Communications
The communications system is used by the pilot and copilot to speak to each other and to the passengers, with the ground stations, and other aircraft. It is also used by the pilot and copilot to listen to communication and navigation radios, and to hear the audio alerts from the integrated avionics. It consists of the systems that follow:
Speech communication
The speech communication is used by the pilot and copilot to talk and listen to ground stations and to other aircraft. It transmits and receives radio signals in the VHF aviation band. It consists of the COM/GPS antenna and uses the VHF COM transceiver of the integrated avionics unit (GIA).
Radio signals for air-to-air and air-to ground communications are transmitted and received through the COM/GPS antenna. The VHF COM transceiver converts between radio signals from the COM/GPS antenna and audio signals from the audio integrating system. Transmit and receive is controlled by the audio integrating system. Tuning, volume, and squelch are controlled by the central display systems.
COM/GPS antenna
The COM/GPS antenna is located on the forward fairing above the cockpit. It is a dual purpose type, bent whip that connects to the VHF COM transceiver and to the GPS receiver of the GIA. The COM/GPS antenna has an omnidirectional radiation pattern, a vertical polarization, and a frequency range of 118.0 to 137.0 MHz
VHF COM transceiver
The VHF COM transceiver is integral with the GIA. It operates in the 118.00 to 136.992 MHz frequency range in 25 kHz or 8.33 kHz increments, giving 760 or 2280 channels respectively. It uses the Amplitude Modulation (AM) technique to modulate the audio signal into a radio signal and to demodulate the radio signal into an audio signal.
The Primary Flight Display (PFD) and Multi-Function Display (MFD) provide control of the tuning, volume, and squelch.
The audio panel processes audio and controls reception and transmission.
Operation
The speech communication system is energized with 28 VDC from the MAIN bus and can be isolated with the GIA COM circuit breaker.
When not transmitting, the VHF COM transceiver is always receiving the frequency tuned through the PFD or MFD. Due to the nature of the AM technique, a certain amount of noise is always present even if there is no radio signal. The purpose of the squelch is to silence the audio from the VHF COM transceiver in the absence of a sufficiently strong radio signal.
The VHF COM transceiver receives the radio signal from the COM/GPS antenna through the COM ANTENNA connection of the GIA. The radio signal is then demodulated into an audio signal by the VHF COM transceiver. The audio signal is supplied to the audio panel.
The audio panel provides the signal to transmit and the audio to the GIA.
The VHF transceiver modulates the amplitude of the selected frequency radio signal with the audio signal. The radio signal is then transmitted through the COM ANTENNA connection of the GIA and the COM/GPS antenna.
Audio integrating
The audio integrating system allows the pilot to communicate with the passengers, and with the copilot when there is one, through their headsets, to listen to communications and navigation radios, and to select radios for reception and transmission.
The system includes the components that follow:
Pilot and Copilot Headset jacks
Passenger Headset jacks
The dual controls kit provides an ICS trigger switch for the copilot when it is installed.
Audio panel
The audio panel (GMA) is located on the pedestal. It is installed in a dedicated quick-change rack to facilitate replacement of the Line Replaceable Unit (LRU). It collects, processes, and distributes audio signals between the pilot, copilot, and passenger headsets, and also with the integrated avionics unit for the communication and navigation radios and Crew Alerting System (CAS).
The audio panel uses digital signal processing to minimize noise. Microphone inputs can be voice activated, if desired. A digital clearance recorder is also available. Its front panel has the controls that follow:
Push-buttons to select audio sources
Push-buttons to select microphones
Push-button to select audio squelch
Push-button to select marker beacon audio mute
Selection indicator lights
Volume/squelch level indicator lights
Volume control knob
Cursor knob
Jack for stereo or telephone input
Intercommunication System (ICS) trigger switch, pilot
The pilot ICS trigger switch is located on the pilot cyclic grip. It consists of a trigger at the front of the cyclic grip and two switches inside the cyclic grip, one for ICS and one for transmit. Pushing down on the pilot ICS trigger switch selects audio from the pilot microphone and commands the VHF COM transceiver to transmit. Pushing up distributes the audio from the pilot microphone to the other active headsets.
Pilot and Copilot Headset jacks
The pilot and copilot headset jacks permits easy connection of the pilot and copilot headsets. They are located in the cockpit on the side of the crew seats at the end of an extension cord connected to the audio integrating system through a quick disconnect.
Passenger headset jacks
The passenger headset jacks permit easy connection of the headsets for three passenger. They are located near the top of the bulkhead at the rear of the cabin.
Operation
The audio panel is energized with 28 VDC from the MAIN bus and can be isolated with the ICS circuit breaker.
The audio panel does a self-test during power-up. All the annunciator lights come on for approximately 2 seconds. When complete, most of the settings are restored to the state they were in at the last shutdown.
The audio signals to and from the headset jacks are processed and distributed by the audio panel according to the selections of the audio panel.
The audio signal for the VHF COM transceiver is supplied from the integrated avionics unit (GIA), through the audio panel.
The audio signal for the NAV receiver is supplied to the audio panel.
The audio signal for the CAS is supplied from integrated avionics unit (GIA), through the audio panel.
Pushing down on the pilot ICS trigger switch applies a ground to the audio panel.
The audio signal from the pilot headset jacks is processed by the audio panel.
The audio panel applies a ground through the Intercommunication System (ICS) trigger switch to the integrated avionics unit (GIA). This enables the COM MIC and causes the VHF COM transceiver to transmit.
The audio signal is applied through the audio panel to integrated avionics unit (GIA) for transmission by the VHF COM transceiver.
Digital communication between the audio panel and the integrated avionics unit (GIA) is through the RS-232 databus.
In case of a failure of the audio panel, COM 1 audio output is routed to the left side of the pilot headset, pilot microphone audio is routed to COM 1 audio input, and pilot ICS trigger switch is operable so the pilot can use the VHF COM transceiver for communication.
Pushing up on the pilot ICS trigger switch applies a ground to the audio panel which distributes the audio signal from the pilot microphone to the other headsets according to the selections of the front panel.
The audio panel lighting is controlled by the cockpit lighting.
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