737 MAX Flight Crew Operations Manual
Communications -
System Description
5.20.8
MN-FLT-OH-201
The audio control panels are used to control voice transmission and receiver
monitoring. When an HF transmitter is keyed after a frequency change, the
antenna tunes. A steady or intermittent tone may be heard through the audio
system. While tuning, the tone can last as long as 7 seconds. If the system fails to
tune, the tone will last more than 7 seconds, to a maximum of 15 seconds. The
antenna is located in the vertical stabilizer.
Note:
Data for the last 100 tuned frequencies is stored in memory. Tuning
duration for these stored frequencies will be very short and a tune tone may
not be noticeable.
1B401 - 1B785, 1B801 - 1B824, 1J590 - 1J594
Both HF radios use a common antenna. When either HF radio is transmitting, the
antenna is disconnected from the other HF radio, and it cannot be used to transmit
or receive. However, both HF radios can receive simultaneously if neither is being
used for transmitting.
Selective Calling (SELCAL)
1B785, 1B786, 1J590 - 1J594
A ground station desiring communications with the flight deck can use the
SELCAL system. SELCAL monitors selected frequencies on VHF and HF radios.
Each airplane is assigned a unique four–letter SELCAL code. When the system
receives an incoming call from a ground station, a two–tone chime sounds and the
respective SELCAL light illuminates.
Cockpit Voice Recorder
1B401, 1B402, 1B801 - 1B824
The cockpit voice recorder uses four independent channels to record flight deck
audio for 120 minutes. Recordings older than 120 minutes are automatically
erased. One channel records flight deck area conversations using the area
microphone. The other channels record individual ACP output (headset) audio
and transmissions for the pilots and observer.
The RIPS (Recorder Independent Power Supply) provides power to the cockpit
voice recorder for 10 minutes after aircraft power is interrupted either by normal
shutdown or by any other loss of power.
ACARS System
The ARINC Communications Addressing and Reporting System (ACARS) is an
addressable digital data link system which permits exchange of data and messages
between an airplane and a ground-based operation center utilizing an onboard
VHF communications system.
The ACARS airborne subsystem provides for the manual entry of routine data
such as departure/arrival information. Also possible is manual entry of addresses
(telephone codes) of parties on the ground for voice communications.
March 1, 2021
