# How well does a composite fuselage deal with lightning?

The metal fuselage of an aircraft, being highly conductive, protects occupants, interior components and fuel from the effects of lightning by conducting the electricity around them.

Some newer aircraft, such as B787 have a fuselage made entirely out of composite materials. The major portion of the 787 is made from carbon fiber plastic laminate.[1] Although carbon fiber conducts electricity it has a higher electrical resistance.[2][3] Higher resistance means more heat generated by the material. And carbon fiber makes up only a percentage of the composite.

Has the composite material been found to have enough conductivity to deal with a direct lightning strike without damage to either the material itself or the contents of the plane? Or do designers deal with lightning strikes in some other way?

[1] Boeing 787 Systems Manual.

[3] Electrical and mechanical properties of electrically conductive polyethersulfone composites. Lin Li and D.D.L. Chung, 1991, State University of New York at Buffalo. [.pdf]

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Note that higher electrical resistance mean a greater potential voltage difference across the parts of the plane. It is this higher voltage which results in higher temperatures. It could also conceivably cause the current to seek alternative, lower resistance paths through the airframe. (Such as perhaps bags of wet meat) – dotancohen Feb 1 at 11:47

## 2 Answers

In case of composite materals, the usual solution adopted for lightning protection is to have a conductive layer in the structure. Basically, this provides a continuous conductive path of low resistance over the entire aircraft exterior (fuselage, for eg), with additional protection in zones where lightning is most likely to attach (for example, the radome). This is achived usually by having a thin conductive copper mesh embedded in the structure.

For example, the 787 uses two main methods to achieve lightning protection.

• The fuselage has a copper mesh 'baked' into it.

• Some of the extremeties have metal foils in it to protect against lightning strike.

The composite layup uses an expanded metal foil (EMF) in its structure to provide lightning strike protection (LSP). The EMF is usually placed above the CFRP layer, as shown below. The EMF is usually a thin, lightweight metal foil embedded in the outer laminate of the CFRP or in the surface film in some cases.

The composite structure layup shown at left consists of an expanded metal foil layer shown at right. This figure is a screenshot from the COMSOL Multiphysics® software model featured in this blog post. Copyright © Boeing. Imge from comsol.com

787 also uses metal foils in some extremties (wings, nacelles etc.). From 787 Systems review:

Wing fuel tank skin surfaces— electromagnetic effects protection. Fasteners with dielectric tops (sealant flush to the outer surface of the fuel tank skin) in combination with copper foil protect against external ignition sources such as lightning and electrostatics on the wing tank skin surface by diverting and distributing current.

The metal mesh is the usual method in practice at present and used in other places too, like helicopter rotor blades and fuselage. However, work is going on for using other methods like conductive composites, conductive surface layers etc.

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Lightning striking modern aircraft with composite structures

Composite parts that are in lightning-strike prone areas must have appropriate lightning protection. [...] Composite structures are less conductive than metal, causing higher voltages.
Lightning protection on airplanes may include wire bundle shields, ground straps, composite structure expanded foils, wire mesh, aluminum flame spray coating, embedded metallic wire, metallic picture frames, diverter strips, metallic foil liners, coated glass fabric, and bonded aluminum foil.
Lightning can also damage composite airplane structures if protection finish is not applied, properly designed, or adequate. This damage is often in the form of burnt paint, damaged fiber, and composite layer removal.

Source: "Lightning Strikes: Protection, Inspection, and Repair", Boeing, 2012

New technology improving the lightning protection of modern aircraft

Traditional methods to protect composite aircraft from lightning strike damage rely on a conductive layer embedded on or within the surface of the aircraft composite skin. This method is effective at preventing major direct effect damage and minimizes indirect effects to aircraft systems from lightning strike attachment, but provides no additional benefit for the added parasitic weight from the conductive layer. [...]
A new multi-functional lightning strike protection (LSP) method has been developed to provide aircraft lightning strike protection, damage detection and diagnosis for composite aircraft surfaces.
The method incorporates a SansEC sensor array on the aircraft exterior surfaces forming a "Smart skin" surface for aircraft lightning zones certified to withstand strikes up to 100kA peak current. SansEC sensors are open-circuit devices comprised of conductive trace spiral patterns sans (without) electrical connections. The SansEC sensor is an electromagnetic resonator having specific resonant parameters (frequency, amplitude, bandwidth & phase) which when electromagnetically coupled with a composite substrate will indicate the electrical impedance of the composite through a change in its resonant response. Any measureable shift in the resonant characteristics can be an indication of damage to the composite caused by a lightning strike or from other means. The SansEC sensor method is intended to diagnose damage for both in-situ health monitoring or ground inspections. [...]
The SansEC sensor smart skin technology provides lightning strike protection, shielding effectiveness, and opportunity to achieve in-situ damage detection and diagnostics for aerospace composite structures along with other potential beneficial functions not available from the standard LSP method.

Source: Szatkowski, G. N. at al.: "Open Circuit Resonant (SansEC) Sensor Technology for Lightning Mitigation and Damage Detection and Diagnosis for Composite Aircraft Applications", NASA, 2014 - Download Link

Further information can be found on the internet, see for example "Playing with Lightning in the Name of Aircraft Safety", NASA.

See also this similar question.

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