B1-05.14 ELECTROMAGNETIC ENVIRONMENT

Electromagnetic Environment (5.14)

• Describes the influence of electromagnetic compatibility (EMC) on maintenance practices for electronic systems
• Describes the influence of electromagnetic interference (EMI) on maintenance practices for electronic systems
• Describes the influence of high-intensity radiated fields (HIRF) on maintenance practices for electronic systems
• Describes the influence of lightning and lightning protection on maintenance practices for electronic systems

Key Definitions

• Electromagnetic Environment (EME) is the totality of electromagnetic phenomena at a given location
• Electromagnetic Compatibility (EMC) is the capability of equipment/systems operating in the intended electromagnetic environment at designed efficiency levels without degradation caused by electromagnetic interference
• Electromagnetic Interference (EMI), as defined by NATO, is any electromagnetic disturbance that interrupts, obstructs, degrades, or limits the effective performance of electronics/electrical equipment
• High-Intensity Radiated Field (HIRF) is man-made electromagnetic radiation generated external to aircraft
• Radio Frequency Interference (RFI), also called radiofrequency interference (RFI), is electromagnetic interference (EMI) within the radio frequency spectrum; a disturbance generated by an external source affecting an electrical circuit via electromagnetic induction, electrostatic coupling, or conduction

Avionic Frequency Bands

• Avionics systems utilize frequency bands spanning from a few kilohertz to several gigahertz
• VHF Omnidirectional Range (VOR) operates from 108 to 118 MHz
• Glideslope systems used during landings operate in the 328 to 335 MHz range
• Distance-Measuring Equipment (DME) operates at just over 1 GHz
• Global positioning, collision avoidance, and cockpit weather radar systems also use the spectrum above 1 GHz

PED Frequency Bands

• Personal Electronic Devices (PEDs) operate at frequencies from 10 to 15 KHz for AM radios and up to 400 MHz for laptop computers
• Emitted frequencies may cover almost the entire range of navigation and communication frequencies used on aircraft
• Therefore the entire system of electronic equipment aboard aircraft is at risk of EMI.

EMI Permeation

• EMI permeation is often caused by inadequate or damaged shielding from servicing or corrosion, which increases resistance of the electrical connection to ground
• Effective shielding relies on good grounding, thus additional system resistance enables wires to pick up interfering signals
• Aircraft antennas can pick up EMI radiated through passenger windows and unshielded openings

Effects of EMI

• Electromagnetic interference can jam sensitive equipment and burn out electric circuits
• EMI can affect everything from fly-by-wire flight control systems to cockpit fuel gauges
• EMI, in extreme cases it can send a plane into an uncommanded dive or shut down a critical avionics system

Electromagnetic Compatibility

• EMC refers to the ability of equipment to operate well in an electromagnetic environment without causing intolerable disturbances to other electrical devices

Elements of an EMC Problem

• Source of electromagnetic phenomenon
• Receptor unable to function due to the electromagnetic phenomenon
• Path allowing the source to interfere with the receptor

Potential Sources

• EMC problems include radio transmitters, power lines, electronic circuits, lightning, lamp dimmers, electric motors, arc welders, solar flares, etc

Potential Receptors

• Radio receivers, electronic circuits, appliances, people, and any object able to utilize or detect electromagnetic energy

Coupling Path

• Methods of coupling include: Conducted (electric current), Inductively coupled (magnetic field), Capacitively coupled (electric field), Radiated (electromagnetic field)
• Coupling paths often utilize a complex combination of these methods

Electric and Magnetic Fields

• Electric fields are created by voltages on conductor areas
• Magnetic fields are created by currents flowing (in loops)
• All electrical signals create both field types, causing conductors to leak signals and allowing external fields to leak in

Leakage and Antenna Effect of Conductors

• Frequencies in daily life range from AC power lines to mobile phones (up to 1.8 GHz)
• Mains rectifiers emit switching noise as harmonics of fundamental frequencies

Mitigation

• At distances greater than one-sixth of the wavelength of the frequencies of concern, E and M fields develop into full electromatic fields
• When wavelength is comparable to conductor length, resonances occur

Conductor Length vs Antenna Efficiency

• At very-high frequency ranges, conductors of very short distances are susceptible to and can be sources of electromagnetic interference.

