TM 1-1510-218-10 3-1 CHAPTER 3 AVIONICS (COMMON)  C  D  T1  T2 Section I.  GENERAL 3-1.  GENERAL DESCRIPTION. This    chapter    covers    the    avionics    equipment installed   common   to   the  C–12C,  C -12D1,   C-12D2,   C-12T1,   and  C-12T2      aircraft.      It   includes   a   brief description   of   the   avionics   equipment,  its  technical characteristics,  capabilities,  and  locations.    Avionics installed  in  the  C-12C  and  D1  aircraft  are  covered  in Chapter  3A.    Avionics  installed  in  the  C-12D2  aircraft are  covered  in  Chapter  3B.    Avionics  installed  in  C- 12T1 and T2 aircraft are covered in Chapter 3C. 3-2.  AVIONICS EQUIPMENT CONFIGURATION. The    avionics    configuration    of    the    aircraft    is comprised   of   three   groups   of   electronic   equipment.   The  communication  equipment  group  consists  of  the interphone, UHF command   C  D     ,  V/UHF  command   T   ,   and   HF   command   systems.      The   navigation equipment group provides the pilot and copilot with the instrumentation  required  to  establish  and  maintain  an accurate  flight  course  and  position  and  to  make  an approach on instruments under Instrument Meteorological    Conditions    (IMC).        The    navigation group   includes   equipment   for   determining   altitude, attitude,    position,    destination,    range    and    bearing, heading reference, groundspeed, and drift angle.  The transponder and radar group includes an identification capability, a position and emergency tracking system, and  a  radar  system  to  locate  potentially  dangerous weather areas.  A Ground Proximity Altitude Advisory System (GPAAS) is also installed. 3-3.  POWER SOURCE. a.     DC   Power.      DC   power   for   the   avionics equipment   is   provided  by  four  sources:  the  aircraft battery,  left  and  right  generators,  and  external  power.   Power is routed through a 50-ampere circuit breaker to the   avionics   power   relay   that   is   controlled   by   the AVIONICS MASTER POWER switch on the overhead control  panel,  Figure  2-15.    Individual  system  circuit breakers and the associated avionics buses are shown in Figures 2-27 and 2-28 and Tables 2-6, 2-7, 2-8, and 2-9.  With the switch in the  ON position, the avionics power   relay   is   de–energized   and   power   is   applied through both the  AVIONICS MASTER POWER #1 and #2  circuit  breakers  to  the  individual  avionics  circuit breakers on the overhead circuit breaker panel, Figure 2-16.    In  the  off  (aft)  position,  the  relay  is  energized and    power    is    removed    from    avionics    equipment.   When   external   power   is   applied   to   the   aircraft,   the avionics  power  relay  is  normally  energized,  removing power from the avionics equipment.  To apply external power to the avionics equipment, move the  AVIONICS  MASTER  POWER  switch  to  the   EXT  PWR  position.   This de–energizes the avionics power relay and allows power to be applied to avionics equipment. b.    AC Power.  Two inverters provide ac power for the avionics equipment.  The inverters supply 115- volt and 26-volt single-phase ac power when operated by the INVERTER #1 or  #2 switches.  Either inverter is capable  of  powering  all  avionics  equipment  requiring ac power.  115 Vac power from the inverters is routed through  fuses  and  transformers  in  the  nose  avionics compartment.    The  transformers  provide  the  required 26 Vac needed by avionics equipment. Section II.  COMMUNICATIONS 3-4.  COMMUNICATIONS DESCRIPTION. The   common   avionics   in   the   communications equipment group consists of the microphone switches, microphone jacks, and headset jacks. 3-5.  MICROPHONE SWITCHES, MICROPHONE JACKS, AND HEADSET JACKS. a.     Microphone Switches.  A bi-level microphone switch, placarded   INTPH  XMIT  /  MIC, is located   on   the   pilot's   and   copilot's   control   wheels, Figure 2-22, and copilot's floor. (1)   MIC    INTPH    /    XMIT     Switch.        Keys selected facility. (a)  INTPH.  When pressed to first detent, keys    interphone    facility    regardless    of    position    of transmitter selector switch. (b)  XMIT.        When    pressed    fully,    keys facility indicated on transmitter selector switch.