5.14 ELECTROMAGNETIC ENVIRONMENT

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SYLLABUS

5.14 Electromagnetic Environment – L2 • Influence of the following phenomena on

maintenance practices for electronic system: – EMC-Electromagnetic Compatibility – EMI-Electromagnetic Interference – HIRF-High Intensity Radiated Field – Lightning/lightning protection

THE INCREASED CONCERN IN RECENT YEARS BECAUSE:

• Greater dependence on electrical and electronic systems for continued safe flight.

• Reduced electromagnetic shielding due greater use of composite materials.

• Increased susceptibility of electrical and electronic systems to HIRF due to increased data bus and processor operating speeds, higher density integrated circuits and cards, and greater sensitivities of electronic equipment;

• Expanded frequency usage, especially above 1 gigahertz (GHz); • Increased severity of the HIRF environment because of an increase in

the number and radiated power of radio frequency (RF) transmitters; and

• Adverse effects experienced by some aircraft when exposed to HIRF.

Ref: AC 20-158

ADVERSED EXPERIENCES

• Gross navigation error in a passenger aircraft in USA (from Newark to San Maarten) caused by a portable tv set used by a passenger.

• Lap top used by a passenger affected the navigation equipment during aircraft take-off and landing.

ELECTROMAGNETIC ENVIRONMENT

• Caused by transmission of electrical energy in space, e.g from radar, radio or TV.

• Like current flowing in a wire, magnetic fields are created.

EMC – Electromagnetic Compatibility

• Electromagnetic compatibility, or EMC means that a device is compatible with (i.e., no interference is caused by its electromagnetic (EM) environment and it does not emit levels of EM energy that cause electromagnetic interference (EMI) in other devices in the vicinity.

• All electric devices or installations influence each other when interconnected or close to each other. Eg your TV set, your GSM handset, your radio and nearby washing machine or electrical power lines.

• The purpose of electromagnetic compatibility (EMC) is to keep all those side effects under reasonable control

EMI ELECTROMAGNETIC INTERFERENCE

• Electromagnetic interference or EMI, also called radio frequency interference or RFI. It is the disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source.

• If avionics equipments are left unprotected, may cause serious flight safety issues.

EMI SOURCES (electromagnetic interference)

Source DOT/FAA/CT-86/4

EMI IN AIRCRAFT

SOURCES OF EXTERNAL INTERFERENCES (note P168)

• Two forms of interference– Conducted interference– Radiated interference

• Sources of interferences– External Electrical Systems e.g brushes, switches– Engines system – ignition system– Inadequate bonding– Faulty static discharger/wicks

CONTROLLING THE INTERFERENCE

• Well Located aerials – the interference to the comm/nav systems like ADF and VHF

• Electronic equipment to be grounded and the related wires to be shielded and grounded.

• The lighting current flows through the outer skin and discharge to the extremity.

• Bonding – all equipments to be bonded together, min R = 0.05 ohms.

• Static Discharger – provide the low resistance path to the admosphere.

WHAT IS HIRF

• It is High Intensity Radiated Fields (electromagnetic energy) external to the aircraft, of a strength sufficient to have adverse affect on aircraft safety.

• Note: The source of energy is external, exclude onboard system and static sources.

LIGHTNING

• The high energy and high voltage can affect the aircraft hardware as well as the data.

• The high transient discharge current can damage the skin of the aircraft and the bonding wires.

PROTECTION AGAINST LIGHTNING

• Aircraft skin• Voltage and current protector at the

equipment• Wire shielding• Static discharger

PERSONAL ELECTRONIC DEVICES (PED)

• Personal Electronic equipments can produce signals that affect electronic equipments.

• However there was no definitive proof of individual cases. Symptoms and failures went off when passengers were asked to switch PED.

• The cautions were founded as some of the sensitive electronic wires run in close proximity to passengers.

EXAMPLE OF HIRF CHECK

TYPICAL INSPECTION (P166)