TM  55-1510-221-10 3.  Cabin  temperature  control  rheo- stat  -  As  required. 4.   Cabin,   cockpit   and   defrost   air knobs  -  As  required. (2.)  Manual  cooling  mode. 1. Bleed  air  valve  switches  -  OPEN, LEFT  and  RIGHT. NOTE For  maximum  cooling  on  the  ground,  set the  bleed  air  valve  switches  to  the ENVIRO  OFF  position. 2. (3.)  Air  conditioning  cold  operation  mode. (Used  if  ambient -25º  C). Cabin   temperature   mode   selector switch  -  MAN  COOL. temperature  is  between  10°C  and NOTE Setting  the  cabin  temperature  mode  selec- tor  switch  to  the  A/C  COLD  OPN  posi- tion  at  ambient  temperatures  below  -25°C may  cause  the  air  conditioning  system  to exceed  the  compressor  low  pressure  limit switch   setting, terminating    compressor operation,  and  thereby  rendering  the  sys- tem   inoperative   for   the   remainder   of   the flight. 1. Bleed  air  valve  switches  -  OPEN, LEFT  and  RIGHT. 2.   Cabin   temperature   mode   selector switch  -  A/C  COLD  OPN. 3.  Cabin  temperature  control  rheo- stat  -  As  required. 4.   Cabin,   cockpit   and   defrost   air knobs  -  As  required. 2-71.  UNPRESSURIZED  VENTILATION. Ventilation   is   provided   by   two   sources.   One source   is   through   the   bleed   air   heating   system   in both   the   pressurized   and   unpressurized   mode.   The second   source   of   ventilation   is   obtained   from   ram air  through  the  condenser  section  in  the  nose through  a  check  valve  in  the  vent  blower  plenum. Ventilation  from  this  source  is  in  the  unpressurized mode  only  with  the  CABIN  PRESS  DUMP  switch in  the  DUMP  position.  The  check  valve  closes  dur- ing  pressurized  operation.  Ram  air  ventilation  is  dis- tributed  through  the  main  ducting  system  to  all  out- lets.  Ventilation  air,  ducted  to  each  individual  eye- ball   cold   air   outlet,   can   be   directionally   controlled by  moving  the  ball  in  the  socket.  Volume  is  regu- lated  by  twisting  the  outlet  to  open  or  close  the valve. 2-72.  ENVIRONMENTAL  CONTROLS. An   environmental   control   section   on   the   over- head  control  panel  (fig.  2-12)  provides  for  automatic or  manual  control  of  the  system.  This  section  con- tains  all  the  major  controls  of  the  environmental function   including   bleed   air   valve   switches,   a   vent blower  control  switch,  an  aft  vent  blower  switch,  a manual   temperature   switch   for   control   of   the   heat exchanger  valves,  a  cabin  temperature  level  control, and  the  cabin  temp  mode  selector  switch  for  select- ing  automatic  heating  or  cooling  or  manual  heating or   cooling.   Four   additional   manual   controls   on   the main  instrument  subpanels  may  be  utilized  for  par- tial  regulation  of  cockpit  comfort  when  the  cockpit partition  door  is  closed  and  the  cabin  comfort  level is  satisfactory. a.   Heating   Mode. (1.) If the cockpit is too cold: 1. Pilot  and  copilot  air  knobs  -  As required. 2. Defrost  air  knob  -  As  required. 3. Cabin  air  knob  -  Pull  out  in  small increments.  Allow  3  -5  minutes after   each   adjustment   for   system to  stabilize. (2.) If the cockpit is too hot: 1. Cabin  air  knob  -  As  required. 2. Pilot  and  copilot  air  knobs  -  In  as required. 3. Defrost  air  knob  -  In  as  required. b.  Cooling  Mode: (1.) If the cockpit is too cold: 1. Pilot  and  copilot  air  knob  -  In  as required. 2. Defrost  air  knob  -  In  as  required. 3.    Overhead  cockpit  outlets  -  As required. (2.) If the cockpit is too hot: 1. Pilot  and  copilot  air  knobs  -  Out as   required. 2-56