13.1 Electromagnetic Interference and Compatibility

13.1 Electromagnetic Interference and Compatibility

Appendix I topic 5.14 Electromagnetic environment sits at Level 2 for Categories B1 and B2/B2L, Level 1 for Category B3, and is not required for Category A, as listed in Commission Implementing Regulation (EU) 2023/989 (applicable from 12 June 2024). Level 2 means general knowledge of theoretical and practical aspects and the ability to apply that knowledge. The current Appendix I gives only the 5.14 heading and category levels. The pre-12 June 2024 detailed description specified the influence on maintenance practices of electromagnetic compatibility (EMC), electromagnetic interference (EMI), high-intensity radiated fields (HIRF) and lightning / lightning protection; those details are used here as historical study scope. This section treats EMI and EMC, coupling paths, and the hangar controls that keep those paths closed: bonding, shielding and filter connectors. HIRF and lightning follow in sections 13.2 and 13.3.

Module 5 is a multiple-choice paper only: B2 sits 72 questions in 90 minutes, B1 sits 40 questions in 50 minutes, and A/B3 sit 20 questions in 25 minutes, with a 75% pass mark, no negative marking, and no essay. The live paper uses three options; the practice items in this chapter use four.

This chapter is independent OpenExamPrep study material. It is not an EASA, EUROCAE, RTCA or type-certificate-holder document.

EMI and EMC

Electromagnetic interference (EMI) is unwanted electromagnetic energy that degrades, obstructs or interrupts the intended operation of electrical or electronic equipment. On an aeroplane the victim may be a VHF receiver, a fly-by-wire computer, a fuel-quantity processor or a cabin public-address amplifier. The source may be on the same aeroplane (a switching power supply, a motor, a digital-clock harmonic, an onboard radar) or outside it (a broadcast mast, another aircraft's radar).

Electromagnetic compatibility (EMC) is the ability of equipment to operate satisfactorily in its electromagnetic environment without introducing intolerable disturbance into that environment. EMC therefore has two faces that maintenance must not confuse:

1. Emission — how much disturbance the equipment (or the aeroplane) puts onto cables and into space.
2. Susceptibility (called immunity when written as a pass criterion) — how well the equipment continues to perform when disturbance is present.

A box that is quiet but fragile still fails EMC. A box that is robust but sprays harmonics onto the 115 V AC bus also fails EMC. Line maintenance cannot redesign the circuit, but it can restore the emission and immunity features that the type design already provided: bonds, shields, connector shells, gaskets and filters.

[!NOTE] Emission versus susceptibility exam trap: Reducing emissions (twisted pairs, filters, clock shielding) does not automatically make the same unit immune to a nearby HF transmitter. The two properties share hardware, but they are specified and tested as separate EMC attributes.

Four coupling mechanisms

Energy does not jump from source to victim by magic. It uses one or more of four classical paths. Hangar work that opens a path re-creates the interference that the design closed.

At low frequency and over short distances, inductive and capacitive coupling dominate (the near field). At higher frequency, or when a cable is long compared with a wavelength, the same harness behaves as an antenna and radiated coupling dominates. Conducted coupling is often the path that carries radio-frequency energy from a bulkhead into a box even when the original field was radiated onto the cable outside.

Bonding

Bonding equalises electrical potential between two conducting parts and provides a controlled, low-impedance path for lightning, HIRF and EMI currents. It is not the same job as hanging a green wire “to earth” and hoping. Aircraft structure is the usual reference; the quality of the bond is the impedance of the joint, not the colour of the jumper.

A bonding strap or bonding jumper is typically a short, flexible, tinned-copper braid with specified terminals. Paint, primer, anodising, corrosion product and loose fasteners insert resistance and, more importantly at radio frequency, inductance. A milliohmmeter check in the AMM confirms the direct-current resistance of the joint. Radio-frequency performance also depends on length: a jumper replaced with a longer lead can pass a DC check and still be a poor RF bond.

Worked example — why jumper length matters at radio frequency

A common teaching estimate treats a straight bonding jumper as having an inductance of about 1 nH per millimetre of length (order-of-magnitude industry teaching, not an EASA Appendix I constant). A jumper 100 mm long then has L ≈ 100 nH = 1.00 × 10⁻⁷ H.

At f = 100 MHz = 1.00 × 10⁸ Hz:

2πf ≈ 6.283 × 10⁸ rad/s

X_L = 2πfL = (6.283 × 10⁸) × (1.00 × 10⁻⁷) ≈ 62.8 Ω

The same jumper may measure 2 mΩ on a milliohmmeter. At 100 MHz the inductive reactance is tens of thousands of times larger than the DC resistance. Lengthening the jumper to 200 mm approximately doubles L and doubles X_L. That is why the AMM shows a maximum length and why technicians must not “improve” a strap by routing it around a new bracket.

Typical AMM bonding-resistance limits are a few milliohms (figures such as 2.5 mΩ appear in many manuals). Quote the task-card limit, not a memorised “Module 5 resistance”.

Shielding and shield termination

A shield (braid, foil, or both) intercepts electric fields and provides a return for induced current so that the inner conductors see a smaller interfering voltage. Coverage, continuity and how the shield ends decide whether the shield is real or decorative.

• 360-degree (circumferential) termination at a conductive backshell or gland clamps the braid all around the connector. That is the usual design intent for radio-frequency and HIRF-critical harnesses.
• A pigtail (a drain wire soldered to the braid and landed on one pin or a lug) adds inductance, exactly as the long jumper did. At VHF and above a pigtail can make the shield almost useless. Do not convert a 360-degree backshell to a pigtail because it is quicker.

Unused connector cavities, missing backshell hardware, broken braid and paint on the mating face all open the shield. After a connector change, restore the backshell, strain relief and any conductive gasket the illustrated parts list shows.

Filter connectors

A filter connector builds capacitance (often a feed-through or π-section) into each pin so that radio-frequency current is shunted to the connector shell rather than entering the box. The filter only works if the shell is bonded to the bulkhead or equipment case. A painted rack rail, a missing star washer or a filter connector hanging on the wires with no structural bond returns the RF to the circuit the filter was meant to protect. Substitute an unfiltered connector only with approved data; pin-for-pin mechanical fit is not EMC fit.

Maintenance implications

• Bond to bare metal as the AMM shows; restore primer and finish only where the drawing allows.
• Replace damaged, frayed or corroded bonding straps; do not daisy-chain extra jumpers unless the design already does.
• Restore shield termination and backshell torque; do not leave a braid floating “for now”.
• Keep filter-connector shells tight to structure.
• Respect wire segregation (power versus sensitive analogue, ignition versus audio).
• After structural repair or a composite patch, restore electrical bonding and any embedded mesh (section 13.2).
• Do not treat a successful power-up as proof of EMC: many interference problems appear only in flight, near a radar, or during HF transmit.

B1/B2 Level 2 means you can name the coupling paths and apply the hangar controls. B3 Level 1 means you recognise that EMC and EMI exist and that bonding, shielding and filters are airworthiness features, not optional tidy-up.