TM 1-1510-225-10 3A-12 4.   Number   2  key   –  Press  to  delete the selected channel. (3)   Shutdown Procedure. 1.   On/off knob  – Press.  The off page will appear and flash to warn the operator that the system is turning off. 2.   After  4  seconds,  the  system  will  turn off. 3A-8.  VHF COMMUNICATIONS TRANSCEIVER. Refer to Chapter 3, Paragraph 3-4. 3A-9.  HF COMMUNICATIONS TRANSCEIVER. Refer to Chapter 3, Paragraph 3-3. 3A-10.  AIRBORNE TELEPHONE SYSTEM. The   airborne   telephone   system   consists   of   a remote   transceiver,   a   telephone   base   and   handset located    in    the    passenger    compartment,    and    an antenna. The   transceiver   operates   in   the  UHF  band  at frequencies   of   454.675   to   454.975   MHz   (receiver section) and 459.675 to 459.975 megahertz (transmitter  section).    Transmitter  power  output  is  a nominal  10  watts.    The  maximum  operating  altitude  is a    nominal    51,000    feet.        Although    the    theoretical maximum  range  at  31,000  feet  altitude  is  220  nautical miles,  the  range  is  essentially  limited  to  line-of-sight, and  may  be  reduced  depending  upon  the  altitude  of the  aircraft,  weather,  type  of  terrain,  and  the  location and  altitude  of  the  ground  transmitter.  The system is protected    by    a    5-ampere    RADIO    PHONE    circuit breaker  located  on  the  right  sidewall  circuit  breaker panel, Figure 2-6. a.   Airborne Telephone System Controls, Indicators, and Functions.   Refer to Figure 3A-4. (1)   Telephone   Base   Unit.        The    following items are located on the telephone base unit. (a)   Hookswitch     –     When     pressed     by placing   handset   in   cradle,   puts   system   in   standby mode and deactivates transmitter. (b)   Intercom  Lamp  (IC)  –  While handset is in cradle, illuminates to indicate power is on.  When handset is removed from cradle, illuminates to indicate system is in intercom mode. (c)   Direct Dial Lamp (D/DlAL) – Illuminates  to  indicate  an  Air/Ground  Radiotelephone Automated  Service  (AGRAS)  station  with  direct  dial service has been selected. (d)   HF  /  BELL  OFF  /  PHONE Switch  – Selects   between   HF   and   PHONE   modes.      Center position selects ringer ON or OFF. (e)   Transmit  Lamp  (TX).    Illuminates to indicate transmitter is activated. (2)   Telephone  Handset.    The following items are located on the telephone handset. (a)   PTT Switch  –  When system is in HF mode,  it  functions  as  a  push-to-talk/release-to-receive switch.  When the handset is in cradle, it functions as a release button to unlock handset from base unit. (b)   Numerical   Keys   –   Press   to   select channels and telephone numbers. (c)   Enter    (#)    Key     –    When    pressed, immediately   enters   a   channel   number   or   telephone number. (d)   Clear    (Q )    Key    –    When    pressed, clears input in progress without entering. (e)   Hookswitch   –   When   pressed,   puts system in standby mode and deactivates transmitter. (f)    Volume  Control   –   Rotates  to  adjust volume of handset speaker. b.  Airborne    Telephone    System    Operation.   The airborne telephone system may be used either in HF mode or in PHONE mode.  The HF mode permits the  handset  to  be  used  as  a  microphone/speaker  in conjunction  with  the  aircraft's  HF  communications  set to provide standard two-way radio communications in the HF band. The PHONE mode provides communications very similar to ground-based telephone service, operating on 13 channels (12 telephone   channels   plus   1   ground-to-air   calling channel). Telephone    calls    may    be    placed    either    with assistance   from   an   operator   at   a   manned   ground station    or    direct    dialed    through    AGRAS    stations.   When  the  handset  is  removed  from  the  base,  the  set will    automatically    scan    each    channel    for    ground stations in range of the aircraft and will select one in the following order of preference. 1.   Idle AGRAS stations.