Indirect Lightning Coupling Methods
In Part 1, there was a brief discussion of how lightning behaves within the structure of the airframe. The currents and voltages associated with these strikes can couple onto aircraft wiring in various ways. Several mechanisms are involved in this transfer of energy.
First, there are significant, rapidly changing currents in the aircraft’s skin due to the strike, with 200 kA possibly induced. High-level, rapidly changing currents generate strong magnetic fields along their paths. Wiring running along the airframe is within the magnetic field’s coupling path, inducing a reverse current in the cable bundle. See Figure 1. For these types of fields, lower-impedance cables and shields tend to be more sensitive to coupling.[1]
Figure 1: Lightning Currents Induced into Cables
Inducing a current inside the airframe requires a few things: seams and discontinuities in the airframe, and a high-frequency current that is not evenly distributed across the airframe. Apertures and other openings can also couple these fields. In an ideal metal tube with uniform current distribution, there should be no magnetic fields inside the airframe. However, this is not the case. When metal frames and structures, such as aluminum air ducts, are in contact with the airframe at various locations across the aircraft, lightning currents can flow through them as well.
Currents will flow around openings such as windows, and likely doors and access panels. This can concentrate currents at these locations. Fields can leak through the openings. Thus, cables routed near apertures may experience higher levels of coupling.
Figure 2: Fields coupled into loop created by pigtails’
Another effect is the use of shielded cables, which are terminated into pigtails. The pigtail creates a loop with some separation from the cables, which can couple these magnetic fields and induce currents in the core wires.[2]
When currents cross seams, joints, and composite fairings, the resulting impedance can induce voltages. Voltages can capacitively couple into cables, inducing voltages, especially on higher impedance lines (sensor and data lines). The amount of displacement current, or capacitively coupled current, will depend on the voltage rise and fall times. In the case of lightning, these can be very fast.
High-frequency rise times in aluminum airframes will induce currents mostly on the aircraft’s surface, as we know it as the skin effect. But the longer the lightning current hangs on, the longer the wavelength, and the greater the depth of currents appearing in the aircraft. Thus, the induced internal currents will have a long-wave appearance, with slower rise and fall times.
With the increased use of carbon fiber on aircraft (carbon fiber composite, or CFC), the distribution of current is found deeper under the surface and passes through the aircraft. The current waveforms induced in cables from deeper structural currents, such as those from CFC airframes, have longer waveforms, lower impedance, and higher amplitudes than those from surface currents on aluminum airframes.
Andy Plumer states, “Since the 1920s, large numbers of natural lightning current waveforms have been recorded at the earth’s surface. Thus, the characteristic waveforms of natural lightning currents are well-known to science.”[3] The typical waveforms used are either double-exponential or damped-sine. The double exponential waveform equation used is:
E = E0(e-αt – e -βt)
Where α and β are time constants used for each waveform, the waveform width is determined by the system’s source and load impedance – namely, whether the source (the airframe) is aluminum or composite fiber. Is the load structural or conductive, and if a cable, is it shielded or does it include a shield or ground? The location of the load will also influence the voltage, E, also known as the Transient Control Test Level. If the load is near the airframe, level 3 is imposed. If it is buried deeper in the structure, levels 1 or 2 may be imposed. Level 5 is reserved for cables and structures routed wings (such as flight control actuators, landing gears, and the like), where very intense and concentrated lightning currents may be found.
Part 3 will look at the waveshapes used.
References
[1] Lightning Protection of Aircraft Handbook, DOT/FAA/TC-22/11, 249-250. Full details of this effect are described throughout the document. Since this is public domain, it is highly encouraged to download and review.
[2] Ibid, 95-96.
[3] Ibid, 28.
