TM 55-1510-219-10 2-65.  SUN VISORS. CAUTION When   adjusting   the   sun   visors, grasp    only    by    the    top    metal attachment   to   avoid   damage   to the plastic shield. A    sun    visor    is    provided    for    the    pilot    and    copilot respectively  (fig.    2-9).    Each  visor  is  manually  adjust able.  When not needed as a sun shield, each visor may be  manually  rotated  to  a  position  flush  with  the  top  of the cockpit so that it does not obstruct view through the windows. 2-66.  RELIEF TUBES. One  relief  tube  is  provided,  located  immediately  aft  of the    cabin    door,    on    the    left    side    of    the    fuselage. Section VIII.  HEATING, VENTILATION, COOLING, AND ENVIRONMENTAL CONTROL SYSTEM 2-67.  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 (fig.  2-21).  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. (1) Bleed airflow 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.  2-18)  and  a  normally  open solenoid  valve  operated  by  the  left  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  device  systems.    These  valves  are  controlled  by the  bleed  air  valve  switches  located  on  the  overhead control panel (fig.  2-18). (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,  2-18).    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, 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    both    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 re 2-53