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. CABIN PRESSURE REGULATOR CON- TROL.— The pressure regulator control is a pneumatic control that provides four modes of cabin pressure operation. In addition to the modes of operation, a test valve is included with three manually set positions: FLIGHT, DIFF ON, and ALL OFF. The test valve is normally lockwired in the FLIGHT position for all cabin pres- surization modes. The DIFF ON position permits aground test of the normal delta-P setpoint. The ALL OFF position permits a ground test of the setpoint of the pressure safety valve. These tests are accomplished with pressure supplied by support equipment. Four pneumatic ports are provided on the pressure regulator control for use with various sensed pressures and the pressure regulator valve. These ports are different sizes to prevent improper plumbing connections. The pressure regulator control contains an isobaric bellows, which is calibrated to maintain an aircraft cabin pressure of 5,000 feet while the aircraft is flying at altitudes between 5,000 and 24,000 feet. The isobaric bellows, which modulates a control pressure, use cabin air as a pressure source and low pressure in the environmental control system compartment as a negative pressure. Control pressure is delivered to one side of the pressure regulator valve diaphragm, and cabin pressure is connected to the opposite side. Because control pressure is normally less than cabin pressure and will decrease relative to cabin pressure, the pressure regulator valve becomes more open to decrease cabin pressure. The pressure regulator control contains provisions for controlling the rate of cabin repressurization when recovering cabin pressure after using the cabin dump mode, or during a rapid descent in altitude. The control pressure modulated by the isobaric bellows is further modulated by the repressurization diaphragm to limit cabin repressurization to an equivalent 4,000 feet per minute change. The pressure regulator valve is held open until normal pressure characteristics are sensed. CABIN LOW-PRESSURE SWITCH.— The low-pressure switch is installed below the center console to sense the cabin absolute pressure. The normally open low-pressure switch closes at 13,000 ± 500 feet and reopens at 11,000±500 feet. The cabin pressurization indicator light on the annunciator panel illuminates when the low- pressure switch closes. The indicator light goes off when the low-pressure switch reopens. CABIN AIR PRESSURE SENSING FIL- TER.— The air pressure sensing filter is located in the line that connects the cabin exhaust air duct, the cabin pressure regulator control, and the cabin pressure regulator valve. The replaceable filter element, which is connected to the air sensing tube, is mounted with clamping rings on the fuselage frame. The filter element is a cylindrical plug of treated paper and fabric in a metal housing. The clamping rings confine the air entry to the dome-shaped end to trap the entry of tobacco tar and dust particles greater than 10 microns in diameter. MAINTENANCE AND INSPECTION Very little maintenance is required on most pressurization and ACSs other than making the required periodic inspections and operational checks. Many of the components are repairable at the depot level of maintenance rather than at 3-34