TM 1-1510-224-10 OPN  position   during   ground   operation,   the   condenser blower   will   be   off   initially,   and   will   remain   off   until compressor   discharge   pressure   equals   the   pressure setting  of  the  blower  control  high  pressure  switch.    The blower   will   remain   in   operation   until   system   pressure drops   to   equal   the   setting   of   the   blower   control   low pressure switch. (6) Air    conditioning    cold    operation    bypass valve.      Selecting   the    AC   COLD   OPN   mode   on   the CABIN  AIR  MODE  SELECT  switch  (fig.    2-15)  permits the operation of the air conditioning system by overriding the  refrigerant  low  pressure  switch.    This  allows  the  air conditioning system to operate in the manual mode.  The operational  life  of  the  compressor  will  be  decreased  by five   hours   each   time   the   air   conditioning   system   is started below 10°C using this mode (AC COLD OPN).  If the  air  conditioning  system  has  been  operating  in  the normal    mode    during    flight,    and    decreasing    ambient temperatures make it necessary to switch to the AC cold operation   mode,   there   will   be   no   degradation   in   the mean time between failure for the compressor. (7) Air   conditioner   cold   operation.      The   air conditioner  cold  operation  mode  is  provided  to  allow  the air conditioner to operate in the event of a hot cabin and an outside air temperature of less than 10C.  At an OAT of    less    than    10C,    a    green    advisory    annunciator placarded AC COLD OPN, located on the caution/advisory  annunciator  panel  will  illuminate.    This advises  the  crew  that  if  air  conditioning  is  required,  the CABIN  AIR  MODE  SELECT  switch  must  be  in  the   AC COLD  OPN  position.    This  causes  the  air  conditioner compressor   to   run   continuously   and   the   temperature controller to operate the bypass valve. b. Normal Operation. (1) Automatic cooling mode. 1. BLEED  AIR    valve  switches  -   ON, LEFT and  RIGHT. 2. CABIN  AIR  MODE  SELECT  switch - AUTO. 3. CABIN AIR TEMP CONTROL rheostat - As required. 4. CABIN, PILOT, COPILOT, and DEFROST AIR knobs - As required. (2) Manual cooling mode. 1. BLEED    AIR    valve    switches    ON, LEFT and  RIGHT. NOTE For  maximum  cooling  on  the  ground,  set  the BLEED  AIR  valve  switches  to  the  PNEU  ONLY position. 2. CABIN  AIR  MODE  SELECT  switch - MAN COOL. (3) Air     conditioning     cold     operation     mode. (Used   if   ambient   temperature   is   between   10°C   and  - 25C.) NOTE Setting  the  cabin  temperature  mode  selector switch    to    the    AC    COLD    OPN    position    at ambient  temperatures  below  -25°C  may  cause the   air   conditioning   system   to   exceed   the compressor low pressure limit setting, terminating compressor operation and thereby rendering    the    system    inoperative    for    the remainder of the flight. 1. BLEED   AIR   valve   switches   -   ON, LEFT and  RIGHT. 2. CABIN  AIR  MODE  SELECT  switch - AC COLD OPN. 3. CABIN AIR TEMP CONTROL rheostat - As required. 4. CABIN, PILOT, COPILOT, and DEFROST air knobs - As required. 2-70.  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   which enters  the  condenser  section  in  the  nose  and  passes through  a  check  valve  in  the  vent  blower  plenum.    The check valve closes during pressurized operation. Ventilation from this source is in the unpressurized mode only, with the CABIN PRESS DUMP switch in the  DUMP position.      The   check   valve   closes   during   pressurized operation.    Ram  air  ventilation  is  distributed  through  the main ducting system to all outlets.  Ventilation air, ducted to    each    individual    eyeball    cold    air    outlet,    can    be directionally  controlled  by  moving  the  ball  in  the  socket. Volume   is   regulated   by   twisting   the   outlet   to   open   or close the valve. 2-71.  ENVIRONMENTAL CONTROLS. An  environmental  control  section  on  the  overhead control    panel    (fig.        2-15)    provides    for    automatic    or manual  control  of  the  system.    This  section  contains  all the     major     controls     of     the     environmental     system, including  bleed  air  valve  switches,  forward  and  aft  vent blower  switches,  manual  temperature  switch  for  control of  the  heat  exchanger  valves,  a  cabin  temperature  level control, and the cabin temperature mode selector switch for    selecting    automatic    heating/    cooling    or    manual heating/cooling. a. Heating Mode. (1) If the cockpit is too cold: 2-70