Electromagnetic Environment
Influence of the following phenomena on maintenance practices for electronic systems, EMC, EMI, HIRF and lightning, together with P-static and the bonding, shielding and grounding used to control them.
Electromagnetic environment, summary notes
- EMI (electromagnetic interference) is unwanted energy coupled into a system; EMC (electromagnetic compatibility) is the design discipline of ensuring equipment neither emits excessive interference nor is susceptible to it.
- HIRF (High-Intensity Radiated Fields) is the external threat from ground radars, broadcast transmitters and shipborne emitters. Aircraft and systems are certified to defined HIRF environments.
- Lightning attaches to extremities and sweeps along the airframe. Direct effects are burning, pitting and structural damage at attachment points; indirect effects are the transients induced into wiring, which is what threatens avionics.
- P-static (precipitation static) builds up as the airframe flies through rain, snow, dust or ice crystals; it discharges as corona from extremities and causes radio and navigation noise. Static wicks/dischargers bleed it away, and bonding keeps the whole airframe at one potential.
- The defences are BONDING (low-resistance connections so all structure is at one potential), SHIELDING (screened cable, with the screen earthed to the manufacturer's specification) and GROUNDING/separation of cable routes. Composite structure needs an embedded conductive mesh or foil because it is not naturally conductive.
- Portable electronic devices are controlled because their emissions can fall in navigation and communication bands.
- ⚠ Exam trap: the greater lightning threat to avionics is the INDIRECT effect, induced transients in the wiring, not the burn mark at the attachment point.
- Bonding resistance
- typically well below 1 Ω, check the specific AMM figure
- Threats vs defences
- HIRF · lightning · P-static → bonding · shielding · grounding
A VHF radio suffers heavy crackling in cloud that clears in still air. What is the probable cause and the remedy?
Precipitation static. Charge accumulating on the airframe discharges as corona and radiates broadband noise. Inspect the static dischargers for damage, contamination or high resistance, and check the bonding of control surfaces and extremities.
Why must a cable screen be terminated exactly as the wiring manual specifies rather than "earthed at both ends for safety"?
Screen termination determines whether the screen carries interference away or forms an earth loop that injects it. Bonding both ends where the design calls for a single-point earth can create a circulating current and make the interference worse, so the specified termination must be followed exactly.
EMI / EMC concept map
Electromagnetic Environment
Electromagnetic environment quiz
Electromagnetic Environment, quiz
1. HIRF stands for:
High Intensity Radiated FieldsHigh Internal Resistance FaultHeated Inlet Ram Flow2. Lightning strike protection of a composite aircraft relies on:
A conductive mesh or foil in the skinThe paint aloneThe avionics software3. EMC is best described as:
Equipment neither emitting excessive interference nor being susceptible to itThe total shielding of every cableThe bonding of the airframe to earth on the ground4. The greater lightning threat to avionics is:
The indirect effect, transients induced into wiringThe direct effect, burning at attachment pointsThe acoustic shock5. P-static is caused by:
The airframe charging as it flies through precipitationGround radar illuminationGenerator ripple6. Static dischargers (wicks) are fitted to:
Bleed accumulated charge from the airframe extremitiesConduct lightning current to the enginesProvide a ground bonding point7. The three principal EMC defences are:
Bonding, shielding and groundingFiltering, fusing and isolatingPainting, sealing and potting8. Composite structure requires an embedded conductive mesh or foil because it:
Is not naturally conductiveIs heavier than aluminiumCannot be painted