AET Communication Systems 3 — Questions and Answers
Question 1: What is the purpose of SELCAL (Selective Calling System) in aviation communication?
- To encrypt pilot transmissions
- To alert a specific aircraft when a ground station wants to communicate (Correct answer)
- To automatically switch between VHF frequencies
- To monitor cabin crew interphone traffic
Correct answer: To alert a specific aircraft when a ground station wants to communicate
SELCAL allows ground stations to alert a specific aircraft via a unique 4-tone code, enabling pilots to reduce cockpit audio monitoring on long-haul flights.
Question 2: Which modulation type is used by standard VHF aviation communication radios for voice transmissions?
- FM (Frequency Modulation)
- AM (Amplitude Modulation) (Correct answer)
- SSB (Single Sideband)
- DSB-SC (Double Sideband Suppressed Carrier)
Correct answer: AM (Amplitude Modulation)
Aviation VHF communication uses AM because it allows multiple signals to coexist on the same frequency and is compatible with legacy ground infrastructure.
Question 3: An aircraft HF radio is being checked and exhibits low output power but correct frequency. The MOST likely cause is:
- Failed frequency synthesizer
- Defective PA (power amplifier) stage (Correct answer)
- Incorrect antenna coupler setting
- Faulty squelch circuit
Correct answer: Defective PA (power amplifier) stage
Low output power with correct frequency typically points to a failure in the PA stage, since frequency generation is handled separately by the synthesizer.
Question 4: What is the frequency range of the aviation VHF communication band?
- 108.0–117.95 MHz
- 118.0–136.975 MHz (Correct answer)
- 225.0–399.975 MHz
- 406.0–420.0 MHz
Correct answer: 118.0–136.975 MHz
Aviation VHF communication operates from 118.0 to 136.975 MHz, while 108–117.95 MHz is reserved for VOR/ILS navigation.
Question 5: A SATCOM system on an aircraft provides communication primarily via:
- Ground-based VHF repeaters
- Geostationary or LEO satellites (Correct answer)
- HF sky-wave propagation
- UHF line-of-sight links
Correct answer: Geostationary or LEO satellites
SATCOM uses geostationary (e.g., Inmarsat) or low Earth orbit satellites to relay voice and data communications globally, including over oceans.
Question 6: During an interphone system check, audio is heard in the cockpit but not at the forward attendant station. Which component is MOST suspect?
- Cockpit audio control panel
- Interphone amplifier serving the forward cabin zone (Correct answer)
- Volume control in the cockpit
- Audio frequency oscillator
Correct answer: Interphone amplifier serving the forward cabin zone
If only one station is silent, the amplifier or wiring serving that specific zone is the most likely fault rather than a shared component.
Question 7: What does the term 'squelch' refer to in an aviation radio receiver?
- A filter that removes sideband noise
- A circuit that mutes the receiver output in the absence of a valid signal (Correct answer)
- A device that boosts weak incoming signals
- A modulator that encodes audio before transmission
Correct answer: A circuit that mutes the receiver output in the absence of a valid signal
Squelch suppresses receiver output when signal strength falls below a threshold, preventing the cockpit from being flooded with static noise between transmissions.
AET Communication Systems 3
What is the purpose of SELCAL (Selective Calling System) in aviation communication?