SELCAL System

SELCAL System

• The word SELCAL is derived from the term selective calling and the SELCAL DECAER is an instrument designed to relieve the pilot and co-pilot from continuously monitoring the aircraft radio receivers.

• The Selcal decoder is in effect an automatic monitor which listens for a particular combination of tuner which is assigned to the individual aircraft. Whenever a properly coded transmission is received from a ground station, the signal is decoded by the Selcal unit which then given a signal to the pilot notifying him that a radio transmission is being directed to the aircraft. The pilot can then listens to his receiver and hear the message. 

• Ground station is equipped with tone transmitting equipment cell individual aircraft by transmitting two pairs of tones which will key only an airborne decoder which is not to respond to the particular combination of tones. When the proper tones are received, the system operates an external alarm circuit to produces a chino, light, buzz, or a combination of such signals.

• When the ground operator wishes to contact a particular aircraft by means of the Selcal unit, he selects the four tone code which has been assigned to the aircraft. The tone code is transmitted by a radio-frequency wave, and the signal can be picked up by all receivers tuned to the frequency used by the transmitter. The only receiver which can respond to the signal and produce the alert signal for the pilot is the receiver and decoder which has been out for the particular combination of tone frequencies.

• Each decoder requires four Vibersponder reads to establish the code for the unit. The reads respond to audio frequencies and cause the generation of a voltage which is processed by the unit. The resulting voltage is used to operate the alarm circuits.

Vibresponder  reads are manufactured with tone frequencies from 31.6 to 977.2 Hz.

High frequency (HF) Communication system

High frequency (HF) Communication system

The HF Communication system provide amplitude nodulation & single side band voice communication between aircrafts and ground or other carets The frequency range of this system is from 2 to 3C MHZ but nest communication stations are in the range from 2 to I8 MHZ system id used for long distance communication (sky wave propagator) The O/P from HF system is also directed to sacral decoders for Air traffic control signals. 

 

Each HF system consists of transceiver, antenna, antenna coupler, heightening arrestor, control panel & necessary relays.

1- Transceiver:-

The transmitter, section operates in a frequency range of 2 to 30 MHZ by me are of frequency selector witches on control panel. Amplification & nodulation (AM or SSB as required) are made on the mice audio.

The receiver operates at the sane channels. The receiver is a crystal- controlled superheat dyne.

2- Antenna coupler:-

Antenna tuning for all transmission frequencies is accomplished automatically by means of an antenna coupler system.

The complete antenna coupler system consists of a lightening arrestor remote coupler unit, coupler controller.       

Initial adjustment of the antenna coupler unit begins when ever a new frequency is selected for the transceiver, during the transceiver tuning cycle; a signal is seat to the antenna – coupler control which focuses Tuning circuits in the antenna coupler to drive to a home position. 

When the transmitter is tuned for transmitting, a sensing coupler to operate & adjust the impedance latch between the antenna & the transmission line so that the amount of reflect power is at a minimum. the sensing & tuning circuits operate  at all times that the transmitter is  operating & therefore continuously  provides  optimum  match between antenna & line, The tuning unit will match any combination of antenna resistance & reactance  to  the characteristic impudence -transmission line  at any  carrier frequency from2 to 30 MHZ

3-Control Panel:

Control panel of HF system consists of frequency selection, selector, and volume control.

4-HF antenna: -

The design of HF antenna varies with the size & shape of the aircraft. Aircraft cruise below 300 M.P.H. generally uses   a long wire antenna. Higher speed aircraft have specially designed antenna probes impelled the vertical Stabilizer.  Regardless of the Typo of antenna, a tuner is used to match the impedance of the transceiver to antenna.   

VHF COMMUNICATION SYSTEM

VHF COMMUNICATION SYSTEM

The VHF Communication  system   provides   amplitude   modulated   voice communication   between   aircraft   and   ground   or   other   aircraft.    The system   is   used   mainly   during   take   off   and   approach.    Communications take   place   in   the   118   to   1 5O   MHZ   frequency   range   with   25   KHZ   sparing between   channels.   The   nature   of   radio-wave   propagation   at   these   fre­quencies   is   such   that   communication   is   Limited   to   the   line   of   sigh distance   and   that's   why   the   communication   range   increases   as   the aircraft   gains   altitude.

