Aviation Communication Systems Safety and Maintenance Guide

TOPIC 4.1.6: COMMUNICATION

Introduction

In the aircraft avionics industry, maintenance personnel encounter several safety issues. The following sections detail hazards related to high voltage, electromagnetic radiation, radioactive materials, and hazardous substances used in equipment construction.

High Voltage Hazards

Radar transmitters and display units contain circuit components that operate with high voltages, often in the range of $1000s$ of volts. A critical danger in radar systems is that certain components retain residual voltage even after the equipment has been shut down.

Protective Measures

• Documentation: High voltage warnings and cautions are highlighted in the relevant equipment maintenance manuals.
• Power Isolation: All power must be switched off from the source before the removal or installation of any component.
• Discharging: Maintenance personnel must discharge any residual voltage on components before handling. If the method is unclear, procedures for discharging these voltages are detailed in the manufacturer's maintenance manual.

Electromagnetic Radiation Hazards

Radiation is defined as any form of energy propagated as waves or streams of charged particles. These include:

• Radio waves
• Heat rays
• Light rays
• Ultraviolet (UV) rays
• X-rays
• Gamma rays

There are two primary types of radiation hazards depending on energy levels: non-ionizing and ionizing radiation. Both pose serious health risks to individuals in the vicinity of radar equipment.

Non-Ionizing Radiation

Non-ionizing radiation has enough energy to move atoms in a molecule or cause them to vibrate but lacks the energy to change them chemically. This means atoms in the air and insulators will not be ionized.

• Conductive Effects: The valence electrons of atoms in conductors are easily dislodged, causing current to flow. This principle is the basis of RF transmission and reception.
• Biological Impact: Insulators and non-conductors (such as the human body) do not have electrons forced from their orbits by radiated $E$ and $H$ waves. The primary health effect from high exposure is heat generation in body tissue (e.g., UV rays causing sunburn).
• Types and Sources: Examples include ultraviolet (UV), visible light, infrared (IR), microwave, radio and television waves, and extremely low frequency (ELF/EMF). In aircraft, sources include radar antennas, transmitting tubes, final stage cavities of transmitters, and damaged or incorrectly installed waveguides.

Ionizing Radiation

Ionizing radiation falls within a range that has enough energy to break chemical bonds. The $E$ and $H$ waves dislodge electrons from atoms in non-conductors (humans), ionizing them and causing cell mutation. A damaged atom or molecule in a cell may then reproduce as a cancer cell.

• Historical Background: X-ray technology was discovered by accident in $1895$ by German physicist Wilhelm Roentgen. While experimenting with electron beams in a gas discharge tube, he noticed a fluorescent screen glowing despite being shielded by heavy black cardboard. He eventually saw the silhouette of his bones when placing his hand in front of the tube.
• Physical Properties: Energy increases as frequency rises. X-rays and gamma rays have very high frequencies (around $100 \times 10^{18} \text{ Hz}$ or $100 \text{ billion billion Hz}$) and very short wavelengths ($1 \times 10^{-12} \text{ m}$ or $1 \text{ million millionth of a meter}$).
• Ionization Process: This is the process where a charged portion of a molecule (usually an electron) is given enough energy to break away from an atom, resulting in a net positive molecule and a free negative electron.

Measuring Radiation Effects: REM

The acronym REM (Roentgen Equivalent Man) is a standard unit measuring the biological risk and effects of ionizing radiation on the human body.

Biological Effects of Exposure

Radiation deposits energy in body tissue, causing cell damage or death. Cells may survive but become permanently abnormal or malignant (cancerous). Damage depends on:

• Total amount of energy absorbed
• Time period and dose rate of exposure
• The specific organs exposed

Latency Periods: Evidence of injury from low/moderate doses may not appear for months or years. Leukemia has a minimum latency of $2 \text{ years}$, while solid tumors have a latency of more than $5 \text{ years}$.

Types of Exposure

• Cosmic Radiation: Originates in outer space (sun and other sources). Secondary cosmic rays account for $45-50 \text{ millirem}$ of the $360 \text{ millirem}$ average annual background radiation.
• Chronic Exposure: Continuous or intermittent exposure to low levels over a long period. Effects include genetic defects, cancer, cataracts, and skin changes.
• Acute Exposure: Large, single doses or series of doses over a short period. This can cause rapid radiation sickness (G.I. disorders, infections, hemorrhage, loss of fluids). Extremely high levels cause death within hours or days.

