TM 1-1510-224-10 Section VIII.  HEATING, VENTILATION, COOLING, AND ENVIRONMENTAL CONTROL SYSTEM 2-68.  HEATING SYSTEM. Warm    air    for    heating    the    cockpit    and    mission equipment compartment and for defrosting the windshield  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 engine  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    floor    outlets    in    the    mission equipment  compartment.    The  environmental  system  is shown in figure 2-28. a. 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   on   the overhead control panel (fig.  2-15), and a normally open solenoid  valve  operated  by  the  right  landing  gear  safety switch. b. 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 on the overhead control panel (fig.  2-15). c. 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  on  the  overhead  control  panel  (fig.  2-15). 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. d. Cabin  Temperature  Mode  Selector  Switch.       A switch placarded  CABIN AIR MODE SELECT - OFF - AUTO - AC COLD OPN 10°C to -25°C - MAN COOL - MAN   HEAT,   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 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. e. Cabin Temperature Control Rheostat.  A control knob placarded   CABIN  AIR  TEMP  CONTROL  -  INCR, located    on    the    overhead    control    panel    (fig.        2-15), 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. f. Manual  Temperature  Control  Switch.    A  switch placarded  CABIN  AIR  MANUAL  TEMP  -  DECREASE  - INCREASE, located on the overhead control panel (fig.      2-15),   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.        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. 2-67