SkyLicence
Chapter V

Communication Systems - VHF, HF, Audio, SATCOM, ACARS

SkyLicence study guide with diagrams.

Diagram — Communication Systems - VHF, HF, Audio, SATCOM, ACARSIntermittent reception(engine-dependent)Ignition system EMICheck ignition shielding integrityLow receiver sensitivityFaulty coax cable orconnectorInspect shield continuity andconnector conditionTransmit works, receiver deadOpen coax at radioconnectorCheck continuity from antenna toreceiver inputLoud hum on one COM positionGround loop in wiringVerify single-point grounding andshield termination

2. HF Communication Systems

VHF vs HF Propagation — Range and Ionosphere VHF vs HF Propagation — Range & Ionosphere Layer F (Long-Distance HF Reflection) Layer E Layer D (Daytime Absorption) Radio Horizon Aircraft (TX) Remote Station BLOCKED VHF (118-137 MHz) Direct Line of Sight (LOS) Range ~200 NM (alt. dep.) HF (2-30 MHz) Ionospheric Wave (Skywave) Reflection via D/E/F Layers Key AME Comparison: • VHF: Superior audio quality, LOS. • HF: Oceanic/Polar, requires antenna coupler (impedance).

Operating Principles: HF radios operate in the 2-30 MHz frequency range and use sky-wave propagation, allowing long-range communication beyond line of sight. These systems are essential for oceanic and remote area operations.

Antenna Couplers: The antenna coupler (tuner) is a critical component that matches the impedance of the antenna system to the transceiver across the wide HF frequency range. A properly functioning coupler:

HF Antenna Coupler — Impedance Matching HF Antenna Coupler — Impedance Matching HF System 2–30 MHz — 50 Ω Transceiver — Long-wire antenna with variable impedance TRANSCEIVER HF 2–30 MHz Fixed impedance: 50 Ω Max VSWR: 2:1 Reflected power protection 50 Ω coaxial cable TX → ← RX ANTENNA COUPLER (Antenna Coupler Unit) SEARCHING FOR MATCH... AUTOMATIC MATCHING Matching network: L (inductance) + C (capacitance) variable Matches: variable Z → fixed 50 Ω 50 Ω line HF Antenna Long wire / whip Variable Z: 5–200 Ω Reflection CONSEQUENCE OF A FAILING COUPLER Weak reception Degraded signal, reduced range Noisy reception Increased noise and interference Risk of transmitter damage Excessive reflected power Cause: the coupler fails to match on the selected frequency → impedance mismatch → high VSWR ■ TX path (transmit) ■ RX path (receive) ■ Reflected power (VSWR) ■ Fixed 50 Ω impedance at transceiver / variable at antenna VSWR ↑
Automatically adjusts tuning for each selected frequency
Maximizes power transfer for transmission
Optimizes signal reception sensitivity
Protects the transmitter from reflected power

Failure Analysis:

Weak/noisy reception with good transmission: Most likely indicates a faulty antenna coupler that fails to tune for reception, causing impedance mismatch and signal attenuation
No transmit power with good reception: Check the microphone key line (PTT) continuity, as the receiver may function independently but the transmitter requires the key line to enable the power amplifier

3. Audio Management Systems

Audio Panel — Central Routing Hub Audio Panel — Signal Routing Center COM 1 VHF 118-137 MHz COM 2 Backup VHF NAV 1 (VOR) Identification NAV 2 (ADF) Identification Beacons Marker (OM/MM) CVR Recorder Alarms Alert AUDIO PANEL AUDIO SELECTOR COM / NAV / ICS COM1 COM2 NAV1 NAV2 PTT Transmission ICS INTERCOM Pilot / Copilot PA PASSENGERS Announcements PILOT HEADSET Headphones + Mic COPILOT HEADSET Headphones + Mic SPEAKER Cabin PA PASSENGERS Cabin speakers CVR Recording SELECTOR FAILURE MODES: Faulty contact → audio loss on 1 position Stuck PTT line → continuous transmission Ground loop → 50/60 Hz hum Legend: → Audio

System Architecture: The audio panel serves as the central switching and routing hub for all aircraft audio signals. It manages:

Multiple COM radio audio inputs/outputs
Navigation receiver audio (VOR, ADF, marker beacons)
Intercom (ICS) communications
Passenger address (PA) system
Cockpit Voice Recorder (CVR) audio feeds

Audio Selector Switches: These mechanical or electronic switches route selected audio to the crew's headsets. Failure modes include:

Failed switch contact: Loss of audio on one specific position while the radio itself remains operational
Stuck key line: Continuous transmission on a selected radio
Ground loops: Induced hum (typically 60 Hz) on specific channels due to multiple ground paths

Intercom Systems: The ICS allows crew communication independent of external radio transmissions. The PTT (Push-To-Talk) circuit:

Keys the selected radio transmitter
Routes microphone audio to the transmitter
Must have continuity for transmission to occur

Microphone Considerations:

A failed microphone element will cause the transmit light to illuminate (indicating PTT circuit function) but no modulation (audio) will be transmitted
Headset microphone issues affect transmission only, not reception

4. Satellite Communications (SATCOM)

System Components: SATCOM systems consist of:

Antenna: High-gain directional or top-mounted blade type
Satellite Data Unit (SDU): Processes voice and data signals
Control Panel: User interface for system operation
Coaxial Cables: Connect antenna to SDU

Signal Acquisition Requirements:

SATCOM — Signal Acquisition and Line of Sight SATCOM — Signal acquisition and line of sight ✓ CLEAR LINE OF SIGHT Signal acquired ✗ HANGAR — BLOCKED No signal CAUSES OF ACQUISITION FAILURE Mountain Physical obstruction Frost / snow On the antenna Cut cable / loose connector FACTORS WITH NO EFFECT ON ACQUISITION Audio panel Does not affect acquisition SIM card / registration Does not affect acquisition Software version v2 Does not affect acquisition Audio configuration Does not affect acquisition ✓ KEY CHECKS: Clear line of sight → Coaxial cable integrity → Tight connectors TROUBLESHOOTING PROCEDURE — NO SERVICE 1 Check line of sight — hangar, obstacles, frost 2 Inspect antenna — damage, frost, corrosion 3 Check coaxial cable — continuity, short circuit 4 Check connectors — tightness, corrosion, pins Signal acquired if all points are compliant Clear line of sight Obstruction Equipment with no effect Reference: TC E-Electronics ch5 — SATCOM
Clear line of sight to the satellite is mandatory
The antenna must be properly connected with intact coaxial cables
Signal acquisition is independent of:
Audio panel configuration (affects audio routing only)
Registration programming (affects billing/network access)
SIM cards (not used in aviation SATCOM)
Software version (causes faults, not acquisition failure)

Troubleshooting 'No Service' Conditions:

5. ACARS (Aircraft Communications Addressing and Reporting System)

System Architecture: ACARS uses VHF radios for data transmission but requires dedicated processing equipment:

Management Unit (MU): Processes data messages and manages communication protocols
VHF Data Radio: Provides the physical RF link
Control Display Unit (CDU): User interface for message composition

Operational Requirements:

Correct aircraft identification (tail number) must be programmed in the MU for network logon
The ground network validates the aircraft identity before allowing data exchange

Failure Analysis:

6. Cockpit Voice Recorder (CVR)

Regulatory Requirements (CAR 605.34):

Part 705 aircraft (commercial air services, MTOW > 5,700 kg): Minimum 2-hour recording duration
Smaller aircraft or Part VI operations: 30-minute minimum

Channel Configuration: CVRs typically record:

Pilot audio channel
Co-pilot audio channel
Cockpit area microphone
Passenger address (if applicable)
Additional crew positions

Failure Analysis:

Single channel missing: Faulty microphone or wiring for that specific channel
All channels affected: Recording head failure, power supply issue, or audio panel configuration error

7. Emergency Locator Transmitter (ELT)

ELT — Distress Alert Chain ELT — Distress Alert Chain (COSPAS-SARSAT) 1. ACTIVATION Impact or Manual ELT 406 MHz 2. SATELLITE COSPAS-SARSAT 3. LUT STATION Local User Terminal (Ground Reception) 4. MCC CENTER Mission Control (Data Processing) 5. JRCC CENTER Rescue Coord. 406 MHz Signal Data Alert FINAL LOCATION Homing 121.5 MHz

Regulatory Requirements (CAR 605.38):