Radiation Protective Measures

To prevent over-exposure to ionizing and non-ionizing radiation:

• Adhere to "NO ENTRY" signage for hazard areas near transmitting radar.
• Do not touch radar antennas of operating equipment with hands or conductive material.
• Inform maintenance personnel in the vicinity of active aircraft radar.
• Do not transmit inside hangars.
• Do not point radar antennas at metal structures within the hazard zone.
• Use artificial loads (dummy loads) or absorption blankets during servicing.
• Ensure power is off and pressure is purged when replacing waveguide components.

Radioactive Materials

Radioactive materials emit radiation spontaneously without the application of power (unlike radar which requires power to radiate). Radioactivity is measured in Becquerels or disintegrations per second.

Particle Types

• Alpha Particle: Positively charged (helium nucleus, mass number $4$, charge $+2$). Low penetrating power; stopped by a sheet of paper or skin.
• Beta Particle: Charged particle (negatively charged is an electron; positively charged is a positron). Can cause skin burns and is harmful if it enters the body. Stopped by thin metal or plastic.
• Gamma Radiation: High-energy, short wavelength. Highly penetrating; best stopped by lead or depleted uranium.

Hazard Labeling and Storage

• White Label: Indicates a very low level of hazard.
• Yellow Label: Indicates a higher degree of hazard; the amount of internal radioactivity is higher.
• Packaging: Strict control over packaging design ensures only safe levels of activity are released during transport.

Radar Components and Radioactive Hazards

In radar installations using a single antenna for both transmit and receive, a duplexer is used to channel power and protect the receiver.

TR and ATR Cells

• TR Cells (Transmit-Receive): Act as electronic switches (gas discharge tubes). When the transmitter pulses, the gas ionizes to short-circuit and protect the receiver. Water vapor is often used for stability in ionization/de-ionization.
• TB or ATR Cells (Anti-Transmit Receive): Prevent received energy from being wasted in transmitter circuitry by causing the transmitter impedance to appear infinite during reception.
• Hazard: Some TR and ATR cells contain radioactive substances. While safe under normal conditions, they present a serious health hazard if the cell is broken.

Safety Precautions for Tubes

• Do not remove the tube from its carton until immediately prior to installation.
• Place the tube in a suitable carton immediately after removal.
• Use extreme care during fitting and removal.
• Follow approved safe disposal procedures.

Hazardous Materials in Aviation

Polychlorinated Biphenyls (PCBs)

Man-made aromatic compounds banned in most countries since the late $1970s$.

• Properties: Colorless, yellow oily liquids, or waxy solids. High flash points ($170-380 \text{ }^{\circ}\text{C}$), high specific gravity ($1.5$, sinks in water), and a sweet solvent odor similar to benzene.
• Uses: Found in oil-filled transformers, capacitors, and pulse forming networks due to excellent dielectric and flame-retarding properties.
• Toxicity: Enters the body via lungs, skin, or G.I. tract. Symptoms include fatigue, headaches, numbness, eye discharge, and skin sores. Long-term effects include liver/kidney damage and birth defects.

Beryllium (Be) Compounds

A silver-grey metal, lighter than aluminum but stiffer than steel. Its melting point is $1285 \text{ }^{\circ}\text{C}$.

• Applications: Used in rocket nose cones, X-ray windows (low absorption), radomes, microwave tubes, and non-sparking hand tools (Beryllium-Copper or BeCu alloy).
• Health Hazards: Beryllium is highly toxic when inhaled as dust or fumes.
  • Acute Beryllium Disease: A type of chemical pneumonia.
  • Beryllium Sensitivity: Immune response triggered in $2-14 \%$ of exposed individuals.
  • Chronic Beryllium Disease (CBD): Inflammation and scarring of lungs; latency period varies from a few months to $30 \text{ years}$ (average $10-15 \text{ years}$).
• Precautions: Do not grind, scratch, file, or acid-clean components suspected of containing Beryllium Oxide. Consider all semiconductors potentially hazardous until proven otherwise.

Communication Measuring Instruments

Signal Generators

Used to test, align, and troubleshoot transmitters and receivers. They produce AC of specific frequencies and amplitudes.

