Probabilities of Catastrophe in Lightning Hazard Assessments

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? There are many figures we do not have, because lightning is too unpredictable an event, because we have too few measurements of the “real” lightning that we are trying to protect against, and because we have too little knowledge of the science behind the whole range of phenomena involved., Despite that we can make some intelligent guesses or estimates, at least to understand where about we are. Such an analysis would be of benefit also in the debate on whether the external threat level is adequate as it is presently defined, in furthering the understanding of the need for fidelity in attachment zoning analysis, and in defining thoroughness necessary in assessing similarity between aircraft and their installations. In this paper an attempt will be made to assess these probabilities, and some of those “guestimates” will be laid out. It is hoped that this will start some discussion and hone both the numbers and the process to something of real use.