, computer redundancy, heat sinks, etc, etc, ad nauseum...that is where the emf design engineers earns their living. And I'm sure that they are doing that on the 787.

Here is a real life accident caused by lightning striking a composite structure. Concerns about this issue are not totally unfounded. They don't always think of everything before they allow an aircraft to fly. Any student of aviation history can cite the examples.

Composites are the future, but I'm sure this issue will rear its ugly head in the future in ways not anticipated.

Quoting Tockeyhockey (Thread starter): As I understand it, normal airliners withstand lightning strikes because they are made of very conductive materials -- mainly metal.

That's the primary reason why the contents of the airliner are protected...it doesn't do a whole lot to protect the aircraft itself. The radome is a common entry point and, by far, the most common exit points are the nacelles, wing tips, and empennage. All of which are composite on most production airliners today. Although it's a legtimate concern, it's a looooooong way from being a new problem.

Quoting Tockeyhockey (Thread starter): does anyone have any insights on what boeing is doing to deal with this problem?

About a year ago there was a fairly extensive piece on this from Boeing. The major mitigation appears to be some kind of bronze mesh in the composite laminate.

Quoting AA737-823 (Reply 4): How much of the 787 fuselage will actually be composite? I think it's just the skin..

Skin, stringers, most frames, spars, leading and trailing edges, floor beams, stanchions...it's most of the primary structure except the really highly loaded things.

Quoting Tockeyhockey (Reply 21): actually, my language on lightning strikes is pretty accurate -- because composites are not conductive, the energy does not flow through them. instead, it expands outward (explodes) and splinters the carbon-based material.

Composites are all conductive at lightning voltages. Some of them are quite conductive at normal voltages too. The conductivity of CFRP is the main reason why you can't bond aluminum directly to CFRP.

Quoting Tockeyhockey (Reply 27): anyone who has taken a high school physics class knows that energy does not flow through composite materials. that is why composites are often used as insulators.

Some types of energy, specifically strain energy, flows *very* well through CFRP.

Quoting YYZYYT (Reply 39): Quoting LMP737 (Reply 6): 3)A thin aluminum foil can be bonded to the outer layer. Is this readily possible?

Yes. You see this on the interior side of the ECS pack bay doors on older 737's.

Tom.

Carbon fiber is conductive. Epoxy, not so conductive. Do you actually get contact between layers of carbon tape in Boeing's CFRP? My totally uneducated wild guess is that the epoxy sheathing the fibers would make CFRP a lousy lightning conductor without the bronze mesh.
The main thing you need in lightning protection is predictability. You need to steer that current where you want it to go. If you had conductive tape, but the tape layers weren't in contact with each other you could get current going everywhere, and jumping across high resistance areas in the fuselage, turning those excited little electrons into calm ones and a lot of high energy photons. (For the "I read a book and now I'm an expert") folks who try to impress people with their knowledge of various forms of energy.
Damn. Now you got me doing it. I hope you're happy.

Quoting Prebennorholm (Reply 30): Not as good as aluminium which is the second best conductor only next to cupper.

Just to pick nits, both silver and gold are better conductors than aluminum, and silver is even better than copper. And both have noteworthy uses as conductors (although with significant limits, given the costs).

Quoting Nomadd22 (Reply 44): Do you actually get contact between layers of carbon tape in Boeing's CFRP?

Given the high fiber fraction and cure pressure, it's probably pretty difficult to avoid having contact between tape layers. Keep in mind that any lightning mesh is going to be embedded in the composite...it's only electrical interface with the outside world (except at fasteners) is via contact with the carbon plies.

Tom.

Quoting Tdscanuck (Reply 43): Quoting Tockeyhockey (Thread starter): does anyone have any insights on what boeing is doing to deal with this problem? About a year ago there was a fairly extensive piece on this from Boeing. The major mitigation appears to be some kind of bronze mesh in the composite laminate.

On the wing box expanded copper/aluminum foil is employed where the skin is attached to ribs and spars by fasteners. Arcing between the skin and fasteners is a concern were integral fuel tanks are. This is further mitigated by fuel tank inerting.

I

Quoting Rheinbote (Reply 47): On the wing box expanded copper/aluminum foil is employed where the skin is attached to ribs and spars by fasteners. Arcing between the skin and fasteners is a concern were integral fuel tanks are. This is further mitigated by fuel tank inerting.

I'd guessed that the bronze mesh wasn't so much to carry the current from the lightning strike to "ground, since there isn't really any ground in an airborne plane, but to keep the charge even throughout the structure so you won't get arcing in places like that. Extra conductors between points like exposed rivets whick would take direct hits and the skin, which could acquire a substantial surface charge would help keep arcing down.
It doesn't hurt to be sitting at 250,000 volts, as long as everything is at that voltage.

Quoting Nomadd22 (Reply 48): I'd guessed that the bronze mesh wasn't so much to carry the current from the lightning strike to "ground, since there isn't really any ground in an airborne plane, but to keep the charge even throughout the structure so you won't get arcing in places like that.

It's there to carry it from the entry to the exit point. The lightning will go to ground (the real ground) and there's nothing the airplane can do about that. However, the entry and exit points will not be the same. The mesh is there to get the current from the entry to the exit safety without frying everything in between.

Tom.

Quoting Tdscanuck (Reply 49): The mesh is there to get the current from the entry to the exit safety without frying everything in between.

I think we're pretty much saying the same thing. The way you carry the current "safely" is to keep all areas of the plane at pretty much the same potential, so you don't get arcing. That means not just making a good current path, but keeping good conductivity throughout the structure so you don't get radically different voltage levels throughout the fuselage.
Planes don't get hit just because they happen to be in the way of a lightning bolt, but because a good conductor gives the arc a shortcut through the air.
If you just put lightning rods on the top and bottom of the plane attached to the frame or copper bar, you'd have a good path from entry to exit, but you'd also have a several hundred thousand volt difference between parts of the plane. You need to be sure all areas of the plane have good electrical connection to all other areas to avoid the localized charges that cause arcing and damage.
The mesh isn't just to give lightning that hits the skin somewhere to go, but to simulate a metal skin and keep the voltage potential constant throughout the fuselage.
Ben Franklin's lightning rods probably burnt down as many houses as they saved, because they actually drew lightning strikes, and few people grounded them well enough to carry the current.