the butterfly valve to maintain the required differential pressure. The cabin pressurization system also makes provision for dumping cabin pressure in an emergency. By setting the cabin pressure switch on the environmental control panel to the DUMP position, the latching solenoids on both the cabin outflow pressure regulating valve and on the cabin safety valve are actuated to the dump position. In addition, the recirculation air shutoff valve will be actuated to the full open position, provided electrical power is available. A secondary method of achieving cabin depressurization is to turn the air-conditioning switch to the OFF/RESET position and select the auxiliary vent mode. This selection will cause the cabin outflow pressure regulator valve to open, but it will not actuate the cabin safety valve to the open position. The repressurization mode of operation is used when returning to the normal mode from the dump mode or during a rapid descent in excess of 4,000 feet per minute. In this mode, the pressure regulator control modulates the rate of cabin repressurization with an integral isobaric and differential pressure control system. The pressure regulator control compares the existing cabin pressure to a lagging cabin pressure reference. If the result of this comparison exceeds the calibrated rate, control pressure output from the pressure regulator control is reduced. This causes the pressure regulator valve to sense a relatively higher pressure on the opening side of its actuating diaphragm, thus allowing the diaphragm to open the pressure regulator valve butterfly. This reduces cabin pressure and the rate of repressurization. Precautions for operating the S-3 cabin pres- surization subsystem on the ground, where the elevation is 5,000 feet or higher, are required because the cabin pressurization subsystem does not have provisions for automatic repressuriza- tion. Therefore, the cabin will be pressurized whenever the ground elevation is above 5,000 feet. To ensure adequate cooling of the internal avionics during operations at ground elevations above 5,000 feet, one of the following steps must be used: 1. Keep the cabin pressurized as in flight. 2. Set the cabin pressure switch to DUMP to ensure a full-open pressure regulator valve and a full-open pressure safety valve. 3. If the outside air temperature is below 80°F, turn the auxiliary vent selector to ON and open the cabin entry door. Component Description The S-3 cabin pressurization subsystem consists of five primary components. Four of them are shown in figure 3-20. The fifth component is located in the cockpit. Each component is discussed in the following paragraphs. If you are to troubleshoot effectively, it is important to know the relationship of each component to the system as a whole. CABIN PRESSURE REGULATOR VALVE.— The cabin pressure regulator valve is a pneumatically actuated butterfly valve mounted in the cabin exhaust ducting downstream of the cabin exhaust fan. The butterfly is spring-loaded to the closed position and diaphragm operated to the open position. The pressure regulator valve consists of the butterfly valve, which is actuated by a pressure-controlled diaphragm, and a solenoid valve to control the air pressure on the diaphragm. The solenoid valve is electrically con- nected to the cabin pressurization switch on the environmental control panel (fig. 3-21). There are three ports leading into the pressure regulator valve diaphragm chamber (fig. 3-20). The first port is located on the spring-loaded closing side of the diaphragm. It admits pressure from the cabin pressure regulator control. The second port is the ambient vent port. It is also located on the spring-loaded closing side of the Figure 3-21.—Environmental control panel. 3-33