(b)  
Make sure the MAN L and the MAN R switches are on OFF.

 (c)  
Set the CMC menu to GROUND TEST.

 (d)  
Set the menu to chapter 21 CPCS (A or B).

 (e)  
Make sure that IN PROGRESS shows on the CMC CDU.

 (f)  
Make sure that PASS shows on the CMC CDU.

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A 747-400 MAINTENANCE MANUAL
 (8)  S 414-021Close the access panel on the aft bulkhead in the bulk cargo compartment.
 (9)  S 864-024Remove the electrical power if it is not necessary (Ref 24-22-00/201).

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 PRESSURIZATION RELIEF VALVES - DESCRIPTION AND OPERATION
________________________________________________________
 1.  General (Fig. 1)_______
 A.  Overpressurization of the airplane fuselage is prevented by two pressure relief valves. The pressure relief valves remain closed during normal operation of the pressurization control system (Ref 21-31-00). If a failure in the pressurization control system causes cabin pressure to increase, the relief valves open to prevent cabin overpressurization and consequent structural damage. The valves are powered by differential pressure and do not require an external power source. Outside ambient pressure is sensed at a remote sensing port, with backup provided by the integral sensing port. The opening of a valve causes the No. 2 pack to trip and on advisory message, PACK 2, to show on the EICAS screen. By selecting STATUS the EICAS screen will show which valve opened, UPR PRESS RELIEF or LWR PRESS RELIEF. If a valve opens and the No. 2 pack does not trip, the advisory message is PRESS RELIEF VLV.
 B.  Negative pressurization of the airplane fuselage is limited by vacuum relief valves. Two of the valves are installed in each cargo door (Ref 52-34-00/00).
 2.  Pressure Relief Valves______________________
 A.  Each pressure relief valve opens to prevent cabin overpressurization. Pressure relief valves are located on lower left side of the fuselage in the forward cargo compartment area. The valve gate is the part of the valve which moves to allow airflow through the valve. The valve gate opens when the servo chamber is vented to ambient by the action of a control diaphragm assembly. There is one normal control diaphragm assembly and one backup control diaphragm assembly integral with each valve. A normally open switch is installed on the valve and causes EICAS STATUS messages UPR RELIEF or LWR RELIEF to show when the valve is open. Airflow through an open valve gate forces blowout doors to open. The blowout doors are flush with the airplane skin and cover the round opening for the pressure relief valve. One set of blowout doors consists of two semicircular segments. The blowout doors remain open after the valve gate has closed. After landing the doors must be closed manually from outside the airplane. This provides positive indication that pressure relief has occurred.
 3.  Operation_________
 A.  Functional Description (Fig. 1)
 (1) Overpressure Relief
 (a) When cabin differential pressure reaches 9.25 |0.15 psi
 (63.77 |1 kPa), the normal poppet opens and vents the servo chamber to ambient. Cabin-ambient differential pressure acts on the servo piston of the valve to power the valve gate open.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

SEE A
PRESSURE RELIEF VALVE (2)
FILTER
A
Pressure Relief Valve Installation Figure 1
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