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CABIN PRESSURE REGULATOR CONTROL.- 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.
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