In the past 10 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 electromagnetic interference (EMI).
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 (high intensity radiated fields), and mainly against lightning indirect effects. Due to the low conductivity, lightning current—which is focused in the low frequency spectrum—will not decay through the composite skin material as quickly as it does in all-metal skin. Aircraft skin is not thick enough, compared to the skin effect for this low frequency spectrum, to reduce substantially the current through a composite structure. Therefore, the diffusion effect of lightning current in composite aircraft structures will be substantially higher than all-metal aircraft, so there will be the developed voltages in systems.
On all-metal aircraft, systems are bonded on the aircraft structure that provides a ground plane. On a composite aircraft, this can only be achieved through a grounding network. The grounding network impedance is mainly driven by its inductance on composite aircraft but also by its resistance, while with an all-metal aircraft the ground plane can be considered purely resistive.
As a consequence, the mutual inductance between the grounding network and the cable shield for a cable with 360° shield termination, or the mutual inductance between the grounding network with the pigtails in addition to the mutual inductance between the grounding network and the core of the cable for a cable with shield terminated with pigtails, will completely change the coupling mechanism of the current induced by the electromagnetic threat on the shield with the core of the cable and lead to a higher induced voltage at the system level in composite aircraft vs. all-metal aircraft, mainly for common mode systems. On a metallic aircraft, the same level of threat coupling to the cable shield will induce a lower voltage at the system level.
Current standards for equipment qualification (RTCA DO 160 or equivalent) still require testing systems for their qualification on a metallic bench test. Unfortunately, those ones are only resistive and will underestimate the induced voltage at the system input level and will not lead to the same test results as when using a grounding network on a composite structure. Consequently, there is an urgent need to improve the qualification standard requirement for them to match with the new reality of composite aircraft.
In the scope of a composite demonstrator project, Bombardier Core Electromagnetic Engineering has conducted a lightning indirect effect measurements campaign on different cylindrical barrels simulating all-metal and all-composite fuselages. The coaxial line return concept used for those tests at an Electronics Test Center (ETC) laboratory (an MPB Technologies company) in Ontario, Canada, develops an equal current density at all points around the barrel circumference since the magnetic field lines produced by the current propagating through the cylinder are made of concentric circles distributed all along the cylindrical barrel.
WF1 and WF5A current waveforms were generated on the different barrels, with the goal being to assess the excitation of different cables within those different barrels for the same current amplitude representative of the same lightning current waveform and to measure the induced voltages at the load level in dummy boxes that simulate equipment.
An aluminum barrel and a composite barrel were submitted to 4.68 KAmps WF5A. A 2-m coaxial cable interconnecting dummy boxes with 50 ohms load was excited with a maximum current of 9.4 A peak to peak in the composite barrel, while in the aluminum barrel the same coaxial was excited with only 384 mA pp. The results showed that the current induced on the coaxial cable in those two equivalent environments that differ by the skin structure is 24.5 times more in the composite structure than in the aluminum one.
The composite aircraft skin will not be thick enough, mainly in the frequency lower spectrum, to attenuate the lightning-induced current through the structure in the same magnitude as an all-metal aircraft. Due to structural skin electrical characteristics—thickness, conductivity, and permeability—lightning current will take some time to enter the structure by diffusion and develop voltages through systems.
With the same thickness for the two comparative structures, the penetration time constant for the aluminum barrel is greater than the one for the composite one. The effect is that the lightning current will stay on the aluminum structure longer and dissipate; this will also allow for a high dissipation of heat. The composite barrel lightning current will stay only for a short time on top of the structure and will not dissipate too much. Consequently, the current developed inside the composite barrel will be higher, and adequate remedies will be required to protect systems.
Following the excitation of the coaxial cable running close to a metallic ground plane inside an aluminum barrel, and the same test on a composite barrel with the same coaxial cable running at the same height above a Signal Return Network (or a Grounding Network) that was installed on the composite structure, the induced voltage was measured at the load level. That induced voltage measured at the 50-ohm load level in the interconnected dummy boxes was found to be 142 mV pp in the metallic barrel and 290 mV pp in the composite one. This result shows that the common mode system appears to be two times more susceptible inside the composite barrel compared to the aluminum barrel.
Moreover, the results highlight that for the same cable interconnecting the same common mode system, the induced voltages at the load level are not proportional to the threat inside a composite barrel compared to an aluminum barrel. In fact, 24 times more current measured on the coaxial cable in the composite barrel (compared to the current observed on the same cable in the aluminum barrel) for the same external threat does not equate to 24 times induced voltage but only two times more in the scope of this test. The explanation is through the coupling mechanism between the grounding network and the coaxial cable.
Currently, RTCA DO-160 and equivalent test standards provide guidance to test and qualify systems for any aircraft (metal or composite) with a metallic ground plane. Following these tests and analysis, it clearly appears that the use of metallic ground plane should not be recommended for the qualification of composite aircraft systems, mainly common mode ones. In fact, testing systems for composite aircraft with metallic ground plane may underestimate the induced voltage at the system input level and cannot meet the required composite aircraft system qualification threat.
This article is based on SAE International technical paper 2011-01-2513 by Dr. Fidèle Moupfouma, Chief Aircraft Electromagnetic Hazards Protection Engineer, Bombardier Aerospace Core Engineering. More information on this paper can be found at http://papers.sae.org/2011-01-2513.