TM 55-1510-220-10 hold the switch at the INCR position, to decrease cabin temperature,   hold   the   switch   in   the   DECR   position. Allow  approximately  30  seconds  per  valve  to  drive  the bypass valves to the fully open or fully closed position. Only one valve moves at a time. (2) With     the     cabin     temperature     selector switch    in    the    MAN    COOI,    position,    the    automatic temperature control system is bypassed.  In the manual cooling  mode,  the  refrigeration  system  is  on,  providing the right engine NI speed is above 65'SY, however, the bypass   valves   may   be   manually   positioned   for   the desired  temperature.    Hold  the  MANUAL  TEMP  switch in the DECR position approximately one minute to fully close air-to-air heat exchanger bypass valves. e. Bleed Air and Vent Control. (1) Bleed  air  entering  the  cabin  is  controlled by   bleed   air   valve   switches   placarded   BLEED   AIR VALVE  OPEN  ENVIRO  OFF  PNEU  &  ENVIRO  OFF. When    the    switch    is    in    the    OPEN    position,    the environmental flow control unit and the pneumatic valve are  open.    When  the  switch  is  in  the  ENVIRO  OFF position,  the  environmental  flow  control  unit  is  closed and the  pneumatic  bleed  air  valve  is  open;  in  the  PNEU  & ENVIRO OFF position, both are closed.  For maximum cooling on the ground, turn the bleed air valve switches to the ENVIRO OFF position. (2) The forward vent blower is controlled by a switch placarded VENT BLOWER AUTO LOW HI.  The HI and LOW positions regulate the blower to two speeds of  operation.    IN  the  AUTO  position,  the  fan  will  run  at low speed except when the CABIN TEMP mode selector switch   is   placed   in   the   OFF   position.      In   the   OFF position, the blower will not operate. (3) Aft   vent   blower   switch.      The   aft   vent blower  is  controlled  by  a  switch  placarded  AFT  VENT BLOWER   OFF   AUTO   ON,   located   on   the   overhead control   panel   (fig.      2-12).      The   single   speed   blower operates  automatically  through  the  cabin  temperature mode selector switch when the aft vent blower switch is placed  in  the  AUTO  position  during  ground  operation and  the  air  conditioner  is  operating.    The  blower  runs continuously   when   the   switch   is   placed   in   the   ON position.      In   the   OFF   position,   the   blower   will   not operate. Section IX.  ELECTRICAL POWER SUPPLY AND DISTRIBUTION SYSTEM 2-72.  DESCRIPTION. The aircraft employs both direct current (DC) and alternating   current   (AC)   electrical   power.      The   DC electrical  power  supply  (fig.    2-23)  is  the  basic  power system    energizing    most    aircraft    circuits.        Electrical power is used to start the engines, to power the landing gear  and  flap  motors,  and  to  operate  the  standby  fuel pumps, ventilation blower, lights and electronic equipment.      AC   power   is   obtained   from   DC   power through inverters.  The single phase AC power system is shown  in  figure  2-24,  and  the  three  phase  AC  power system  is  shown  in  figure  2-25.    The  three  sources  of DC power consist of one 20 cell 34-ampere/hour battery and two 400-ampere starter-generators.  DC power may be   applied   to   the   aircraft   through   an   external   power receptacle  on  the  right  nacelle  (refer  to  Section  XII  for GPU requirements). The 3tarter-generators are controlled   by   generator   control   units.      The   output   of each    generator    passes    /    through    a    cable    to    the respective   generator   bus   (fig.      2-23).      Other   buses distribute  power  to  aircraft  DC  loads,  and  derive  power from     the     generator     buses.          The     generators     are paralleled   to   balance   the   DC   loads   between   the   two units.    When  one  of  the  generating  systems  is  not  on line, if no fault exists,  aircraft  DC  power  requirements  continue  to  be supplied  from  one  or  the  other  generating  source,  but not    from    both.        Most    DC    distribution    buses    are connected  to  both  generator  buses  but  have  isolation diodes     to     prevent     power     crossfeed     between     the generating    systems,    when    connection    between    the generator buses is lost.  Thus, when either generator is lost  because  of  a  ground  fault,  the  operating  generator will  supply  power  for  all  aircraft  DC  loads  except  those receiving   power   from   the   inoperative   generator's   bus which  cannot  be  crossfed.    When  a  generator  is  not operating, reverse current and over-voltage protection is automatically  provided.    Two  inverters  operating  from DC power produce the  required single-phase AC power. Three  phase  AC  electrical  power  for  inertial  navigation system   and   mission   avionics   is   supplied   by   two   DC powered  mission  inverters.    AC  power  may  be  applied through an external power receptacle located on the left nacelle.  The mission power system is shown in figure 2- 26. 2-73.  DC POWER SUPPLY. One  nickel-cadmium  battery  furnishes  DC  power when the engines are not operating.  This Change 2   2-59