Natural Sources of EMI

• EMI has been a factor in aircraft construction since the 1930s
• Shielded cabling was introduced to protect communication systems from reciprocating engines and magneto ignitions
• Man-made electromagnetic noise is generated by motors, generators, and other machinery
• Naturally occuring radio noise originating from atmospheric disturbances (including lightning) and extraterrestrial sources (such as sunspots) can also degrade performance of electronic equipment

Lightning Strikes and Lighting Protection

• The heating and magnetic forces produced by the high currents of a lightning strike can cause structural damage (direct effects) and can induce transients which may damage or disrupt electrical equipment (indirect effects).

Electrical Bonding

• Facilitates safe conduction of lightning currents through the airframe
• Achieved through normal airframe riveted/bolted joints and additional bonding provisions

Man-Made Sources of EMI

• Communications signals may also interfere with the operation of sensitive electronic equipment
• Banning intentional radio frequency (RF) emitters like CB radios, remote-controlled toys, and walkie-talkies occurs on commercial airlines

Personal Electronic Devices

• Personal Electronic Devices (PEDs) produce signals in the 1-MHz range that could affect avionics
• Commercial passenger jets contain up to 150 mi of electrical wiring, it is extremely important for passengers to heed regulations for use of potentially disruptive electronic devices

Effect of Airframe

• The airframe can compound the effects of internal/external EMI by concentrating signals and broadcasting interference

High-Intensity Radiated Field

• High-Intensity Radiated Emissions (HIRF) encompasses man-made sources of external electromagnetic radiation that can interfere with safe flight
• HIRF does not include interference among on-board systems (EMC issue) or effects caused by PEDs; it excludes lightning or static electricity (Electrostatic Discharge - ESD effects)

EMI Management/Addressing EMI Problems

• Determine the mechanism used for energy transfer for the affected device
• Radiation, Conduction or Induction

Radiated Electromagnetic Interference

• Radiated electromagnetic energy entering an adjacent device is difficult to identify/control, but is most affected by addressing one of these options
• Option 1 - Remove/Reduce the source
• Option 2 - 'Harden' the target
• Option 3 - Separate the devices

Effective Shielding of Avionic Devices

• Effective shielding of avionic devices must anticipate both the radiated susceptibility and radiated emissions

EMI Shielding/Shielding From EMI Types

• Radio Frequency Interference (RFI) - Use a combination of foil wrap and high-coverage braided shield
• Crosstalk - Isolate the wires with a shield or altering the lay length
• Electromagnetic Interference (EMI) - Employ a ferrous metal braid or quality grounding technique
• Electronic Environment - Shield the signal cable with a non-ferrous braid with moderate coverage
• High Voltage Transients - Shield with a non-ferrous shield and proper isolation

Cable Shielding

• In interconnected applications, wires and cables are shielded to reflect or conduct noise to ground, preventing interference

Braided Shields

• Provide exceptional structural integrity.
• Effective at minimising low-frequency EMI at audio and RF ranges

Foil Shields

• Are made from aluminium foil laminated to a polyester or polypropylene film.
• Provide 100% cable or component coverage

Multi-Shielding

• Multi-shielding provides superior attenuation that is effective from the kilohertz to the gigahertz frequency range. and internal cross-coupling.
• Shielding provides electrostatic protection, metal armour protection and reduces the effects of Electromagnetic Pulses (EMPs)

EMC/EMI Problems

• An increase in the shielding of the major units, may reduce the intensity. The addition of Radio Frequency (RF) filters to wiring entering and exiting the equipment can improve the situation

EMI Minimisation/Balanced Circuits/Twisted Wires

• Using twisted wires balances current and impedance for each earth connection, with transformer taps earthed
• Advantages include prevention of electrical noise and minimised cross-talk
• Disadvantages include electromagnetic interference vulnerability dependent on twisting schemes staying intact during cable installation

PCB Continuous Ground Plane

• Continuous ground planes under high-speed signal lines or printed over circuitry on PCBs reduce EMI production, emissions, and crosstalk

Structure Shielding

• It may be used in electronic assemblies to cover sensitive components
• Structure shielding protects circuits from lightning, High-Intensity Radiated Field (HIRF) & EMI by providing a low impedance path

Static Discharging

• Low-resistance paths disperse at airframe extremeties
• Deterioration causes increased static interference

This lesson explores electromagnetic interference (EMI). It covers topics such as typical frequency ranges for switch-mode power converters and how conductors contribute to EMI. It also considers how signal wavelengths and clock frequencies relate to electromagnetic field development.