TM  55-1510-221-10 2. Cabin   air   knob.   Close   in   small increments.   Allow   3   -   5   minutes after   each   adjustment   for   system to  stabilize.  If  CABIN  AIR  knob is completely closed  before obtaining satisfactory cockpit comfort,  it  may  be  necessary  to place  the  aft  vent  blower  switch  in the   ON   position   to   activate   the aft  evaporator  to  recirculate  cabin air. c. Automatic  Mode  Control.  When  the  AUTO mode   is   selected   on   the   cabin   temperature   mode selector  switch,  the  heating  and  air  conditioning  sys- tems  are  automatically  controlled.  When  the  temper- ature  of  the  cabin  has  reached  the  selected  setting, the  automatic  temperature  control  allows  heated  air to  bypass  the  air-to-air  exchangers  in  the  wing  center section.  By-pass  air  is  hot.  Heat  exchanger  air  is cooled   to   approximately   30°F   above   ambient   (out- side)  air.  The  warm  bleed  air  is  mixed  with  the cooled  air.  The  rear  evaporator  picks  up  recirculated cabin  air  only. NOTE The  automatic  mode  control  works  as described,  when  the  cabin  is  being  cooled by  bleed  air.  However,  when  the  cabin  is heated  with  bleed  air  and  the  selected temperature  is  reached,  hot  bleed  air routes  through  the  heat  exchanger  for cooling   in   order   to   maintain   the   desired temperature. (1.)  When   the   automatic   control   drives the  environmental  system  from  a  heat  mode  to  a cooling  mode,  the  bypass  valves  close.  When  the  left bypass   valve   reaches   a   fully   closed   position,   the refrigeration  system  will  begin  cooling,  provided  the right  engine  N₁   speed  is  above  65%.  When  the bypass  valve  is  opened  to  a  position  approximately 30º  from  full  open,  the  refrigeration  system  will  turn Off. (2.)  The  CABIN  TEMP  -  INCR  control provides   regulation   of   the   temperature   level   in   the automatic  mode.  A  temperature  sensing  unit  in  the cabin,  in  conjunction  with  the  control  setting,  initi- ates   a   heat   or   cool   command   to   the   temperature controller   for   desired   cockpit   and   cabin   environ- ment. d. Manual  Mode  Control.  With the cabin tem- perature  mode  selector  in  the  MAN  HEAT  or  MAN COOL  position,  regulation  of  the  cabin  temperature is   accomplished   manually   with   the   MANUAL TEMP  switch. (1.)  In  the  MAN  HEAT  mode,  the  auto- matic  system  is  overridden  and  the  system  is  con- trolled  by  opening  and  closing  the  bypass  valves (two)  with  the  MANUAL  TEMP  -  INCR  -  DECR switch.  To  increase  cabin  temperature,  hold  the switch  at  the  INCR  position,  to  decrease  cabin  tem- perature,   hold   the   switch   in   the   DECR   position. Allow  approximately  30  seconds  per  valve  to  drive the  bypass  valves  to  the  fully  open  or  fully  closed position.  Only  one  valve  moves  at  a  time. (2.)  With  the  cabin  temperature  selector switch  in  the  MAN  COOL  position,  the  automatic temperature   control   system   is   bypassed.   When   the left  bypass  valve  reaches  a  fully  closed  position,  the refrigeration  system  will  begin  cooling,  provided  the right  engine  N₁   speed  is  above  65%.  When  the bypass  valve  is  opened  to  a  position  approximately 30º  from  full  open,  the  refrigeration  system  will  turn off.  Hold  the  MANUAL  TEMP  switch  in  the  DECR position  approximately  one  minute  to  fully  close  air- to-air  heat  exchanger  bypass  valves. (3.)  Bleed   air   entering   the   cabin   is   con- trolled  by  bleed  air  valve  switches  placarded  BLEED AIR  VALVE  -  OPEN  -  ENVIRO  OFF  -PNEU  & ENVIRO  OFF.  When  the  switch  is  in  the  OPEN position,  the  environmental  flow  control  unit  and the  pneumatic  valve  are  open.  When  the  switch  is  in the  ENVIRO  OFF  position,  the  environmental  flow control   unit   is   closed   and   the   pneumatic   bleed   air valve  is  open;  in  the  PNEU  &  ENVIRO  OFF  posi- tion,  both  are  closed.  For  maximum  cooling  on  the ground,   turn   the   bleed   air   valve   switches   to   the ENVIRO  OFF  position. (4.) The  forward  vent  blower  is  controlled by  a  switch  placarded  VENT  BLOWER  -  AUTO  - LOW  -  HI.  The  HI  and  LOW  positions  regulate  the blower   to   two   speeds   of   operation.   IN   the   AUTO position,  the  fan  will  run  at  low  speed  except  when the  CABIN  TEMP  mode  selector  switch  is  placed  in the  OFF  position.  In  the  OFF  position,  the  blower will  not  operate. (5.) The  aft  vent  blower  is  controlled  by  a switch  placarded  AFT  VENT  BLOWER  -  OFF  - AUTO  -  ON.  The  single  speed  blower  operates  auto- matically  through  the  CABIN  TEMP  mode  selector 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. 2-57