• FG-30 Sweep Function Generator: A multi-purpose unit with a frequency range of $0.5 \text{ Hz}$ to $3 \text{ MHz}$ ($2.5 \text{ MHz}$ for Ramp and Pulse). Functions include six output waveforms and a pulse/sweep generator.
• Attenuators: Calibrated resistor networks used to regulate output voltage and prevent overloading the test circuit.

Multimeters

• Analogue Multimeters: Generally $20,000 \text{ }\Omega / \text{V}$ for DC and $1,000 \text{ }\Omega / \text{V}$ for AC. Feature a "zero adjust" dial. Useful for testing potentiometers/rheostats due to circuit loading.
• Digital Multimeters (DMM): Most common for aircraft technicians. Features include auto-ranging, high sensitivity, and elimination of parallax error. Some versions measure frequency, capacitance, diode junctions, and temperature.

specialized Measuring Tools

• AF Power Meters: Measures power delivered to an internal load. The Marconi AF power meter measures up to $10 \text{ W}$ across five ranges. Logarithmic scales are referenced such that $0 \text{ dB} = 1 \text{ mW}$.
• RF Directional Wattmeter: Measures forward and reflected power in coaxial lines. It uses plug-in elements for specific power/frequency ranges. A diode rectifier detect energy across a known impedance.
• VSWR Measurement: Calculated using the formula substituting forward and reflected power or via a VSWR nomograph (plotting X and Y axis for forward/reflected power).
• Frequency Counter: One of the most accurate means of measuring frequency, displaying numerical counts of pulses over time.
• Oscilloscope: Displays signals in the time domain, showing the shape of video pulses.
• Spectrum Analyser: Observes signals in the frequency domain, separating components into individual frequencies and power levels.
• Time Domain Reflectometer (TDR): Feeds energy impulses into a system to locate faults by analyzing reflections. It determines the distance to the fault and the nature (resistive, inductive, or capacitive) of the discontinuity.

General Communication Installation Considerations

• Weight & Balance: Equipment weight can affect the aircraft center of gravity.
• Cooling: Requires adequate clearance and forced air-cooling to prevent exceeding ratings.
• Cables: Maximum lengths must be observed; types must protect against interference.
• Antennas: Must avoid deleterious mutual interference. Separation, frequency filtering, and polarization (E field direction) are utilized to prevent cross-coupling.
• Controls: Must be visible and accessible from the normal seated position of the crew.
• Compass: Magnetic compass deviation must not exceed permissible limits after installation.
• HF Radiation: High potential for electric shock from HF antennas; do not transmit while maintenance (doping/painting) is occurring nearby.

Regulatory Requirements: CAO 108.34 (Antennas)

Specific standards for antenna installation include:

• 9.3: A tensioning device or weak link must be fitted at the aft end of fixed wire antennas.
• 9.8: Leads between HF couplers and antennas should be as short as practicable (ideally $< 150 \text{ mm}$).
• 9.9: Coaxial cables must not have loss in excess of $3 \text{ dB}$.
• 9.10: All antennas must be connected to the airframe by a DC path or fitted with a static leak.

Electromagnetic Interference (EMI)

EMI manifests as undesirable voltages/currents from other sources, including lightning, external RF fields, or internal systems.

Coupling Modes

• Radiated EMI: Propagates through air.
• Conducted EMI: Couples through common wiring or metallic structure connections.
• Static/Electrostatic Induction: Caused by precipitation static (frictional contact in atmosphere). Sparks between unequal potentials generate wide-band RF noise.

Mitigation Strategies

• Radio Bonding: Use short jumpers ($R < 0.003 \text{ }\Omega$). Use aluminum alloy or cadmium-plated copper for aluminum structures; brass/bronze for steel.
• Static Dischargers: Fitted to trailing edges of mainplanes and stabilizers to provide an easy path for discharge to the atmosphere.
• Suppression: Capacitors provide a low resistance RF path across brushes/contacts in motors and generators. Ignition wires use metallic braid and shielded primary leads.
• Twisted Pairs: Used to reduce capacitive/inductive pick-up ($400 \text{ Hz}$ power noise) and crosstalk (interference between signal cables).
• Screening: Metallic shields earthed at one end (to avoid Earth Loops) are vital in audio systems to prevent mains interference.