TM 55-1510-220-10 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.      2-12)   provides   for   automatic   or manual control of the system.  This section contains 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 selecting automatic heating or cooling or  manual  heating  or  cooling.    Four  additional  manual controls   on   the   main   instrument   subpanels   may   be utilized for partial 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. 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 systems are  automatically  controlled.    When  the  temperature  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.    The  warm  bleed  air  is  mixed  with  the  cooled air.  The rear evaporator picks up recirculated cabin air only. (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   300   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,  initiates  a  heat  or cool command to the temperature controller for desired cockpit and cabin environment. d. Manual     Mode     Control.          With     the     cabin temperature  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  automatic system  is  overridden  and  the  system  is  controlled  by opening  and  closing  the  bypass  valves  (two)  with  the MANUAI, TEMP INCR DECR switch.  To increase cabin temperature, 2-58