Composites in airliners and Lightning

[deSitter]

It struck me like a bolt today (haha) that the electrical protection offered by an all-metal can in the sky is not going to be present on something like the 787 - this would seem to mean that lightning might find its way inside the structure, particularly as the components age and gaps appear, or worse, create a situation at metal-composite interfaces where great heat might be generated, e.g. if the composite breaks down electrically under an enormous potential applied across it in a lightning strike.

I assume this problem is understood by aero-engineers. Right?

-drl

[piggybank]

As far as I remember, composite helicopter blades (as an example) use a metallic mesh in the skin construction and the whole assembly has a metal bonding strap to equalise any electrical charge between the head and blades. I was in a Puma in 1980 travelling from Natuna Island at night to Singapore with a medevac, when we built up St Elmos fire.

An interesting 'scenic' experience, when the electricity discharged from the helicopter, it was like looking down a cheap torch beam, brighter at the edges than the centre. Maybe I have digressed a bit, but yes the designers will certainly have thought it out as its a problem that has been known about for years.

[glad rag]

Bla Bla Bla, Seattle know everything, we don't need bonding in our 787 wing tanks 'cos we will inert them with N2 Bla Bla Bla.

Simple really, don't think, just do.:bored::bored::bored:

[Intruder]

Dunno where you got that impression...

A friend of mine is an engineer in the fuel systems shop at Boeing. The different types of bonding required in the 787 are a major issue, and are NOT being disregarded!

[muduckace]

As of recent Airbus had operated with composite materials that now Boeing is adapting out of a need to compete. Airbus sets guidlines for bonding of composite materials to the airframe, none the less the A300 stab/rudder failure over Long Island makes me suspicious of the ability to inspect MAJOR structrual components made out of composite material.

Alumininum and alloys are easily inspected for structural degredation as extirior cracks are visible to simple testing procedures way prior to catastorphic failure. Metals dissipate static electricity easily with the means that are in current use.

Composite materials I can only hope (eg: 787) have allready had thurough process engeneering to allow for the same static dissipatition abilities and new inspection procees to find stress damage prior to failure.

[Michael Birbeck]

The following summary, relevant to composites, caught my eye when reading about a CAA recommendation when reviewing the loss of a Puma helicopter

http://www.eurocopter.com/site/FO/doc/rotor_j/41/pa-41-22-00.pdf

With respect to fixed wing aircraft there are examples of loss due to explosive delamination of the wing after lightning strike (these losses relate to aircraft not certified against lightning strike, e.g. gliders).

The following accident report gives a good lightning summary (although the loss here may be partly attributable to the different conductivity of aluminium versus steel).

http://www.pas.rochester.edu/~cline/ASK%20lightning%20strike/ASK%20accident%20report.htm

The use of different composites in new commercial aircraft (reported 50% by weight for Boeing's dreamliner, for example) opens up a whole new area of research in thermal and electrical conductivity and is an active area of materials research which means that although enough is known to mitigate against the known risks and thereby obtain certification, the book is still open and will continue to be so as composites become integral to the whole of the aircraft (acoustic damping, thermal transfer and the electrical system). :hmm:

Each lightning strike will be a minor test but this is true of the current generation of commercial aircraft (made of more traditional materials). The risks are not significantly greater but are not negligible either.

In an ideal world we would (literally) avoid the potential for lightning strikes.

[Denti]

There was a fatal accident last year in germany where a Diamond DA-42 IFR and known icing approved light piston twin crashed after a lightning strike. Sadly the final report is not out yet and the preliminary is in german only.

However it seems from the report that the twin in question received a lightning strike which lead to instant explosive delamination of the elevator. Remember, that type of aircraft is IFR approved and has inlaid metal meshing within the composites to prevent damage by lightning strikes. Of course that type is a JAR/FAR 23 aircraft and not 25, so the requirements differ by quite a bit, however it is a full composite aircraft that should be able to withstand lightning strikes.

[dixi188]

We lost a 6 inch by 6 inch triangle of metal elevator tip due to a strike a few years ago. The entry point put a small 1 inch burn on the radome. When the radome was opened the inside had exploded for at least 1 foot accross.

If something like this happened on a composite aircraft how would the maintenance engineers inspect for this sort of damage?

(A300-B4 with all metal stab by the way).

[Michael Birbeck]

Tap testing (initially) has been the cheapest method for testing composite structures. Damage is inferred from audible differences in the structure.This is an art as much as a science and is prone to human error. In safety critical areas this might result in greater expense based upon a conservative approach to judging the health of a part or a structure (replace rather than assume OK - false positives).

Thermographic and ultrasonic testing is also used in many cases (particularly if visual/tap inspection shows problems like debonding).

Currently a new range of non destructive inspection techniques (NDI) is evolving e.g. a test program is running under the auspices of the FAA and this is resulting in the definition of standards for testing composite laminates etc. The aim of defining standards is to allow the continued use of standard tools without impacting day to day operations while ensuring safety, reducing false positives and keeping costs within industry tolerances.

Tools like the Boeing MAUS system (resonance mode) are used to test composite materials in a number of industries.

The following PDF gives an insight into what people like NASA and the FAA are doing to ensure safety right across the fleet spectrum including next generation aircraft using a high % (by weight) of composite materials.

http://www.aeronautics.nasa.gov/nra_pdf/aad_technical_plan_c1.pdf