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Advanced commercial aircraft increasingly use more composite or hybrid (metal and composite) materials in structural elements and, despite technological challenges to be overcome, composites remain the future of the aviation industry. Composite and hybrid aircraft today are equipped with digital systems such as fly by wire for reliable operations no matter what the flying environment is. These systems are however very sensitive to electromagnetic energy. During flight, aircraft can face High Intensity Radiated Fields (HIRF), static electricity, or lightning. The coupling of any of these threats with airframe structure induces electromagnetic energy that can impair the operation of avionics and navigation systems. This paper focuses on systems susceptibility in composite aircraft and concludes that the same electromagnetic rules dedicated to all metal aircraft for systems and wiring integration cannot be applied directly as such for composite aircraft.

The lightning represents a fundamental threat to the proper operation of aircraft systems. For aircraft protection, Electromagnetic Compatibility requires conductive structure that will provide among all, electromagnetic shielding and protection from HIRF and atmospheric electricity threat. The interaction of lightning with aircraft structure, and the coupling of induced energy with harnesses and systems inside the airframe, is a complex subject mainly for composite aircraft. The immunity of systems is governed by their susceptibility to radiated or conducted electromagnetic energy. The driving mechanism of such susceptibility to lightning energy is the exposure to the changing magnetic field inside the aircraft and IR voltage produced by the flow of current through the structural resistance of the aircraft. The amplitude of such magnetic field and IR voltage is related to the shielding effectiveness of the aircraft skin (wiremesh, composite conductivity).

In order to guarantee systems immunity, lightning induced electromagnetic energy has to be lower than the system's susceptibility threshold. This can be achieved, if the aircraft structure provides a good protection against lightning current as well as against its electromagnetic induced field. Moreover such a structure is also required to constitute a ground plane that guarantees very low common mode impedance between all grounded systems in order to keep them at the same electrical potential. The interaction of lightning with aircraft structure, and the coupling of induced energy with harnesses and systems inside the airframe, is a complex phenomenon, mainly for composite aircraft. Composite structures are either not conductive at all (e.g., fiberglass) or are significantly less conductive than metals (e.g., carbon fiber).

During its flight an aircraft can be struck by lightning and the induced high current will require a highly conductive airframe skin structure in order for it to propagate through with minimum damage. However an aircraft skin is generally coated with paint and the airframer does not always have control on the paint thickness. Paint thickness generates heightened concerns for lightning strike on aircraft, mainly because most of coatings dedicated to that purpose are non-conductive. Using insulating material or non-conductive coating with certain thickness may contribute to or increase damage inflicted by the swept stroke lightning energy, even on metallic structures Due to its high relative permittivity, a non-conductive paint or coating on a fuselage skin surface will contribute to slow down the lightning current propagation through structure. With this comes the risk of increasing heat that will favor structural damage and possible melt through.

In the past ten years, the all composite commercial aircraft has become a reality and the need for the aircraft designer to consider electromagnetic threats has also grown. Aircraft systems are now designed with miniaturized electronic components, which make them more sensitive to EMI; and it turns out that the safety of flight relies on the functionality of some of these systems. Composite materials (Polymer Matrix Composites, PMCs) are characterized by their low conductivity that greatly reduces the shielding effectiveness of the aircraft structure and consequently the protection of systems against HIRF, and mainly against lightning indirect effects.

Electromagnetic threat could be an issue for aircraft safety. HIRF and lightning are the more known threats. However, even if the scientific community seems to focus on the increasing danger of passengers' electronics devices (PEDs) on board aircraft with respect to interference with aircraft systems, Electrostatic Discharge (ESD) appears to be a more harmful threat for aircraft systems than PEDs. ESD charging and discharging phenomena may lead to interferences with communication and navigation systems. More, ESD also generates E field pulses and voltages transients that could be fatal to some electronic systems components. With composite aircraft, because of the lack of skin conductivity, ESD could become a very high concern for systems immunity.

The advances in electronics and software since two decades have increased the use of digital technology in modern aircraft. Because digital electronics are powered with low level energy and are made of miniatured components, they are very sensitive to electromagnetic energy. It takes a single glitch to change the entire information. Therefore, insuring the proper protection of critical systems against Electromagnetic energy is a must during early design phase of the modern aircraft. All the onboard systems that govern aircraft functions are interconnected by hundreds of meters of wires and bundles. Moreover, the immunity of any signal on board aircraft is for at least 50% guaranteed by the cable shielding effectiveness and the shield termination. Wiring acts like an antenna: it can either radiate or collect energy. If not protected, it may pick up any electromagnetic noise in the surrounding; consequently, unwanted energy can modulate signals and cause system malfunction.

During an aircraft flight, water droplets, snow or precipitation will impact the fuselage and the wing surfaces, and develop static charges on the structure. Because of composite materials, those charging particles can produce potential differences between surfaces of different aircraft zones. The transients generated by the discharge can couple into onboard electronic/electrical systems and cause no found faults or upsets. Electrostatic Discharge (ESD) phenomenon resulting from corona effect can generate very high electric field that can couple with wiring bundles and generate communications disruption during flight. ESD effects have to be properly addressed during an aircraft design, as are Lightning and HIRF. This paper presents some theoretical considerations and experimental data based on full aircraft ESD ground tests at Bombardier facilities in Mirabel (Montreal).