(2)
Both the ac manual mode and the dc manual mode provide direct control to the outflow valve through separate electrical motors. The manual modes back up the automatic and standby systems.
(3)
In the automatic mode, the system accepts manual inputs of planned flight altitude and landing altitude before takeoff. It then determines the lowest possible cabin altitude that can be maintained during airplane high altitude flight and schedules any necessary change in cabin pressure during the flight without any action by the crew. The system can also be reset if a change in planned flight altitude or landing field altitude is made.
(4)
Cabin pressure rate of change is automatically controlled.
(5)
The system provides a cabin altitude of approximately 8000 feet with the airplane at cruise altitude.
(6)
Barometric correction for automatic and standby pressure control assures that cabin pressure and landing field pressure are approximately the same at landing.
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Jun 20/87 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 21-31-0 Page 1
K16725
Pressurization Control System Component Location 564
21-31-0 Figure 1 May 01/99
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(7)
A selector switch permits overriding the automatic and standby systems to directly control the position of the outflow valve electrically. Alternate ac or dc control over the valve is provided.
(8)
The STANDBY system automatically takes over to control cabin pressure according to cabin altitude and rate of cabin pressure change settings if certain limits are exceeded in AUTO. In addition, a manual transfer can be made from AUTO to STBY, if desired, by use of the mode selector switch.
B. Operation of the pressurization control system requires signals into the pressurization control system from direct ambient sense, the cabin pressure barometric correction selector, and the landing gear ground safety sensor in the automatic mode, and from the cabin pressure barometric correction selector, the No. 1 air data computer (No. 2 air data computer on airplanes with SB 21-1119), and the landing gear ground safety sensor in the standby mode. Refer to Air Data Pressure Instruments, Chapter 34, for description of the air data computer and to Landing Gear Safety Sensors, Chapter 32, for description of the landing gear ground safety sensor. Electrical circuit protection for the pressurization control system is provided by circuit breakers on the P6 circuit breaker panel.
2. Pressure Control Panel
A. The pressure control panel includes four groupings of dials, switches, and indicators used in setting up or changing the operation of the pressurization control system. One grouping is for the automatic (AUTO) method of operation, another is for the semiautomatic or STANDBY method, the third is for the MANUAL method, and the fourth is to allow choosing the desired method of control and function available.
B. For the AUTO method there is an adjusting knob and indicator (LAND ALT) for setting destination landing field altitude and another knob and indicator (FLT ALT) for setting cruise altitude.
C. Instruments for STANDBY consists of a cabin altitude adjusting knob (CAB ALT) and indicator and a cabin rate of pressure change (CAB RATE) knob. There is a pip mark on the panel for the CAB RATE knob, which marks the setting for approximately 300 feet per minute rate of change. CAB RATE range is from 50 to 2000 sea level ft/min.
D. The manual method utilizes a two-position self-centering toggle switch for controlling the position of the outflow valve. The switch has momentary contacts closed in the OPEN and CLOSE positions for driving the outflow valve open or closed respectively. Releasing the switch breaks the contacts. A position indicator is provided for monitoring outflow valve position.
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May 01/99 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 21-31-0 Page 3
E. The fourth section of the panel includes a five-position mode selector switch, CHECK, AUTO, STBY, AC and DC MAN, and a two-position FLT-GRD toggle switch. The FLT-GRD switch is used in conjunction with the selector switch and landing gear sensor switch to control minimum cabin differential pressure when the selector is in AUTO, cabin pressurization on the ground when the selector is in AUTO or STDBY, outflow valve positioning rate when the selector is in MAN and to check the rate limit fault detector of the auto transfer when the selector is in the CHECK position. When the selector is in either MAN position the cabin pressurization is controlled directly by the use of the manual OPEN-CLOSE switch to actuate the outflow valve under either ac or dc control, depending on the position of the selector switch.
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