2. Operation
A. The pressurization control system is normally operated in the automatic mode. Any change in the operating mode causes illumination of one or more lights on the forward overhead panel. The operating mode may be selected by positioning the selector switch. The STANDBY or MANUAL lights will illuminate as applicable. A loss of power during operation in the AUTO or STANDBY mode may cause illumination of the AUTO FAIL light. An automatic transfer from automatic to standby mode causes illumination of the AUTO FAIL and STANDBY lights. The AUTO FAIL light will go out if the selector is switched to STBY.
B. With the pressurization control in AUTO, the flight altitude selected corresponds to the cruising altitude of the airplane. The OFF SCHED DESCENT light will illuminate if the airplane starts to descend prior to coming within .25 PSI of the pressure selected by the FLT ALT selector.
C. There is an outflow VALVE position indicator below the MANUAL light on the pressurization control panel. During manual mode operation the outflow valve is positioned manually with the toggle switch.
D. The cabin altimeter and differential pressure indicator are combined in one instrument on the pressurization control panel. The cabin altimeter senses cabin pressure and displays equivalent altitude in FEET. The other scale indicates pressure differential from cabin to ambient in PSI.
E. The rate of cabin altitude change is indicated on the CLIMB indicator, and is applicable to airplane ascent or descent. The CLIMB indicator senses rate of cabin pressure change and therefore reads differently at different cabin altitudes for the same pressure change. For example, a change of 500 sea level feet per minute would read approximately 725 ft/min at a cabin altitude of 10,000 feet.
F. The aural warning system provides an intermittent horn signal if cabin altitude exceeds approximately 10,000 feet during flight (Ref 31-26-0, Aural Warning and Call Devices). The ALTITUDE HORN CUTOUT switch may be used to stop the sound signal.
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21-33-0Page 2 Figure 1 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Jun 20/84
HEATING - DESCRIPTION AND OPERATION
1. General
A. Heating includes that portion of the air conditioning system which supplies heated air to the passenger cabin and cargo compartments. Heating is accomplished by use of conditioned airflow and distribution ducting, and on Passenger/Cargo Convertible Airplanes, heating blankets.
B. Cabin heating is accomplished by conditioned air distribution. The overhead distribution duct provides heat to the cabin. On Passenger/Cargo Convertible Airplanes, electric heating blankets are installed in the main cargo door to provide heating in the door area (Ref 21-41-0, Passenger Cabin Heating).
C. Cargo compartment heating is accomplished by exhausting passenger cabin air around the compartment walls (Ref 21-43-0, Cargo Compartment Heating).
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Feb 01/74 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 21-40-0 Page 1
PASSENGER CABIN HEATING - DESCRIPTION AND OPERATION
1. General
A. Heating of the passenger cabin does not exist as a specific heating system. Whether heating or cooling is required is determined by the temperature control system, 21-61-0, which either lowers or raises the temperature of the cabin by mixing air from the air cycle system with hot air bypassing the air cycle system (Ref 21-50-0, Cooling).
B. Maintaining a relatively consistent temperature throughout the passenger cabin is accomplished through the passenger cabin air distribution system. Conditioned air enters the passenger cabin from a slotted spaced nozzle in the overhead distribution duct which gives proper air circulation throughout the cabin (Ref 21-23-0, Conditioned Air Distribution System).
C. On Passenger/Cargo Convertible Airplanes, electric heating blankets are installed in main cargo door area to provide warmth in the door area.
2. Main Cargo Door Heating Blankets
A. On Passenger/Cargo Convertible Airplanes, three electric heating blankets are installed in the main cargo door behind the lining to maintain warmth in the cargo door area. The cargo door is lined and insulated so that it appears similar to the rest of the passenger cabin interior sidewall (Ref Chapter 52, Main Cargo Door Lining and Insulation).
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