cooling. At MODE AUTO, the modulating valve is turned on and the cabin temperature is set by the TEMP knob. When at FLOW HIGH, the ECS will operate at high volume from the air-cycle machine. HIGH is used primarily for cooling. Cabin temperature, duct temperature, and the setting of the TEMP knob are all compared in the cabin temperature controller. The MAN position bypasses the cabin and duct control valve by a spring-loaded valve to center HOT-COLD switch. Placing the momentary beep switch to HOT or COLD causes the control valve to move toward open or close as long as the switch is held, allowing manual temperature control. NOTE: Use of the manual mode of the ECS requires pulsing of the HOT-COLD toggle switch followed by a waiting period to judge the magnitude of temperature change. Excessive manual input may cause ECS shutdown and/or APU failure (if APU is the air source). The ECS will automatically shut down under the following conditions: 1. Engine contingency power is selected with the contingency-power switch (CNTGY PWR) on either collective stick. 2. In any position of the AIR SOURCE ECS/START switch, when starting either No. 1 or No. 2 engine. When the AIR SOURCE ECS/START switch is placed to ENGINE, the ECS will also shut down when 1. a turbine gas temperature (TGT) of 856 ± 5°C is reached, 2. either engine ANTI-ICE switch is placed O N ,  o r 3. when the DE-ICE MASTER switch is placed to AUTO and ice is detected. AVIONICS COOLING The total aircraft avionics system requires the dissipation of approximately 12 kilowatts of heat. Units cooled by the external air system are main- tained at 15° to 27°C. Units cooled by ambient cabin air require an ambient temperature below 29°C. Two fans provide cooling air for the mission avionics. One fan is located on the right side of the cabin at the base of the mission avionics rack, and the other is located on the left side of the cabin at the base of the sensor operator’s (SO’s) console. Fan control is provided by the mission power (MSN PWR) switch, located on the lower center console on the MSM SYS (mission systems) panel, and by a 27°C temperature-sensing switch, located at each fan inlet. When the MSN PWR switch is placed in either PRI or SEC position and the fan inlet temperature is above 27°C, the fans run to bring in outside air for circulation through the respective avionics areas. Backup cooling for the avionics is provided by the ECS. If the ECS is operating, the modulating valve will automatically go to the full-open position when the temperature switches at the fan inlets sense a temperature of 54°C or greater. Conditioned cabin air may be circulated through the avionics system by removing the thermal/acoustic panels for backup cooling. Power is supplied from the No. 1 ac primary bus and No. 2 ac primary bus through the (SO) circuit breaker panel by two circuit breakers, marked LH RACT BLOWER and BLOWER, RH RACK. AIRCRAFT PRESSURIZATION SYSTEMS Learning Objective: Recognize the purpose and function of aircraft pressurization systems to include maintenance and troubleshooting operations. As aircraft became capable of obtaining altitudes above that at which flight crews could operate efficiently, a need developed for complete environmental systems. Air conditioning could provide the proper temperature and supplemental oxygen could provide sufficient breathable air. The one problem was that not enough atmospheric pressure exists at high altitude to aid in breathing, and even at lower altitudes the body must work harder to absorb sufficient oxygen through the lungs to operate at the same level of efficiency as at sea level. This problem was solved by pressuring the cockpit/cabin area. PRESSURIZATION SYSTEM The area of an aircraft to be pressurized must be free from all air leaks. This is accomplished by the use of seals around tubing, ducting, bolts, rivets, and other hardware that pass through or pierce the pressuretight area. All panels and large structural components are assembled with 3-30