CABIN  TEMPERATURE  CONTROL SUBSYSTEM A  cabin  air  temperature  control  sensor  is located mid cabin adjacent to the TACCO cooling air inlet. Its purpose is to ensure adequate airflow over the sensor, which measures cabin tempera- ture and sends a signal, along with a signal from the  cabin  air  temperature  selector,  to  the  cabin air temperature control. The cabin air temperature control  then  directs  the  cabin  temperature  control modulating  valve  to  maintain  a  selected  tempera- ture in the cabin. During normal cruise, cabin air temperature  is  controlled  by  mixing  water  separa- tor  cold  air  with  hot  bleed  air.  The  control  also acts as an anticipator to stabilize response from the supply duct sensor to the cabin temperature demand. System Operation The   cabin   temperature   control   subsystem cools  the  bleed-air  supply  by  air-cycle  refrigeration and ram air mixing to provide a cabin temperature within the range of 60° to 80°F during steady and mild transient conditions. The cabin temperature is maintained within ±3°F of the selected value and  a  temperature  differential  of  10°F  between the   floor   level   and   the   head   level.   Humidity control  ranges  from  a  relative  humidity  of  5  to 70 percent. The cabin exhaust air, after passing through  the  internal  avionics  and  the  sonobuoy and  weapons  bays,  is  exhausted  overboard. Cabin air temperature is monitored by a sensor mounted on the aisle next to the cooling air inlet at the TACCO side console. The sensor measures the flight station air temperature and generates a  signal  that  is  transmitted  to  the  cabin  air temperature control. Additionally, a signal from the temperature select switch is sent to the control. The  cabin  air  temperature  control  senses  the  inlet duct  temperature  and  compares  the  signals  to modulate  the  cabin  temperature  control  valve. Based  on  this  comparison,  it  allows  the  proper amount of hot bleed air to enter the mixing muff at  the  conditioned  air  outlet.  The  cabin  air temperature  control  acts  as  an  anticipator  to stabilize  the  response  of  the  supply  duct  air temperature  to  cabin  temperature  demands.  It also  minimizes  cabin  air  supply  duct  temperature changes  because  of  bleed-  or  ram-air  temperature change.  (See  figure  4-9.) In the manual mode, the automatic controls are overridden to provide manual control of the cabin temperature control valve. Since the cabin air   temperature   control   is   bypassed,   the 160°±5°F   limit   on   the   cabin   air   temperature control is raised to 185°±15°F, as sensed by the cabin air high-temperature limit thermostat. If the pilot has selected the temperature select switch position  for  which  this  185°±15°F  is  exceeded, the cabin temperature control subsystem will cycle open  and  closed  until  manual  control  is repositioned  or  conditions  change  to  reduce maximum  supply  temperature. The augmented air system provides ram air, as required, to supplement the conditioned bleed air and to provide auxiliary ventilation. This ram air  is  drawn  from  the  ram-air  scoop  located  in  the base  of  the  vertical  stabilizer.  The  ram  air  is injected  into  the  cabin  air  distribution  ducting downstream from the mixing muff at the junction between the water separator discharge air and the cabin  temperature  control  valve. During unpressurized flight up to 3,500 feet (+1000 or –500 feet) with a ram air temperature between     20°±6°F     and     72°±6°F,     ram     air supplements  the  conditioned  bleed-air  flow  to  the cabin.  When  operating  in  the  automatic  mode, the  ram-air  shutoff  valve  controls  the  duct-to- cabin  pressure  differential  to  7.5  ±  2  inches  of water to prevent flooding the cabin with ram air when the aircraft is flying at high speeds. The  ram-air  shutoff  valve  is  also  used  to provide   auxiliary   ventilation   by   securing   the refrigeration  package  and  relying  on  the  pilot- operated   auxiliary   vent   switch   to   adjust   the ram-air shutoff valve. (See figure 4-10.) With the air-conditioning  switch  set  to  OFF,  setting  the auxiliary  vent  switch  to  ON  closes  the  cabin recirculating  air  temperature  control  valve  and opens the cabin pressure regulator valve. If the setting of the ram-air shutoff valve is such that ram  pressure  fails  to  satisfy  cabin  exhaust  fan requirements, the negative pressure relief valve opens. This draws additional ambient air from the environmental  control  system  compartment  to compensate  for  any  airflow  deficiencies. In the event of an automatic shutdown of the air-conditioning or pressurization system during single-engine   waveoff,   the   cabin   air   supply temperature  may  change  because  the  ram-air shutoff  valve  opens.  Operation  is  restored  by setting   the   air-conditioning   switch   to OFF/RESET and then back to ON, or by setting the  auxiliary  vent  switch  to  modulate  the  ram airflow. 4-14