COMMUNICATIONS, VHF COM
Description
Aircraft Operations Manual
4/3.1
KO
PAGE
1
Apr 01/15
KING MEMORY
1.
GENERAL
There are two identical VHF communication sys-
tems independent of each other. The frequency
range is 118.000 to 136.975 MHz selectable in 25
kHz steps.
With Mod. No. 2938 the 25 kHz space has been
modified with three frequencies, each with 8.33 kHz
spacing. These frequencies are referred to as
“channels”, since the displayed frequency is not the
actual frequency tuned. For example, a frequency
with 25 kHz spacing, as 118.000 MHz, can now be
selected with 8.33 kHz spacing by, in this case, se-
lecting 118.005 on the control unit. If 118.010 is se-
lected the actual frequency tuned is 118.008 MHz.
118.017 MHz is tuned by selecting 118.015 etc.
This method ensures that the equipment will always
operate in the proper spacing mode. In practice this
will not cause any confusion, since the pilot will be
asked to tune a channel as if it was a frequency.
The control unit is also modified with three decimal
places to ensure proper frequency/channel selec-
tion.
The control units are located on the center pedes-
tal. The COM 1 antenna is mounted on the top of
the fuselage and the COM 2 antenna is mounted on
the bottom of the fuselage.
Due to fuel tank probe wiring, fuel quantity indica-
tion may momentarily change (less than 1% of full
scale indication) when transmitting on COM 2.
NOTE
The transmission (PTT button keying) is recorded
on the Digital Flight Data Recorder (DFDR) via the
Flight Data Acquisition Unit (FDAU). The commu-
nication will also be recorded by the Cockpit Voice
Recorder via the Audio Integrating System.
2.
MAIN COMPONENT AND SUBSYSTEMS
2.1
Control unit
The control unit contains a microprocessor which
decodes selector and switch position and generates
a frequency information which is transferred to the
transceiver as a data code.
Two frequencies can be selected and displayed. By
means of a transfer button the operator can select
one frequency to be active and the other one as
standby. The active one will be indicated by a ”T”
when the Press To Transmit (PTT) button is oper-
ated.
A programmable Memory facility is also contained
in the control unit. The memory provides eight pre-
programmed frequencies, by momentarily pressing
the CHAN button and thereafter, choose frequency
by means of the frequency selector.
The control unit is provided with a squelch function
which can be disable by pressing the PUSH TEST
button.
The displays are dimmable and controlled by a pho-
tocell to give the adequate brightness.
2.2
Transceiver
The transceiver produces an amplitude modulated
signal of the selected frequency and this signal is
directly fed to the antenna. In reception mode, the
selected frequency is demodulated and the gener-
ated audio signal is transferred to the Audio Inte-
grating System for distribution.
Antenna
VHF COM
Transceiver
Selected
frequency
Control
Unit
Audio Integrating System
Digital Flight Data Recorder
Cockpit Voice Recorder
Fig. 1 VHF COM
−
schematic
