A Complete Manual for Conducting International Flight Operations

HF Communications Basics

HF Communications Basics

HF stands for High Frequency and it pertains to the long range communication system that is required to be operating on oceanic flights. The HF has the advantage of skywave propagation, which allows HF radio waves to be reflected off the ionosphere and back to the earth’s surface. This eliminates the need for repeaters as are required for VHF or UHF communications systems.

Prior to initiating an transmission on the HF, the frequency shall be entered and the microphone switch shall be depressed, at which time a loud single tone will become audible indicating that the HF antenna is being tuned to the appropriate frequency. Care must be exercised to ensure that both units are not tuned to the same frequency as damage may occur.


Crewmembers shall review their individual HF manuals to determine proper HF usage. When selecting a frequency from a list of multiple frequencies a key rule of thumb is, the higher the sun is in the sky the higher the frequency that should be selected. At night, the lower the sun, the better the lower frequencies will function. This phenomenon is due to the sun ionizing particles in the ionosphere which thickens the ionosphere which in turn reflects higher frequencies more effectively.

Advantages of HF over VHF Systems

  1. Ability to make contacts over long distances and varied terrain without being limited to line of sight
  2. Atmospheric moisture not absorbing the signals, thus eliminating the need for repeater stations to maintain useful signal strength
  3. Physic obstructions, whether natural or man-made, not attenuating the signal as much as with VHF
  4. Flutter, such as that which occasionally plagues the VHF frequencies is absent in HF
  5. Unlike VHF a large number of frequencies is available (280,000)

HF System Codes

Most long range aircraft have two HF systems which utilize a single antenna. There are many different HF installations, but they share similar functionality. Some of the common codes amongst HF systems are the following:

MAN (Manual Mode): This in the normal position for operations, the display shall indicate MANUV indicating manual mode for upper sideband voice.


MAR (Maritime Mode):


PGM (Program Mode):


SBY (Standby Mode):


TST (Test Mode): This mode selects built-in test and performs an HF system diagnostic

SQL (Squelch adjustment): This mode silences undesired background noise when voice communication is not present, SQL 0 will all transmission noise through.


DSBL: Disables the SQL circuit (squelch)


UV (Upper Sideband Voice):


LV (Lower Sideband Voice):


UD (Upper Sideband Data):


LD  (Lower Sideband Data):


Revision date: July 29, 2015

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