Chapter 11: Electromagnetic Environment

EMC and EMI

An aircraft is crowded with electronics, transmitters and wiring, and it flies through radar beams, radio transmissions, lightning and charged precipitation. Two terms describe how well its systems cope:

• Electromagnetic compatibility (EMC): the ability of equipment to work as intended in its electromagnetic environment without suffering unacceptable interference and without causing unacceptable interference to anything else.
• Electromagnetic interference (EMI): unwanted electromagnetic energy that degrades the performance of equipment — noise in radios, false indications, corrupted data, computer resets.

• Conducted interference travels along wires — power supply ripple and switching transients, for example. Capacitive coupling transfers fast voltage changes between conductors that run close together; inductive coupling transfers changing currents through their magnetic fields. Radiated interference arrives as electromagnetic waves, and long aircraft cables make efficient receiving antennas.
• Interference is narrowband (a single frequency, such as a transmitter) or broadband (spread over many frequencies, such as sparking contacts, motors or lightning).
• Equipment is designed and tested to environmental standards — EUROCAE ED-14 / RTCA DO-160 — for its emissions and its susceptibility to conducted and radiated energy, power transients, lightning-induced transients and electrostatic discharge. The aircraft installation is then tested as a whole.
• Portable electronic devices (PEDs) carried by passengers and crew are possible EMI sources; operators assess and control their use, and aircraft are increasingly certified as PED-tolerant.

HIRF — high-intensity radiated fields

HIRF is the electromagnetic environment created by powerful transmitters outside the aircraft: radars (airport, weather, military and ship-borne), radio and television broadcast stations, satellite uplinks, and the radars of other aircraft. Close to such transmitters the field strength can reach hundreds or thousands of volts per metre.

• The certification HIRF environment covers frequencies from about 10 kHz to 40 GHz. Low frequencies couple into the airframe and long cables; high frequencies enter through apertures — windows, joints, access panels and composite areas.
• Older analogue systems tended to degrade gradually; digital systems can be upset — a corrupted bit, a computer reset or a frozen display. Because fly-by-wire and FADEC systems are critical, they must be shown to keep working in the HIRF environment (EASA CS-25.1317, FAA AC 20-158A).
• Protection by design: the conductive airframe acts as a Faraday cage; composite structure has conductive meshes or foils added; wiring is shielded, overbraided, routed and separated; connectors have conductive backshells; equipment has filters and shielded cases; access panels in protected areas may have conductive gaskets.

Lightning

• A transport aircraft is struck on average about once a year. Most strikes are triggered by the aircraft itself, which bridges the gap between charged regions of a cloud — typically in cloud, near the freezing level, during climb and descent.
• A strike consists of several high-current return strokes plus continuing current. Peak currents are typically 20–30 kA, and severe strokes reach about 200 kA with rise times of microseconds. The channel is around 30 000 °C.
• The current enters at one extremity and exits at another; the aircraft simply becomes part of the lightning channel. Occupants are protected because the current flows through the conductive outer skin — the Faraday-cage principle.

• Zone 1 — surfaces where initial attachment is likely: 1A with low probability of the flash 'hanging on' (nose, radome, wing and tailplane tips, engine nacelles), 1B with a high probability of hang-on (tail cone and trailing tips), and 1C transition areas.
• Zone 2 — the swept-stroke zones aft of zone 1, where the channel reattaches as the aircraft moves: 2A low hang-on, 2B high hang-on (trailing edges).
• Zone 3 — the rest of the aircraft, which carries the current between entry and exit but should not be struck directly.

• Direct effects: burning, pitting and melt-through at attach points; punctures and delamination in composites and radomes; arcing and welding at hinges, bearings and poorly bonded joints; heating of bonding straps and conductors; magnetic forces and shock waves; and the danger of sparks in fuel tanks, which is why fuel systems are carefully bonded and designed against ignition sources.
• Indirect effects: the rapidly changing current produces intense electric and magnetic fields that induce transients in aircraft wiring, which can upset or damage electronics. Composite aircraft need extra protection because composite skin shields far less than aluminium.
• Protection: conductive skins and lightning strike protection meshes or foils in composites; diverter strips on radomes to lead the current over the surface to the structure; thorough bonding of all parts; shielded and twisted wiring; and transient voltage suppressors (TVS diodes, varistors) and filters at equipment inputs.

Precipitation static

Flying through rain, snow, ice crystals or dust charges the airframe by friction — precipitation static (P-static). The charge escapes as corona discharge from sharp points and edges, producing broadband radio noise that can blank out HF, ADF and VHF reception. Static dischargers (static wicks) on wing, tail and control-surface trailing edges bleed the charge off gently through high-resistance tips, away from the antennas. They do not protect against lightning (and are often damaged by it). Missing or damaged dischargers are a maintenance item, sometimes allowed for dispatch by the MEL.

Bonding, shielding and wiring practices

• Bonding joins conductive parts with low-resistance connections, so that they stay at the same potential. It provides a safe path for lightning and fault currents, prevents static build-up and sparking (especially in fuel areas), provides a current return path, makes shields and antenna ground planes effective, and reduces radio interference. Bonding resistance is measured with a low-resistance bonding meter (milliohmmeter) — typically only a few milliohms are allowed; the AMM gives the limit.
• Shielding surrounds conductors with braid or foil connected to structure. It stops electric-field (capacitive) coupling and, when grounded at both ends and terminated all round, magnetic and high-frequency coupling as well. A shield is only as good as its termination: a 360° backshell bond is best; long pigtails make the shield ineffective at high frequency.
• Twisted pairs carry a signal and its return close together, so external magnetic fields induce equal and opposite voltages in each half-twist that cancel out.
• Separation: power, signal, sensitive and RF wiring are routed separately and kept apart as the wiring practices manual specifies; changing the routing can create interference.
• Filters and ferrite cores suppress conducted interference — never remove them.

Maintenance practices

• Treat every bonding jumper, shield termination, backshell, conductive gasket, filter and ferrite as part of the aircraft's HIRF and lightning protection. Many aircraft have specific L/HIRF inspection tasks in their maintenance programme.
• When refitting a component, restore the bond: clean paint, anodising and corrosion from the contact faces, use the specified hardware, measure the resistance, then re-protect the joint against corrosion.
• Repairs to composite structure must restore the conductive protection layer; repainting a radome must use approved materials and thicknesses and must not cover diverter strips.
• Do not reroute wiring, splice shields with unapproved methods, leave shields unterminated or remove ferrites.
• After a lightning strike: carry out the AMM conditional inspection — look for entry and exit marks (pits, burns, holes) especially at extremities, inspect the radome and diverter strips, static dischargers, bonding jumpers, control-surface hinges and bearings, composite panels and fuel-system areas; carry out the required avionics and system checks (and a compass check if specified). Record the findings.