Numerical Methods for Zoning Computation

Zoning consists in establishing lightning strike zones to locate and classify surfaces on an aircraft which are exposed to a part of the lightning current components. The current standard used to certify aircraft is empirical and qualitative, and fails to predict certain features, such as lightning attachment on the middle of the wing. Furthermore, the standard will be difficult to apply to the next generation of aircraft having geometry, engines and fuselage material that will be very different from current designs. Two approaches have been developed to elaborate a zoning around an aircraft. An empirical developed by BAe is based on the rolling sphere model. The input parameter is the radius of the sphere which is evaluated by service lightning strike experience for a given aircraft. The second approach is based on the physical description of the lightning strike on an aircraft. From the physical modelling of lightning discharge, Onera has developed a general method to compute a probabilistic zoning. This method takes into account the fundamental processes occurring during a lightning strike on an aircraft. The attachment process is computed from the aircraft geometry and the atmospheric electric field direction leading to the lightning inception. The results of this computation give the initial points on the fuselage where a lightning can develop and their probability of inception as a function of the skin geometry and the field direction. These inputs are used in a swept model to compute, for each attachment point, the lightning attachment point displacement due to the aircraft motion, the airflow and the lightning channel geometry. The model is based, for computing power purposes, on a macroscopic description of the lightning channel during the continuous current phase. For a given single aisle aircraft, we compute and record in a database several million cases of lightning strikes. By using the distribution of lightning stroke arrival times, the probability that a specific zone of this aircraft will be struck by a stroke is computed.