Aerospace, Military & Defence
Managing EMI, grounding and lightning strike protection in today’s modern aircraft
T
he aerospace industry is a leading user of carbon/kevlar reinforced polymer or similar composite
materials. The major advantage of these is their extremely low weight compared to metals, which delivers several benefits such as greater fuel efficiency, reduced engine size or cost, and the ability to carry a greater weight of passengers or cargo. In combat aircraft, lower weight leads to improved performance and agility, while reduced metal content can help in achieving a low radar footprint. The advantages of lower weight have
resulted in a few technical challenges since the composites themselves are not intrinsically electrically conductive. Modern aircraft have numerous electrical systems capable of generating EMI, which can potentially disturb the operation of critical systems such as navigation, fly-by-wire and engine management. These include, fluorescent lights, light switches, dimming circuits, AC-powered window heaters, motors and generators, data cables and power lines running through the aircraft, and transmitters such as radio and radar. Externally, storms are a major source of potentially disruptive electrical interference and can cause physical damage to the aircraft through lighting strike impact. Phenomena such as vibration and airflow can result in hazardous electrostatic charges accumulating on the outside of the aeroplane, due to friction. A conventional, metal airframe allows designers to take advantage of the natural Faraday cage it forms to protect equipment against interference originating inside or outside the aircraft. There are
many opportunities to ground items of equipment reliably by connecting directly to a convenient surface ground. To cope with a direct lightning strike, a conventional metal airframe is designed to conduct the energy, by means of electrical continuity, towards an exit point. The airframe itself is able to distribute most of the strike energy throughout the outer fuselage, to prevent damage to electrical systems and internal equipment.
Recovering lost properties Designing-in structures built with composites means compromising the EMI shielding and lighting strike protection inherent in their metallic predecessors. To restore lost shielding, bonding or grounding properties, woven or non- woven copper-aluminium mesh, or an expanded foil, can be embedded in composite structures. The embedded metal is specified to provide an optimal combination of electrical conductivity, weight, and corrosion resistance. Solid metal strips can be used in the radome area, to handle very high concentrations of lightning energy. The optimum solution is typically a combination of grounding and shielding materials.
Embedded conductors, however, cannot solve all the electrical challenges that come with increasing use of composites. It is very challenging to ensure reliable electrical continuity between individual composite panels after the airframe is assembled and still promote conduction of lightning energy. Electrical modules are bonded or grounded by direct connection to the
airframe. These connections have extremely low impedance, however connections to the mesh can fail to meet the low impedance requirement because of environmental stresses caused by vibration and temperature variation. These locations, where grounded or bonded modules connections are made (figure 1), the exposed metal mesh can be vulnerable to environmental exposure (temp, humidity, salt fog, oxidation) that increases electrical impedance.
Applied performance enhancement
Additional metal cannot be added to overcome these challenges. The increased weight would negate the advantages of the novel materials. Hence, there is a need for effective, lightweight technologies that will enable designers to optimise conductivity in specific areas of the airframe. This can be achieved through a variety of conductive materials, such as fairing compounds, adhesives, gap fillers, coatings and greases. A silver-bearing conductive coating can be applied to complement the embedded metal mesh. This can improve the shielding performance of an enclosure and enhance lightning strike protection. The coating may be in the form of a two-component epoxy paint such as Parker Chomerics CHO-SHIELD 596, or a polyurethane coating such as CHO-SHIELD 4994, which has high solvent and wear resistance to be used externally. This coating is compatible with many primers and top-coat systems. In areas where high corrosion protection is needed such as access panels with mating
EMI shielding gaskets, a copper-based urethane coating such as CHO-SHIELD 2002 is idea. It is formulated to remain stable and inhibit corrosion when exposed to salt fog, temperature and humidity. Aircraft antennas pose an exceptional challenge in shielding and grounding, as they act as a lightning rod. Effective grounding of the antenna can be achieved through the use of an expanded woven Metalastic EXP-URE EMI gasket material. Electrically conductive grease is typically applied at ground connections, to support reliable electrical connectivity under temperature and vibration. Careful attention must be paid to viscosity and surface-wetting properties, when formulating greases for aerospace applications. Parker Chomerics CHO-LUBE E117, which has resistivity better than 40m /cm, is an example of an aerospace- grade grease. At locations requiring electrical continuity and environmental protection, a sealant such as Parker Chomerics CHO- BOND 2165 or CHO-BOND 1019 may be applied. Airframe features typically treated include screw holes, fasteners, antenna connection points and exposed conductors on external areas such as the wings, fuselage or tail. Where conductive gaskets are used to promote electrical continuity between composite components, a conductive sealant can be applied to provide improvement in continuity. These areas are typically around wheel wells, engine mounts, wings and the tail section, where high vibration occurs (figure 2). Composite materials have been
replacing metal structures throughout the aircraft industry primarily to save weight and improve fuel economy. Their lack of electrical conductivity of these composite materials is a notable disadvantage. Aerospace composites have evolved to recoup some of the interference protection provided by the metals they have replaced. High-performance conductive compounds such as Parker Chomerics fairing compounds, adhesives, gap fillers, coatings and greases, can play an important supporting role in ensuring the safety and reliability of modern aircraft.
Figure 1. Direct bonding of electrical connection to exposed metal mesh
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Figure 2. Conductive gaskets and sealants used to promote electrical continuity between metal-laminated composite structures
parker.com Components in Electronics March 2020 21
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