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

cannot be ‘decoded’ by the receiver. Radio systems are usually the most susceptibleto EMI as they are specifically designed to receive electromagnetic signals (that is,radio waves), but any electrical or electronic system may be affected by EMI, andthese effects depend on the characteristics and function of the affected system. Forexample, an analogue electrical instrument may show incorrect readings oroscillations.Digital signals, including those on digital databuses, can also be affected by EMIbut in a different way. A low level of interference can be eliminated because of itstypically different nature to the nature of the real signals, while higher levels candisrupt the flow of data in a way that is generally detectable by the receivingsystem. This is because the digital signals need to adhere to very strict criteriaincluding the waveform shape (that is, the timing of pulses, voltage levels, andvoltage rise/fall times) and often the validity of the data itself. This is the case withthe ARINC 429 databus, which in addition to strict waveform criteria, uses a paritybit to aid the receiving system to detect any corruption to the signals (section 3.4.7).Any significant disruption due to the direct effects of EMI would result in a largenumber of invalid ‘words’ and random effects on the data values.Typically, a high-power system such as a power supply would not be affected bynoise from a relatively low-power computer system, whereas a computer systemmight be vulnerable to noise from the power supply, electric motors, othercomputer systems, and so on.Electrical and electronic systems operate by sending signals between components,devices and other systems, and these interfaces can be similarly affected byspurious signals. The system could react to these spurious signals as though theywere actual signals, or be unable to decode them, with resulting abnormalbehaviour. For example, EMI occurring to a set of wires carrying electrical currentfor an aircraft’s electronic flight control system may be treated as real inputs,resulting in spurious movement of the flight control surfaces.Similar effects may occur within a single device if one or more of its internalinterfaces are exposed to EMI. A computer may shut down or ‘hang’ (or otherwisebehave unpredictably) if a faulty electrical circuit generates spurious radio signalsthat are inadvertently picked up by another circuit.Different aircraft, systems or components of the same design may exhibit varyinglevels of susceptibility to EMI due to factors such as shape, slight variations inmanufacture, or ageing. For example, a particular wiring bundle may be routedslightly differently from one aircraft to another, which could change both its length(which in turn affects its impedance and resonant frequency) and its locationrelative to the aircraft’s structure and other equipment (which in turn affects theamount and type of electromagnetic emissions to which the bundle would beexposed). Other factors such as the location and electrical properties of seams andapertures, as well as the shape of electronics bays, can affect the electromagneticenvironment around electronic units and wiring.Although EMI has been an ongoing issue in aircraft design and has been known toregularly cause anomalous behaviour in airborne electrical systems, relatively fewaircraft accidents have been attributed to EMI. This is in part due to the difficulty ofproving whether EMI was a factor if the affected system is damaged or destroyed,and in part due to the usually indirect effect of EMI on safety (that is,communication and navigation systems may be disrupted, but the integrity of theaircraft’s structure and control systems are rarely affected).- 257 -