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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?

The certification of fuel systems is an important aspect of the overall certification of an aircraft against lightning hazards and embraces almost all disciplines needed to deal with the lightning interaction. Both direct and indirect effects are encountered and some understanding of the physics of fuel combustion and probability theory concerning ignition is also necessary to appreciate the factors influencing the safety or otherwise of a design. At one time the Western Air Forces were losing one aircraft every other year, due to lightning related fuel explosions. Similar accidents occur to Civilian aircraft, albeit less frequently. After a brief review of aircraft fuels and their flammability and some discussion of combustion processes, the paper considers factors affecting Minimum Ignition Energy (MIE) encountered during flight profiles, such as oxygen enrichment, temperature and pressure.

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.

In order to demonstrate comprehensively for every cable to every flight safety critical equipment on an aircraft, that the qualification test levels were greater by a margin than the levels of induced threat on the aircraft, it is necessary to measure the current or voltage on all of those cables during whole aircraft tests. In this paper an approach is described which relies on a statistical distribution of induced threat amplitudes in the cables within an airframe to reduce the number of cable measurements needed, and fill in for cables that cannot be measured. The method also offers, for the first time, a viable approach to the substantiation of a similarity argument.

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.

This paper describes the latest ideas on a modified process and methods for the clearance of aircraft to the lightning external environment. It covers the whole sequence from the specification of equipment qualification requirements to the demonstration that these are within the electromagnetic environment in which the equipment is installed in the event of strikes up to the severity of the internationally agreed full-threat level. In particular, the process includes computational modelling and whole aircraft/system testing as standard tools, requiring additional levels of confidence where either one is not available or un-feasible. It also contains methods proposed to improve the integrity of whole aircraft testing; an essential concomitant of the availability of computational techniques and the need to validate them. The reasons for changes to currently accepted practice will be explained and justified.