TM 55-1510-220-10 tion,  but  is  used  to  reduce  the  possibility  of  power  loss due   to   icing   or   other   conditions.      Each   engine   has   a separate autoignition control switch and a green indicator light  placarded  No.1  IGN  ON  or  No.2  IGN  ON,  on  the annunciator panel. Autoignition is accomplished-by energizing the two igniter plugs in each engine. NOTE The  system  should  be  turned  OFF during  extended  ground  operation to   prolong   the   life   of   the   igniter plugs. a. Autoignition  Switches.    Two  switches,  located  on the  overhead  control  panel  (fig.    2-12),  placarded  ENG AUTO  IGN-ARM  control  the  autoignition  systems.    The ARM     position     initiates     a     readiness     mode     for     the autoignition   system   of   the   corresponding   engine.      The OFF    position    disarms    the    system.        Each    switch    is protected  by  a  corresponding  START  CONTR  No.1  or No.2   5ampere   circuit   breaker   on   the   overhead   circuit breaker panel (fig.  2-27). b. Autoignition   Lights.      If   an   armed   autoignition system  changes  from  a  ready  condition  to  an  operating condition  (energizing  the  igniter  plugs  in  the  engine)  a corresponding green annunciator panel light will illuminate.  The annunciator panel light is placarded No.1 IGN ON  or No.2 IGN ON  and indicates that the igniters are energized.  The autoignition system is triggered from a  ready  condition  to  an  operating  condition  when  engine torque drops below approximately 20%.  Therefore, when an   autoignition   system   is   armed,   the   igniters   will   be energized continuously during the time when an engine is operating   at   a   level   below   approximately   20%   torque. The     autoignition     lights     are     protected     by     5-ampere IGNITOR  CONTR  No.1  or  No.2  circuit  breakers,  located on the overhead circuit breaker panel (fig.  2-27). 2-32.  ENGINE STARTER-GENERATORS. One  starter-generator  is  mounted  on  each  engine accessory drive section.  Each is able to function either as a  starter  or  as  a  generator.    In  the  starter  function,  24 volts  DC  is  required  to  power  rotation.    In  the  generator function, each unit is capable of 400 amperes DC output. When the starting function is selected, the starter control circuit  receives  power,  through  the  respective  5-ampere START  CONTR  circuit  breaker  on  the  overhead  circuit breaker   panel,   from   either   the   aircraft   battery   or   an external  power  source.    When  the  generating  function  is selected,  the  starter-generator  provides  electrical  power. An   automatic   starter   cutoff   control   is   installed   in   each starter-generator  to  provide  automatic  termination  of  the start  cycle  when  engine  speed  reaches  44%  NI.    Engine speed   is   sensed   by   a   magnetic   sensor   located   in   the starter-generator.  For additional description of the starter- generator system, refer to Section IX. 2-33.  ENGINE INSTRUMENTS. The engine instruments are vertically mounted near the center of the instrument panel (fig.  230). a. Turbine  Gas  Temperature  Indicators.    Two  TGT gages  on  the  instrument  panel  are  calibrated  in  degrees Celsius    (fig.        2-30).        Each    gage    is    connected    to thermocouple  probes  located  in  the  hot  gases  between the  turbine  wheels.    The  gages  register  the  temperature present    between    the    compressor    turbine    and    power turbine for the corresponding engine. b. Engine Torquemeters.    Two  torquemeters  on  the instrument  panel  indicate  torque  applied  to  the  propeller shafts  of  the  respective  engines  (fig.  2-30).    Each  gage shows   torque   in   percent   of   maximum   using   2   percent graduations and is actuated by an electrical signal from a pressure    sensing    system    located    in    the    respective propeller     reduction     gear     case.          Torquemeters     are protected   by   individual.      0.5-ampere   circuit   breakers placarded    TORQUE    METER    No.1    or    No.2    on    the overhead circuit breaker panel (fig.  2-27). c. Turbine  Tachometers.    Two  tachometers  on  the instrument  panel  register  compressor  turbine  RPM  (N1) for  the  respective  engine  (fig.    2-30).  These  indicators register  turbine  RPM  as  a  percentage  of  maximum  gas generator     RPM.     Each     instrument     is     slaved     to     a tachometer generator attached to the respective engine. d. Oil    Pressure/Oil    Temperature    Indicators.    Two gages on the instrument panel register oil pressure in PSI and oil temperature in °C (fig. 2-30).  Oil pressure is taken from the delivery side of the main oil pressure pump.  Oil temperature is transmitted by a thermal sensor unit which senses the temperature of the oil as it leaves the delivery side of the oil pressure pump.  Each gage is connected to pressure  transmitters  installed  on  the  respective  engine. Both    instruments    are    protected    by    5-ampere    circuit breakers,  placarded  OIL  PRESS  and  OIL  TEMP  No.1  or No.2, on the overhead circuit breaker panel (fig.  2-27). Change 2       2-27