The   advantage   of   VHF   comm. is   that   the   signal   is   not   often   dis­torted   or   rendered   unintelligible   by   statics   or   other   types   of   in­terference.

A typical VHF comm. system   consists   of   transceiver,   antenna, control   panel   with   associated   microphones   and   load speakers.

A simplified VHF comm. system   block   diagram   is   shown   in   figure  

VHF   SYSTEM   BLOCK   DIAGRAM

1)   Receive   mode

The   receiver   is   tuned   by   a   tunning   signal   from   the   control panel   according   to   the   frequency   selected,    so   the   received signal   from   ground   station   is   fed   to   receiver   through   antenna relay   which   connects   the   antenna   to   the   receiver   circuits   in case   of   reception    (usually   relaxed   curing   reception) . The   si­gnal   is   mixed,    detected   &   amplified   in   the   receiver   section, delivered   to   the   aircraft   audio   system.

2)   Transmit   mode

The mic   audio   signal   is   fed   to   the   modulator   in   which   inoc­ulation   &   Power   amplification   is   done   in   the   transmitter   sec­tion which   is   tuned   to   the   frequency selected   on   the   control

INTERPHONE SYSTEM

INTERPHONE SYSTEM

The Interphone System provides two way communications between the flight crew members, cabin attendants and maintenance personnel. The system is classified as two subsystems, the service interphone and the flight interphone.

A. Service Interphone System

The service interphone system provides communication between around crew personnel. Cockpit crew and cabin attendants. The system is composed of an amplifier, a number of handset jacks located through out the airplane, handsets at the attendant's stations and associated microphones & loudspeakers at the flight crew stations. The handset jacks are connected to the service interphone amplifier through a service interphone switch. This service interphone switch is placed to the OFF position before take off to avoid any interference with the communication sys­tems. The attendant's stations are connected directly to the service interphone amplifier. The flight crew stations are con­nected to the service interphone amplifier provided that proper selection on their associated audio selector panels has been mode.

Out put of the service interphone amplifier is fed to all the service interphone stations.

B. Flight Interphone System

The flight interphone system provides inter communication be­tween the control cabin crew members. In some aircrafts control cabin crew members can communicate with certain service inter­phone handset jacks (specially the jack located at the external power panel) through the flight interphone system.

The system has a separate flight interphone amplifier which amplifies the interphone audio signals and delivers it to all the flight in­terphone stations.

Associated   with   the   interphone   system   the   following.....

Audio Selector   Panel (ASP)

Each   flight   crew member    (captain,   copilot,   engineer,   observer . . .} has   an   audio selector   panel.   Sometimes   the   electronic   compartment is   provided   with   an   audio   selector   panel.   The   audio   selector   panel allows   the   flight   crew   members   to   select   any   communication   facility {interphone,   passenger   address,   HF,   VHF...}    for   transmission   or   re­ception   or   both   transmission   &   reception.   Also   the   ASP   allows monitoring   of   all   navigation   receiver's   call   signs    (identification to­nes)   such   as   ADF,   VOR,   DME,   marker,   etc.

Microphones

Most aircrafts   are   equipped   with:

Handheld   mike   with   a   PTT   in   the   mike   assembly   .

Boom mike   integrated   with   the   complete   headset.

Oxygen    (oronasal)   mask   mike   in   the   oxygen mask   .

 When   using   boom   or   oxygen   mikes   PTT   is   made   either   from   the   audio selector   panel   or   the   control   wheel    (for   pilot   &   copilot).

Loudspeakers

Pilot & Copilot   are   provided   with   loudspeakers   at the   cockpit ceiling   to permit   monitoring   of- radio   signals   without   wearing   head-phones.

Radio signals   can   be   heared   also   in   headphones   and   headsets. Other audio signals   can be   heared in the telephone headsets.