ENGINE FLUSH AIR INTAKE

PRESSURE VESSEL (CABIN/COCKPIT) JET PUMP

BLEED AIR

Issue 1 - 20 March 2001 Page 4-27 4.9.3 The Isobaric Mode

In this mode the cabin pressure is maintained at a specific cabin altitude as flight altitude changes. The cabin pressure controller begins to close the outflow valve as the aircraft climbs to a chosen cabin altitude. The outflow valve then opens or closes (modulates) to maintain the selected cabin altitude as the flight altitude changes up or down. The controller will then maintain the selected cabin altitude up to the flight altitude that produces the maximum differential pressure for which the aircraft structure is rated. At this point the constant differential mode takes control.

4.9.4 The Constant-Differential Pressure Mode

Cabin pressurisation puts the aircraft structure under a tensile stress as the cabin pressure expands the pressure vessel. The cabin differential pressure is the ratio between the internal and external air pressures. At maximum constant-differential pressure as the aircraft increases in altitude the cabin altitude will increase but the internal/external pressure ratio will be maintained. There will be a maximum cabin altitude allowed and this will determine the ceiling at which the aircraft can operate.

4.9.5 Cabin Air Pressure Regulator

The pressure regulator maintains cabin altitude at a selected level in the isobaric range and limits cabin pressure to a pre-set pressure differential in the differential range by regulating the position of the outflow valve. Normal operation of the regulator requires only the selection of the desired cabin altitude and cabin rate of climb the adjustment of the barometric control.

Cabin Pressure Regulator Figure 25 ACTUATOR DIAPHRAGM OUTFLOW VALVE BAFFLE PLATE BASE REFERENCE CHAMBER HEAD PILOT DIAPHRAGM

ISOBARIC METERING VALVE ADJUSTER CONTROL

BAROMETRIC CAPSULE STATIC ATMOSHERE CONNECTION

ADJUSTER CONTROL DIFFERENTIAL METERING VALVE SOLENOID DUMP VALVE RESTRICTOR

Page 4-28 B1 Mod 11.04 Issue 30 Jan 2003

JAR 66 CATEGORY B MODULE 11.04 AIR CONDITIONING AND CABIN PRESSURISATION

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engineering

The regulator shown in Figure 25 is a typical differential pressure type regulator that is built into the normally closed air operated outflow valve. It uses cabin altitude for its isobaric control and barometric pressure for the differential control. A cabin rate of climb controller controls the pressure change inside the cabin. There are 2 main sections to the regulator, the head and reference chamber and the base with the outflow valve and diaphragm. The balance diaphragm extends outward from the baffle plate to the outflow valve creating an air chamber between the baffle plate and the outer face of the outflow valve. Cabin air flowing into this chamber through holes in the side of the outflow valve exerts a force against the outer face of the valve which tries to open it. This force is opposed by the force of the spring around the valve pilot which tries to hold the valve closed. The actuator diaphragm extends outward from the outflow valve to the head assembly creating an air chamber between the head and the inner face of the outflow valve. Air from the head and reference chamber exert a force against the inner face of the outflow valve helping the spring to hold the valve closed.

The position of the outflow valve controls the amount of cabin air that is allowed to flow from the pressure vessel and this controls the cabin pressure. The position of the outflow valve is determined by the amount of reference chamber air pressure that presses on the inner face of the outflow valve.

4.9.6 Isobaric Control System

The isobaric control system of the pressure regulator shown in Figure 26 incorporates an evacuated capsule, a rocker arm, valve spring and a ball type metering valve. One end of the rocker arm is connected to the valve head by the evacuated capsule and the other end of the arm holds the metering valve in a closed position. A valve spring located on the metering valve body tries to move the metering valve away from its seat as far as the rocker arm allows.

When the cabin air pressure increases enough for the reference chamber air pressure to compress the evacuated capsule the rocker arm pivots around its fulcrum and allows the metering valve to move away from its seat an amount proportional to the compression of the capsule. When the metering valve opens reference pressure air flows form the regulator to atmosphere through the atmospheric chamber.

Issue 1 - 20 March 2001 Page 4-29

Isobaric Control Operation Figure 26

When the regulator is operating in the isobaric range, cabin pressure is held constant by reducing the flow of reference chamber air through the metering valve. This prevents a further decrease in reference pressure.

The isobaric control responds to slight changes in reference pressure by modulating to maintain a constant pressure in the chamber throughout the isobaric range of operation. Whenever there is an increase in cabin pressure the isobaric metering valve opens which decreases the reference pressure and causes the outflow valve to open which then decreases the cabin pressure.

4.9.7 Differential Control System

The differential control system of the pressure regulator (Figure 27) incorporates a diaphragm a rocker arm, a valve spring and a ball type metering valve. One end of the rocker arm is attached to the head by the diaphragm which forma a pressure sensitive face between the reference chamber and the atmospheric chamber.

EVACUATED BELLOWS ISOBARIC METERING VALVE

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JAR 66 CATEGORY B MODULE 11.04 AIR CONDITIONING AND CABIN PRESSURISATION

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engineering

Differential Pressure Mode Figure 27

Atmospheric pressure acts on one side of the diaphragm and reference chamber pressure acts on the other. The opposite end of the rocker arm holds the