5.14 Electromagnetic Environment

Question 1

electric field strength can be measured in terms of

Answer 1

volts per metre

Question 2

Electrostatic induction occurs when

Answer 2

a charged object redistributes charges to another object due to the attraction of opposite charges.

Question 3

magnetic field strength is measured in

Answer 3

amperes per metre

Question 4

As part of the EMC process, designers must look at two components:

Answer 4

Emission - what does the component emit?
Susceptibility - how susceptible is the component to outside emissions?

Question 5

According to MIL-STD-461, Line Replaceable Units (LRU) must be designed with EMI-sensitive electronics located on one side of the enclosure, forming a

Answer 5

“Faraday cage” shielded from EMI effects . This ensures that sensitive microelectronic devices are operated in a protected environment

Question 6

Cabling must be shielded and grounded to be protected against:

Answer 6

External radiated interference from passenger electronic devices, e.g., laptops and mobile phones
High-Intensity Radiated Fields (HIRF)
Lightning strikes

Question 7

Electromagnetic Interference (EMI) can be defined as

Answer 7

the presence of unwanted voltages or currents that can adversely affect the performance of an electrical/electronic system

Question 8

The effects of EMI include:

Answer 8

-Errors in indications
-Unwanted noise on audio signals
-Random patterns on electronic displays
-Repetitive ‘buzzing’ on intercom and cabin phone systems
-Desensitising of radio and radar receivers
-False indications in radar and navigation equipment
-Nuisance triggering of alarms

Question 9

There are many sources of EMI throughout the aircraft these include

Answer 9

-Fluorescent lights
-Radio and radar transmitters
-Power lines
-AC powered window heat controllers
-Motors/generators, particularly if they are DC with commutators and brushes
-Switching and light dimming circuits
-Microprocessors
-Pulsed high-frequency circuits
-Data bus cables (but not fibre optic cables)
-Static discharge and lightning

Question 10

In electromagnetic field radiation, energy is transmitted through what

Answer 10

electrically nonconductive paths, such as air, plastic materials, or fibreglass

Question 11

The need for protection of modern electrical and electronic systems from HIRF is required because of the following factors

Answer 11

Dependence on these systems used for the continued safe flight and landing of the aircraft
Increased use of composite materials (reducing the natural Faraday cage protection of metallic structures)
Increased complexity of digital systems (faster operating speeds, higher-density integrated circuits)
Expanded frequency usage of microwave energy
Increased quantity and power of transmitters
The number of RF transmitters
Electrically initiated explosive cartridges e.g. fire bottle cartridges, helicopter winch cable cutter cartridges

Question 12

High Intensity Radiated Fields (HIRF) are generated by certain Radio Frequency (RF) sources that are external to the aircraft. These fields disrupt electronic components and systems within the aircraft via currents that are induced from these fields into the aircraft’s wiring

Answer 12

.

Question 13

Electromagnetic interference can jam equipment, burn out electrical circuits, cause false readings on instruments, cause a false command in a flight control system (especially fly-by-wire), detonate a squib, or even cause an explosion of fuel vapour

Answer 13

.

Question 14

a common HIRF device found in aircraft galleys is a

Answer 14

microwave

Question 15

HIRF interference can also be caused by high-power radio and microwave towers, including Air Traffic Control (ATC) primary surveillance radars.

Answer 15

.

Question 16

aircraft protection from HIRF includes

Answer 16

HIRF protection features are incorporated inside the LRU.

Protection devices such as filter pin connectors, discrete filter capacitors and transient protection devices (transorbs) are installed within LRUs on one or more of the LRU interface circuits

Question 17

what is HERP

Answer 17

hazard of electromagnetic radiation to personnel is the danger to personnel from the absorption of electromagnetic energy by the human body

Question 18

in relation to HIRF and maintenance personnel

it is important to ensure all precautions are taken when

Answer 18

Working on or maintaining high energy sources such as radar systems, HF etc.
Fuelling and defueling of aircraft
Replacing fire extinguishers, fire extinguisher cartridges, helicopter winch cable cutter cartridges etc.

Question 19

aircraft have been required to comply with HIRF requirements since when

Answer 19

early 1992

Question 20

CS 23.1308 High Intensity Radiated Fields (HIRF) Protection

Answer 20

.

Question 21

Each electrical and electronic system that performs a function whose failure would prevent the continued safe flight and landing of the aeroplane must be designed and installed so that

Answer 21

the function is not adversely affected during and after the time the aeroplane is exposed to HIRF environment.

The system automatically recovers normal operation of that function, in a timely manner, after the aeroplane is exposed to HIRF environment.

The system is not adversely affected during and after the time the aeroplane is exposed to HIRF environment.

Question 22

most lightning strikes occur when

Answer 22

during the climb and descent phases of flight between
5,000 and 15,000ft

Question 23

likelihood of a lightning strike decreases significantly at what altitude

Answer 23

above 20,000ft

Question 24

Around 70% of lightning strikes occur during the presence of rain

Answer 24

.

Question 25

During a lightning strike, voltages can be superimposed on unprotected signals. These can increase the signal level up

Answer 25

to 500 times

Question 26

The currents produced by this phenomenon can be

Answer 26

300 000 times higher under normal conditions

Question 27

to protect against the indirect effects of a lightning strike, certain design features are implemented which are

Answer 27

Protected components are segregated, wire lanes are separated, and system components are protected by filters, which only allow spike free currents to access them

The filters have to be able to neutralise power with a magnitude of 500 kW.

Question 28

The direction of a lightning discharge can be calculated by an

Answer 28

Automatic Direction Finder (ADF) receiver using a loop antenna.

Question 29

Precipitation is termed ‘hard’ or ‘soft’ depending on the type of particles

Answer 29

Hard precipitation refers to relatively dry particles such as snow, ice, hail, and sand.

Soft precipitation refers to wet particles, such as rain and sleet.

Question 30

Optimum siting of dischargers can only be determined by

Answer 30

a thorough investigation of the characteristics of the aircraft type.

however in general the wing, tail and fin tips are the locations of the greatest potential gradient

Question 31

Measures to protect against lightning strikes and other forms of electromagnetic interference include

Answer 31

Proper grounding and shielding with inductances kept to as low a value as possible.
Grounded shields for external computer cables that run right up to the main grounding point.
Symmetrical or separated ground connections for all critical signals.
The separation of critical electronic circuits from interference-prone areas.
The eradication of voltage surges.

Question 32

to keep EMI contained by following the necessary maintenance procedures.

to ensure the continuing safety of the aircraft:

Answer 32

Bonding checks must be correctly carried out.
Bonding terminals must be correctly torque loaded.
Crimping compounds must be used where specified.
Cable screens must be correctly terminated and earthed.

Question 33

how can aircraft and their systems be protected against High-Intensity Radiated Fields (HIRF)

Answer 33

Through the use of Electromagnetic Interference (EMI) filters, electrical shielding of cables, good aircraft electrical bonding, and fitted electrostatic dischargers.

Question 34

which way do the magnetic lines of force travel if the current is flowing toward the receiver

Answer 34

anti-clockwise