ELTs installed after February 1, 2009, must be 406 MHz type with a 121.5 MHz homing beacon
121.5 MHz only ELTs are no longer accepted due to cessation of satellite monitoring
243 MHz is military frequency, not for civilian use

Battery Maintenance:

Replace per manufacturer's recommendations (typically 2-5 years)
If no expiry date is marked, consult manufacturer's instructions
Voltage checks or self-tests are insufficient to determine battery capacity
Battery replacement is not automatically required after accidental activation unless specified by manufacturer

Installation and Inspection:

Mounting bracket must be free from corrosion
ELT must be securely mounted to ensure proper crash activation
Corroded brackets must be replaced per manufacturer's data
Accidental activation requires notification to ATC or rescue coordination centre

Important Formulas, Regulations, and Procedures

Key Regulations

Standard Procedures

Pre-flight SATCOM Checks:

92.Verify satellite visibility (not in hangar)
93.Check antenna connections
94.Confirm system power-up
95.Verify signal acquisition

HF Troubleshooting Sequence:

97.Verify power supply voltage (affects both transmit and receive)
98.Check antenna coupler operation
99.Test microphone key line continuity (if no transmit)
100.Inspect antenna system integrity

ELT Accidental Activation Procedure:

102.Verify ELT is functional
103.Reset the ELT
104.Notify nearest ATC facility or rescue coordination centre
105.Document the event

Common Relationships Between Concepts

System Interdependencies

109.VHF Radio ↔ Audio Panel: The audio panel routes received audio to headsets and microphone audio to the selected transmitter. A fault in either system can mimic failure in the other.
110.ACARS ↔ VHF Radio: ACARS relies on VHF radio for the physical RF link but requires the Management Unit for data processing. A functional VHF radio does not guarantee ACARS operation.
111.SATCOM ↔ Antenna System: SATCOM signal acquisition depends entirely on antenna integrity and line of sight. Audio panel configuration has no effect on satellite connectivity.
112.CVR ↔ Audio Panel: The CVR receives audio feeds from the audio panel. However, individual microphone failures affect only their respective CVR channels.

Cross-System Troubleshooting Principles

Cross-System Troubleshooting — TX, RX, Bidirectional Cross-System Troubleshooting — Transmission, Reception, or Bidirectional Fault RADIO FAULT SYMPTOM Does the radio transmit and receive? No / partial Yes Bidirectional Fault (TX and RX both faulty) Possible causes: Unstable or missing power supply Damaged antenna or coaxial cable Faulty processor / main unit Check power → antenna → cables → unit Unidirectional Fault (TX or RX only) Which direction is faulty? Transmission or reception? Transmission Reception Transmission Fault Possible causes: Faulty microphone PTT circuit (line) Faulty transmitter stage PTT Reception Fault Possible causes: Antenna cable cut Receiver input Panel audio routing Motor Symptoms Motor ignition EMI Legend: Transmission fault flow Reception fault flow Bidirectional fault flow Tip: a 50/60 Hz hum indicates a ground loop
Transmit-only failures typically indicate issues in the transmit path (microphone, PTT circuit, or transmitter stage)
Receive-only failures suggest problems in the receive path (antenna cable, receiver input, or audio routing)
Bidirectional failures point to common components (power supply, antenna system, or main processor)
Engine-dependent symptoms strongly suggest electromagnetic interference from ignition systems
Position-specific symptoms (e.g., only COM2 affected) indicate wiring or switch issues specific to that position

Impedance and Signal Integrity

The 50-ohm standard is consistent across VHF systems:

Radio output/input: 50 ohms
Coaxial cable: 50 ohms
Antenna: 50 ohms

Any deviation from this standard creates mismatch losses that affect both transmission efficiency and reception sensitivity. This principle applies uniformly to VHF communication, navigation, and some HF systems.

This chapter provides the foundational knowledge required for AMEs to diagnose, maintain, and certify aircraft communication systems in compliance with Canadian Aviation Regulations and industry standards.

Chapter 4Aircraft Fabric CoveringChapter 6Navigation Systems - VOR, ILS, DME, ADF, GPS, GNSS, Transponder

Ready to test this chapter?

Practice with exam-aligned questions and timed simulations.

Start Practicing Free