TM 55-1510-220-10 Section VIII. HEATING, VENTILATION, COOLING, AND ENVIRONMENTAL CONTROL SYSTEM 2-68.  HEATING SYSTEM. a. Description.    Warm  air  for  heating  the  cockpit and mission avionics compartments and warm windshield  defrosting  air  is  provided  by  bleed  air  from both    engines.        Engine    bleed    air    is    combined    with ambient air in the heating and pressurization flow control unit in each nacelle.  If the mixed bleed air is too warm for cockpit comfort, it is cooled by being routed through an   air-to-air   heat   exchanger   located   in   the   forward portion  of  each  inboard  wing.    If  the  mixed  bleed  air  is not    too    warm,    the    air-to-air    heat    exchangers    are bypassed.      The   mixed   bleed   air   is   then   ducted   to   a mixing plenum, where it is mixed with cabin recirculated air.  The warm air is then ducted to the cockpit outlets, windshield  defroster  outlets,  and  to  the  floor  outlets  in the  mission  avionics  compartment.    The  environmental system is shown in figure 2-22. (1) Bleed  air  flow  control  unit.    A  bleed  air flow control unit, located forward of the firewall in each engine  nacelle  controls  the  flow  of  bleed  air  and  the mixing of ambient air to make up the total airflow to the cabin  for  heating,  windshield  defrosting,  pressurization and  ventilation.    The  unit  is  fully  pneumatic  except  for an    integral    electric    solenoid    firewall    shutoff    valve, controlled   by   the   bleed   air   switches   located   on   the overhead control panel (fig.  2-12) and a normally open solenoid valve operated by the right landing gear safety switch. (2) Pneumatic   bleed   air   shutoff   valve.      A pneumatic  shutoff  valve  is  provided  in  each  nacelle  to control the flow of bleed air to the surface, antenna and brake  deice  systems.    These  valves  are  controlled  by the  bleed  air  valve  switches  located  on  the  overhead control panel (fig.  2-12). (3) Bleed  air  valve  switches.    The  bleed  air flow  control  unit  shutoff  valve  and  pneumatic  bleed  air shutoff valves are controlled by two switches placarded BLEED   AIR   VALVE   OPEN   ENVIRO   OFF   PNEU   & ENVIRO  OFF,  located  on  the  overhead  control  panel (fig.    2-12).    When  set  to  the  open  position,  both  the environmental  flow  control  unit  shutoff  valve  and  the pneumatic   shutoff   valve   are   open;   when   set   to   the ENVIRO  OFF  position,  the  environmental  flow  control unit shutoff valve 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. (4) Cabin  temperature  mode  selector  switch. A  switch  placarded  CABIN  TEMP  MODE  MAN  COOL MAN   HEAT   OFF   AUTO   A/   C   COLD   OPN   -25°C   to 10°C,  located  on  the  overhead  control  panel,  controls cockpit  and  mission  avionics  compartment  heating  and air   conditioning.      When   the   cabin   temperature   mode selector switch is set to the AUTO position, the heating and air conditioning systems are automatically controlled.  Control signals from the temperature control box   are   transmitted   to   the   bleed   air   heat   exchanger bypass valves.  Here the temperature of the air flowing to    the    cabin    is    regulated    by    the    bypass    valves controlling    the    amount    of    air    bypassing    the    heat exchangers.        When    the    cabin    temperature    mode selector switch is set to the AUTO position, the heating and air conditioning systems are automatically controlled.      When   the   temperature   of   the   cabin   has reached the temperature setting of the cabin temperature control rheostat, the automatic temperature control allows hot air to bypass the air-to-air exchangers. When the bypass valves are in the fully closed position, allowing  no  air  to  bypass  the  heat  exchangers,  the  air conditoner     begins     to     operate,     providing     additional cooling.    When  the  cabin  temperature  mode  selector switch  is  set  to  the  A/C  COLD  OPN  position,  the  air conditioning   system   is   in   continuous   operation.      The cabin  temperature  control  rheostat,  in  conjunction  with the cabin temperature control sensor, provides regulation of cockpit and mission equipment compartment  temperature.    Bleed  air  heat  is  added  as required   to   maintain   the   temperature   selected   by   the cabin temperature control rheostat. (5) Cabin   temperature   control   rheostat.      A control knob placarded CABIN TEMP INCR, located on the    overhead    control    panel    (fig.        2-12),    provides regulation     of     cabin     temperature     when     the     cabin temperature mode selector switch is set to the AUTO or the  A/C  COLD  OPN  position.    A  temperature  sensing unit  in  the  cabin,  in  conjunction  with  the  setting  of  the cabin  temperature  control  rheostat,  initiates  a  heat  or cool   command   to   the   temperature   controller   for   the desired     cockpit     or     mission     avionics     compartment environment. (6) Manual   temperature   control   switch.      A switch placarded MANUAL TEMP INCR DECR, located on the overhead control panel (fig. 2-54 '