Composites and lightning discussion (extracted from AF A330 thread)
 

Speculation - even in some cases here, informed speculation - is just that. However, as a general comment - and not connected to this tragedy, journos - the TWA 800 tragedy in 1996 caused a review of aircraft designs related to fuel tanks, hence FTS/CDCCL (and SFAR 88). One of the considerations was/is, of course, lightning as well as on-board causes such as compromised or ageing sytems, bonding, wiring or tank components (it is a very complicated subject). Bonding is fundamental to protection against electrical faults and is sometimes more difficult to assure with composites.
SFAR 88 and equivalent JAA/EASA documents required TCHs to assess their designs with respect to ignition sources. This spawned a whole raft of ADs and requirements such as training for those involved in continuing airworthiness functions (including maintenance), the need to maintain design characteristics (in maintenance, maintenance documents and planning functions) and the need to adhere to approved data when maintaining and planning maintenance.
(For example, as part of the assessment required under SFAR 88, some arc-testing on fuel tank components related to FQIS revealed that components passing 2000v arc tests when new arced at less than 50v when over 30 years old).
To iterate: a general comment only. Let's wait for the official report.

I've read some of the previous posts regarding the susceptibility of composite materials from lightning strike.

Because weight is a major consideration in aeronautic design, aircraft structures, like wings, flight controls are made of light-weight yet strong composite materials. These materials are made up of several layers of different composites, and in these layers the materials often have a different orientation to increase strength. But, because these modern composites exhibit strongly anisotropic electrical and thermal conductivities and because they have low conductivity compared to metals, when the high electric currents due to a lightning strike flow through them, they experience a high temperature rise and are vulnerable to heating damage. The heat flowing through the composite structure also has an effect on aircraft parts close to the location of the strike.

One probably cause maybe...

IGGS
 

No IGGS on A330/340 as far as I know.

Last July (July 12th 2008 if I remember well), FAA made it compulsory on new programs to have "tank inerting systems" installed, so both B787 and A350 will be equipped.

I haven't heard of any commercial airliner being equipped apart from the 787 and A350.

Not quite sure what the A340 has to do with this thread? A/c in question was a 330.

It's coming soon on the 777.

Reply to #203:

I hope it's not a fuel tank explosion either! :eek:

I have worked within the EMC field for some years and was only last week reading about the (theoretical) problems faced by aircraft designers with regards to lightning hitting carbon fibre sections.

Lightning Strike Protection For Composite Structures: COMPOSITESWORLD.COM

well actually no...
 

The 787 *must* have fuel tank inerting - along with two other EMI protection schemes - interference-fit fasteners and metallic earthing paths in the wing skin. It needs these because of the very poor electrical conductivity in CFRP and cannot be certified without them. This is not just a lightning strike issue, it is to cope with component failure currents also.

Airbus have the same requirements on the A350. Implying that Airbus have a more lax approach to limiting in-tank ignition is incorrect.

I also recall that hull losses attributed to lightning strike include two Boeing a/c and no Airbus airframes (Aviation Losses from Lightning Strikes - National Lightning Safety Institute)..

Recently incorporated on newly delivered 737NG.

Commercial aircraft with a lot of composites will probably be equipped with metallic grids within the composite material, to lead the current towards a metallic skeleton (though light) that is located within the fuselage to duct the heavy voltage current through the fuselage (and wings TBC) without damaging the structure of the aircraft.
http://www.flightglobal.com/articles/2007/09/28/217062/metallic-strips-will-ensure-electrical-continuity-in-a350s-carbon.html

^^ but it depends on how well the structures are bonded with each other. When lightning hits a composite structure, which is the reason why lighting strikes cause burn marks on the skin. After a lightning strike, repairs must be done properly to regain the conductive path, but if this is not done correctly it may cause alot of problems to the structure.

Just to clarify the physics - in a metal, the current must flow over the surface - so everything inside is completely protected in an all-metal airplane (assuming good corrosion maintenance). In CFC, a poor conductor, the current can penetrate into the body of the material and create tremendous heat, destroying the material through explosive fragmentation and vaporization. Once a tall tree in my front yard took a large lightning strike. The 100 ft tree was reduced to a 10 ft stump and pieces of it were found a block away. No piece larger than a football was found. The remaining wood fibers were splayed out like an old paint brush. The same thing can happen to CFC, completely disrupting air flow around the damaged area. My guess is that one way or the other, composites are involved here, either through direct structural failure or otherwise, and we're looking at an industry-changing event.

-drl

@203
 

Not completely true newer B737 also havd nitrogen systems in fuel tanks.

CFC
 

Desitter wrote

The lightning hypothesis still remains a possibility and will beg many questions about composite aircraft materials but any investigation requires the discovery and retrieval of the aircraft and more specifically any composite components that might have been compromised by a lightning strike.

Given the huge area that will need to be traversed and the likelihood of mid air disintegration, the chances of definitive component failure mode identification remain pretty low I think.

Given the Cb and turbulence hypothesis, other material failure modes e.g. failure under high bending moment will also, of course, feature as possible causes.

Truth is we just don't know and the thought that over 200 people have probably lost their lives is a very depressing one tonight.

B737NG Nitrogen Generation System
 

A bit off the topic but as previosly mentioned the latest B737NG has Nitrogen Enriched Air pumped into the Centre Wing Tank Vent cavity. The flow varies with aircraft altitude and flight phase..........

No doubt they will collect more Data and the puzzle solved.............:confused:

I feel for the family and friends:sad:

CFC-related concerns
 

Technical concerns [related to] composite fuselage:

QUOTE: "Remember, an airplane acts as a FARADAYS cage whereby the electronic loads stay on the outside of the cage!"

Not when it's got panels made of carbon fibre, it ain't!!

Lighning will jump through the carbon fibre and strike any piece of metal it can find behind!

The A330 has a horizontal tail made from carbon fibre which contains a huge fuel tank.

The wings, I believe have carbon fibre sections, these too contain fuel tanks!

The A330 doesn't have a "nitrogen fuel tank inerting" feature, that the carbon fibre boeings have, in order to reduce the risk of fuel tank explosion!

(I'm not saying that this is the cause.)

True about carbonfibre,


BUT


the carbonfibre in the tail or in any other part has a metal mesh inside for exactly this reason. Do you really believe authorities would certify an airplane with a possible explosive tailplane?

Dont think so....

composites found on aircraft arent really that much of a concern during lightning strike as long as they are bonded properly particularly after doing composite repair...

CFRP and the A330
 

Whilst it is true that the A330 uses carbon fibre materials, where required they incorporate a conductivity network to cope with lighting strikes. This design principle has been used since the A310.

Metal mesh or not, if there is any moisure within the composite, it will explode when struck by lightning.
The moisture will instantly expand to 800 times its normal volume causing it to fail.
The same mechanism caused the damage to the afore mentioned tree.

And finding moisture in a composite panel is not all that uncommon, especially if the panel has been damaged (impact) by tooling or other methods.