TM 55-1510-222-10 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 equipment compartment. The environmental system is shown in figure 2-24. (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 electronically controlled    with    an    integral    electric    solenoid    firewall shutoff   valve,   controlled   by   the   bleed   air   switches located  in  the  overhead  control  panel  (fig.  2-13)  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 engine  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 in the overhead control panel (fig. 2-13). (3) Bleed  air  valve  switches.    The bleed  air  flow  control  unit  shutoff  valve  and  pneumatic bleed air shutoff valves are controlled by two LEFT and RIGHT  switches  placarded  ENVIRO  &  PNEU  BLEED AIR  PNEU  ONLY  ON,  located  in  the  overhead  control panel (fig. 2-13).  When set to the ON position, both the environmental  flow  control  unit  shutoff  valve  and  the pneumatic   shutoff   valve   are   open;   when   set   to   the PNEU  ONLY  position,  the  environmental  flow  control unit shutoff valve is closed, and the pneumatic bleed air valve is open; in the ENVIRO & PNEU position, both are closed.    For  maximum  cooling  on  the  ground,  turn  the bleed air valve switches to the PNEU ONLY position. (4) Cabin temperature mode selector  switch.  A  switch  placarded  CABIN  AIR  MODE SELECT OFF AUTO AC COLD OPN 10°  to -25°  C MAN COOL-MAN   HEAT   located   in   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 temperature of the cabin air, 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 admitting   hot   air   into   the   cabin.      When   the   bypass valves are in the fully closed position, allowing no air to bypass  the  heat  exchangers,  the  air  conditioner  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  AIR  TEMP CONTROL INCR located in the overhead control panel (fig.   2-13),   provides   regulation   of   cabin   temperature when the cabin temperature mode selector switch is set to   the   AUTO   or   the   AC   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 equipment compartment environment. (6) Manual temperature control switch.  A switch placarded CABIN AIR MANUAL TEMP DECREASE INCREASE located in the overhead control panel (fig. 2-13), controls cockpit and mission equipment   compartment   temperature   with   the   cabin temperature mode selector switch set to the MAN HEAT or   MAN   COOL   position.      The   manual   temperature control     switch     controls     the     cockpit     and     avionics equipment    temperature    by    providing    a    means    of manually changing the amount that the bleed air bypass valves    are--opened    or    closed.        To    increase    cabin temperature, the switch is held to the INCR position.  To decrease  cabin  temperature,  the  switch  is  held  to  the DECR  position.    Approximately  30  seconds,  per  valve, is  required  to  drive  the  bypass  valves  to  the  fully  open or  fully  closed  position.    Only  one  valve  moves  at  a time. (7) Forward    vent    blower    switch. The   forward   vent   blower   is   controlled   by   the   switch placarded  VENT  BLOWER  FWD  AUTO  LOW  HIGH, located in the overhead control panel (fig. 2-13).  In 2-58