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). Consequently, current from a lightning strike will not linger long time on aircraft skin (outside) but will enter the structure and seek metal paths available to propagate through. This is why the protection of a composite aircraft from lightning strike induced effects is a real challenge, and cannot be improvised. A good knowledge of the physics related to the coupling mechanism of lightning energy and a composite aircraft including systems inside, is essential in order to provide adequate protection to the airframe and its systems with minimum cost and weight.
When a metallic aircraft is struck by lightning, the current that will couple to cables interconnecting systems inside that aircraft will be governed by the aircraft structure, the cable shield electrical characteristics, and of course the lightning threat parameters: current amplitude, wave shape and propagation time.
For a composite aircraft it becomes more complex, because more parameters will manage the lightning current induced on harnesses and its effects on systems.
In the same lighting environment, two equivalent airframes with one made of composite and one all metal, lightning current and its induced effects on systems, will be very different and higher on composite aircraft from those observed on a metallic aircraft. Consequently, systems and harnesses will require more protection on a composite aircraft compared to a metallic aircraft.
This paper conducts a theoretical analysis supported by some experimental data on the interaction of lightning with composite aircraft versus all metal aircraft. The paper also assesses the induced effects on different types of systems.