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currents and voltages that are expected to be induced by other systems, so that the
magnitude of any unintentionally induced signal is too low to have an undesirable
effect on the system.
Emissions can be divided into two types: conducted and radiated. Conducted
emissions travel along wire interconnects between systems or parts of a system.
Radiated emissions travel through free space and are generated by time-varying
electrical signals in a conductor. However, radiated emissions can induce currents
in electrical wiring, and currents in wiring can emit electromagnetic radiation.
A system may produce both conducted and radiated emissions over a range of
frequencies and varying magnitudes. Conversely, a system may also be susceptible
to conducted or radiated interference over a range of frequencies and magnitudes.
Those characteristics of a system’s emissions and susceptibilities are a consequence
of the physical properties of the system, mostly by design. For example, a piece of
electrical equipment may be enclosed in a metal housing that prevents internal
radiated emissions from emanating outside the unit and also shields the unit from
external radiated emissions. However, the system’s physical properties may change
over time as a result of environmental effects and ageing, or if there is a hardware
fault. For example, an electrical connection may degrade and result in undesired
emissions.
In an aircraft, emissions may originate from a number of sources:
• from aircraft systems
• from personal electronic devices (PEDs) carried by passengers or crew, or active
electronic devices in the aircraft’s cargo
• from external artificial sources such as radar sites and communications facilities
• from natural sources such as electrical storms, rain particles, electrostatic
discharge, and solar and cosmic radiation.
Electromagnetic compatibility standards
The A330 aircraft type was certified to meet European and US airworthiness
requirements. As such, the aircraft and equipment were required to be resistant to
EMI, demonstrated through aircraft and equipment design and testing.
US Federal Aviation Regulations required the systems to be resistant to
electromagnetic field strengths of 50 to 3,000 volts per metre (V/m), depending on
frequency. At very low frequencies (VLF), the limit was 50 V/m.
Equipment may be vulnerable to conducted and radiated emissions, and special test
methods can be used to determine the susceptibility to both types. Conducted
susceptibility tests induce interference on the wiring interfaces of the equipment,
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while radiated susceptibility tests subject the equipment to high strength radio
waves.
Conducted susceptibility tests covered the 10 kHz to 400 MHz range (such as audio
and VLF frequencies), and radiated susceptibility tests covered the 30 MHz to 18
GHz range (such as high frequency radio and radar frequencies).
As part of the certification of the ADIRU design, a sample ADIRU was tested to
limits specified by the DO-160C standard. The limits for that standard were 100
V/m field strength for radiated susceptibility and 150 milliamperes (mA) induced
current for conducted susceptibility.
Potential sources of EMI in the geographical area
As shown in Figure 11, the three known related events occurred within 1,000 km of
Learmonth. The operator reported that its A330 aircraft conducted 9,149 sectors in
2008. Approximately 19 per cent of those sectors were flights between Perth and
Singapore or Hong Kong and passed in relatively close proximity to Learmonth.
Approximately 29 per cent of its A330 flights passed within 1,500 km of
Learmonth. In addition, other A330/A340 operators conducted regular flights
between Asian locations and Perth.
Given that the events all occurred in a broadly similar geographical area, the
investigation reviewed information concerning potential sources of EMI in the area.
Harold E. Holt Naval Communication Station
The 7 October 2008 occurrence occurred within 170 km of the Harold E. Holt
Naval Communication Station near Learmonth, Western Australia. The station
transmitted at a frequency of 19.8 kHz which is within the VLF band. The
transmission power was about 1 megawatt using an omni-directional antenna.
The station transmitted almost continuously with the exception of weekly
maintenance periods, and was transmitting at the time of the three A330 ADIRUrelated
occurrences under investigation (12 September 2006, 7 October 2008, and
27 December 2008).
The Australian Department of Defence advised that:
• no equipment malfunctioned near to or during the time of the events
• the frequency of 19.8 kHz had been in use for over 10 years
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