CAUTION:  DO NOT APPLY POWER FOR MORE TIME THAN NECESSARY TO VERIFY BLANKETS ARE OPERATIONAL BECAUSE OF THE POSSIBILITY OF GROUND OVERHEAT.

(11)
Place CARGO DOOR HEAT switch to OFF and check that there is no voltage between terminal No. 8 and ground.

(12)
Remove electrical power if no longer required.

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CONTROL CABIN HEATING - DESCRIPTION AND OPERATION
1.  General
A.  Heating of the control 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. Refer to Cooling, 21-50-0.
B.  Maintaining a desirable temperature in the control cabin is accomplished through the control cabin air distribution system. Conditioned air enters the control cabin from several outlets located to give proper air circulation throughout the cabin. Refer to Control Cabin Conditioned Air Distribution System, 21-22-0.
501 
Nov 15/68  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  21-42-0 Page 1 

CARGO COMPARTMENT HEATING - DESCRIPTION AND OPERATION
1. General
A.  Cargo compartment heating is accomplished by exhausting passenger cabin air around the compartment walls.
B.  Air from the passenger cabin, after picking up heat from the passenger cabin, is exhausted through foot level grilles in the passenger cabin sidewall. This air then flows around the walls to keep the cargo compartment from becoming cold as a result of outside air temperature. Air is induced through the walls of the aft cargo compartment by exhausting the air through the cabin pressurization outflow valve located to the right of the airplane centerline, at the underside of the airplane, approximately opposite the aft entry door.
2.  Forward Cargo Compartment Heat Distribution
A.  Control cabin air is drawn through a duct located in the P6 circuit breaker panel and is utilized for equipment cooling. This duct, on the right side of the airplane connects into a system of ducts and manifolds that lead to an inlet plenum. Blowers are installed in the inlet plenum to act as air movers. During ground operation or when the control cabin is unpressurized, air is drawn through the ducts past a flow control valve, and is discharged overboard through the exhaust port. (See figure 1.)

501  Forward Cargo Compartment Heating Airflow Diagram 
Nov 15/68  Figure 1  21-43-0 
Page 1 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  In flight, normal operation of the equipment cooling system will cause the automatic flow control valve of the equipment cooling system to go closed. As a result all air passing through and around the equipment will be discharged under the cargo compartment floor. From under the floor the air, heated from passing through and around the equipment, travels up the sidewalls and circulates between the cargo compartment insulation and lining heating the cargo compartment. This air is then collected in collector shrouds and is exhausted overboard through the forward outflow valve. The forward outflow valve works with the cabin pressurization outflow valve so that its performance is in conjunction with the pressurization system.
3.  Forward Outflow Valve
A.  The forward outflow valve is normally in the open position. During flight, air from the equipment cooling system circulates between the cargo compartment insulation and lining to heat the cargo compartment before it is discharged overboard through the forward outflow valve. Upon demand from the cabin pressurization outflow valve to close, in order to conserve cabin pressure, the forward outflow valve will close. The forward outflow valve position is controlled by a pair of limit switches located in the cabin pressurization outflow valve actuator. (Refer to Pressurization Control System.)
B.  The forward outflow valve is located under the passenger cabin floor, next to the skin in the forward cargo compartment at approximately body station 370.
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FORWARD OUTFLOW VALVE - REMOVAL/INSTALLATION
1.  Remove Forward Outflow Valve
A.  Check that FORWARD OUTFLOW VALVE circuit breaker on circuit breaker panel P6-4 is open.
B.  Remove the left portion of the forward bulkhead panel in the forward cargo compartment  (Fig. 401).
C.  Remove electrical connector from valve.
　
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