In-flight upset - 154 km west of Learmonth, WA, 7 October 2008,

Electromagnetic compatibility designModern electronic systems, particularly those installed in large-capacitycommercial aircraft, are specifically designed to be resistant to EMI and tominimise emissions that could affect other systems.Design features that can reduce both susceptibility and emissions include dedicatedcurrent return wiring, low-resistance single-point grounding, conductive shieldingof circuits, the use of shielded and/or twisted pair wiring, categorisation andphysical separation of wiring, capacitive and inductive filters on system inputs andoutputs, and other circuit and hardware design techniques. Digital systems may alsoimplement parity, checksums and other integrity checks on data.EMI testingThe various means by which electromagnetic energy can pass from one place toanother, and the number of factors which affect it, give rise to very complexrelationships that can be difficult to characterise. For this reason, EMI testing (asopposed to various forms of analysis) has generally been considered the mostrepresentative means by which an aircraft or system may be evaluated for resilienceto EMI.Testing of a system’s susceptibility to EMI can take two forms:• Source-victim testing, which is specific to a particular ‘victim’ (affected) systemand one or more ‘source’ (emitting) systems, often carried out when particularsystems are known or suspected to have affected another system.• Susceptibility testing, which subjects a system to an electromagneticenvironment that comprises a range of frequencies. The frequency ranges, fieldstrengths, and conducted currents are chosen to be representative of theenvironment to which the system is expected to be exposed in service. This formof testing is normally carried out as part of a system’s qualification to ensurethat it is resilient to a defined electromagnetic environment.An advantage of susceptibility testing is that if a system is later subjected toemissions from an unanticipated source, it will most likely have previously beenshown to be resilient to those emissions as long as the susceptibility testingencompassed the relevant frequency ranges and field strengths.Conducted susceptibility tests induce interference on the wiring interfaces of theequipment, while radiated susceptibility tests subject the equipment to high strengthradio waves. Typically, conducted susceptibility tests covered the 10 kHz to400 MHz range, such as audio and very low frequency (VLF) frequencies. Radiatedsusceptibility tests covered the 30 MHz to 18 GHz range, such as high frequencyradio and radar frequencies.The strength of an electromagnetic field can vary significantly with factors such aslocations and distances from the energy source, the presence or absence of metallicor other conductive objects, the quality of the electrical connections (such asgrounding of electrical units), the presence of any corrosion, manufacturingvariability and so on. Because of this, EMI testing will not always revealsusceptibilities, although most confounding factors can be mitigated by usinghigh-power signals during susceptibility testing.- 258 -