CHAPTER

23

Communications

CHAPTER 23 Communications

777 AIRCRAFT MAINTENANCE MANUAL

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HIGH FREQUENCY COMMUNICATION SYSTEM -INTRODUCTION

General
The high frequency (HF) communication system permits voice communication over longer distances than line-of-sight radio systems. The HF system is for communication with ground stations or with other airplanes during long over-water flights. The HF system operates in the HF aeronautical communication band between 2.000 MHz and 29.999 MHz.
Long Range Communication
The HF system uses sky waves for long range communication. The HF signals pass into the ionized layers of the atmosphere. The ionized layers refract the signals toward the earth. The earth reflects the signals back toward the ionized layers. Sky wave distances depend upon frequency, time of day, and airplane altitude. Skips are the areas between sky waves reflections.
Short Range Communication
HF signals also go directly to ground stations and other airplanes.
Abbreviations and Acronyms
*
ACP -audio control panel

*
AM -amplitude modulation

*
AMU -audio management unit

*
CAPT -captain

*
CMCF -central maintenance computing function

*
CMCS -central maintenance computing system

*
DFDAF -digital flight data acquisition function

*
F/O -first officer

*
F/OBS -first observer

*
PSIGA -pounds per square inch gage absolute

*
RTP -radio tuning panel

*
sens -sensitivity

*
seq -sequence

*
USB -upper side band

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HIGH FREQUENCY COMMUNICATION SYSTEM -INTRODUCTION

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HF SYSTEM -GENERAL DESCRIPTION

General
The HF communication system uses two HF systems. Each system has these components:
*
A common HF antenna. The antenna transmits and receives RF signals in the HF range

*
An HF antenna coupler. The antenna coupler matches the impedance of the antenna and transmission line to the output of the transceiver at the selected frequency

*
An HF communication transceiver. The transceiver operates in the AM or upper side band (USB) mode.

WARNING: DO NOT TRANSMIT WITH THE HF COMMUNICATION SYSTEM WHILE THE AIRPLANE IS REFUELED OR DEFUELED. AN EXPLOSION CAN CAUSE INJURIES TO PERSONNEL AND DAMAGE TO THE AIRPLANE.
WARNING: MAKE SURE PERSONNEL STAY A MINIMUM OF 6 FEET AWAY FROM THE VERTICAL STABILIZER WHEN THE HF SYSTEM TRANSMITS. RF ENERGY FROM THE HF COMMUNICATION ANTENNA CAN CAUSE INJURIES TO PERSONNEL.
HF System Connections
The HF communication system connects with these components:
* Three radio tuning panels (RTPs). The RTPs send tuning
data and mode information to the HF transceivers

*
The audio management unit (AMU). Microphone audio and push-to-talk (PTT) discretes go to the transceiver through the AMU. Received audio goes from the transceiver to the speakers and headsets through the AMU

*
The SELCAL decoder. The SELCAL decoder alerts the flight crew when the airplane receives a call.

The HF communication system also connects with the airplane information management system (AIMS) for these functions:
*
Digital flight data acquisition function (DFDAF). The DFDAF in the AIMS cabinet receives the microphone PTT signal for input to the flight recorder

*
Central maintenance computing function (CMCF). The CMCF gets fault reports from the HF system and records this information in a fault history. This shows the maintenance crew the faulty component or function.

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777 AIRCRAFT MAINTENANCE MANUAL

HF SYSTEM -GENERAL DESCRIPTION

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HF SYSTEM -FLIGHT DECK COMPONENT LOCATIONS
HF Interface Component Locations
These are the components in the flight deck that have an interface with the HF communication system:
*
Radio tuning panels (RTPs)

*
Audio control panels (ACPs).

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HF SYSTEM -FLIGHT DECK COMPONENT LOCATIONS

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