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The airworthiness certification authorities specify overall probability levels for catastrophic and less severe effects on aircraft and their occupants. In lightning standards concerning threat levels and zoning for lightning attachments we speak of high and low probabilities. But, despite the certification authority’s overall figure, only one attempt has been made to interpret what that figure means for lightning protection. That one attempt was made under the EC funded FULMEN programme to estimate the degree of accuracy needed in the process of aircraft lightning attachment zoning. Without some figures, how do we know how good our designs have to be. Furthermore, as the number of flight-safety critical systems on our aircraft increases, how does the probability of failure of each change to ensure the overall figure remains the same?

In the frame of the European project CATE (Composite and Advanced Aircraft Technologies Electromagnetic Protection), a full CFC mock-up has been designed and manufactured. Both measurements and computations have been carried out to investigate and to assess the coupling mechanisms throughout CFC structures. Different aspects have been covered: effects of apertures, effects of metallic devices, effects of protection on cables, … The understanding of the physical phenomena involved enables the formulation of general rules of protection which can be applied on very large aircraft at lower cost.

It is a view held by many, but not all, that whole aircraft testing for measuring levels and waveforms of induced cable currents and voltages should be carried out at moderately high levels of imposed external threat. The reason for this proposal is that the are a number of effects occurring during the passage of a lightning strike that will result in different cable transient amplitudes and wave-shapes, depending upon whether certain current or voltage thresholds are exceeded. These are the so-called non-linear effects. In this paper we argue that it is neither practicable nor technically correct to attempt to emulate these effects and test at very low injected current levels.

Lightning induced current and voltage pulses are defined in international standards as arising from three distinct coupling mechanisms: capacitive, inductive and resistive. These mechanisms at their simplest give rise to distinct characteristics in the induced wave-shapes relative to the lightning current pulse that caused them. It has long been the practice to decide from a particular induced wave-shape, which was the likely induction mechanism, and compare it in terms of peak amplitude only with the relevant qualification test waveform. This approach fails to take account of the fact that almost all induced waveforms are actually a sum of two or all of the coupling mechanisms, that the coupling is not simple but gives rise to much more complex wave-shapes than the qualification standards would imply, and that there are other critical parameters apart from the peak amplitude. It may also disguise the effect of possible building/rig resonances in test results.