Lightning Transient Response And Margin Analysis Of Aircraft Circuits

Abstract

Procedures from the Federal Aviation Administration (FAA) establish a margin between lightning induced transient levels, and equipment transient qualification levels, as a basis for certification of aircraft electrical/electronic equipment. The electromagnetic environment applied to equipment by existing qualification standards is an approximation of the aircraft environment, complicating analysis of the margin. A circuit model of aircraft transient behavior is used to compare the aircraft and equipment qualification environments, with elements extracted from data from aircraft lightning simulation tests. Margins are plotted as a function of equipment load impedance and transient source impedance on the basis of peak voltage, power, and energy. The range of validity for margin values is shown to be restricted by the characteristics of the generator source impedance. Introduction Lightning research over the last decade has defined the repetitive current pulses encountered by aircraft. The spread of electronics in aircraft systems from the faucet control to critical flight systems has prompted new aviation regulations to insure electromagnetic compatibility with lightning. Procedures have been developed by the FAA through the work of SAE and RTCA comittees and their counterparts in Europe, to address lightning effects on aircraft electrical/ electronic equipment. These are outlined in Advisory Circular AC 20-136 (1). The external aircraft lightning environment has been defined by waveforms of current such as the single stroke, multiple stroke, and multiple burst waveforms. The steps in the procedure provide that lightning induced transient levels are limited to the transient control level (TCL), and that aircraft equipment tolerate transients up to the equipment transient design level (ETDL). The ratio of ETDL to TCL is defined as the margin (Figure 1). The TCL and ETDL encompass a wide range of circuit responses with differing waveforms. The margin must be based on one or more waveform characteristic. Procedures for qualification of airborne equipment contained in DO-160C section 22 (2) were adopted to verify the ETDL. The induced transients were represented as Thevenin equivalent sources, with short wave, long wave, and oscillatory waveforms, and five ohm or twenty ohm source resistances. The procedures did not address all of the new requirements and have been under revision by the SAE and RTCA committees. The recent approach has been to EQUIPMENT TRANSIENT DESIGN