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.

diaphragm. The third port is located on the opening side of the diaphragm. A sensing line is attached to the third port to connect the cabin pressure regulator control and the cabin pressure exhaust duct. The pressure admitted to the diaphragm through the third port is equivalent to cabin air pressure. The difference between the pressure on either side of the diaphragm causes the pressure regulator valve to modulate between the open and closed positions.

CABIN PRESSURE REGULATOR SAFETY VALVE.- The pressure regulator safety valve is an independent, pneumatically operated, balanced type of poppet valve that limits cabin-to- ambient pressure differentials to 7.07 (+ 0.2 and - 0.0) psi. If the difference between cabin pressure and ambient pressure reaches the calibrated limit, the change in pressure acting on the limit control diaphragm overcomes the metering valve spring-load and allows the metering valve to open. This also opens a passage in the cabin pressure safety valve head, which causes the head pressure to be slightly lowered. Since the cabin pressure is greater than head pressure, it opens the pressure-balanced main poppet to allow cabin air to be vented overboard. When the cabin pressure differential is restored to normal, the limit control metering valve closes, and the pressure safety valve returns to its normally closed position.