TM  55-1510-221-10 2-l  2)  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  over- head  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  over- head   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  condi- tioning   systems   are   automatically   controlled.   Con- trol  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  control- ling  the  amount  of  air  bypassing  the  heat  exchang- ers.  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 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  condi- tioning  system  is  in  continuous  operation.  The  cabin temperature   control   rheostat,   in   conjunction   with the  cabin  temperature  control  sensor,  provides  regu- lation   of   cockpit   and   mission   equipment   compart- ment  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, 2-54 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  tempera- ture  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. 2-12),   controls   cockpit   and   mission   avionics   com- partment   temperature   with   the   cabin   temperature mode  selector  switch  set  to  the  MAN  HEAT  posi- tions.   The   manual   temperature   control   switch   con- trols   cockpit   and   mission   avionics   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  tem- perature,  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  for- ward  vent  blower  is  controlled  by  a  switch  placarded VENT  BLOWER  -  AUTO  -  LO  -  HI,  located  on  the overhead  control  panel  (fig.  2-12).  In  the  auto  posi- tion  the  fan  will  run  at  low  speed  except  when  the cabin  temperature  mode  selector  switch  is  set  to  the OFF  position,  in  this  case  the  blower  will  not  oper- ate. (8.)   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-l  2).  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. The  blower  runs  continuously  when  the  switch  is placed  in  the  ON  position,  In  the  OFF  position,  the blower  will  not  operate. b.   Automatic   Heating   Mode. 1. Bleed  air  valve  switches  -  OPEN,  LEFT  and RIGHT. 2.  Cabin  temperature  mode  selector  switch  - AUTO. 3.   Cabin   temperature   control   rheostat   -   As required. 4.  Cabin,  cockpit  and  defrost  air  knobs  -  As required