757 MAINTENANCE MANUAL
 (6)  S 082-026Slowly remove the spring scale from the valve door.
 (7)  S 212-024Make sure the valve door fits tightly against the airplane skin.

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757 MAINTENANCE MANUAL
 PRESSURIZATION INDICATION AND WARNING SYSTEM - DESCRIPTION AND OPERATION
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General (Fig. 1)
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 A.  The Pressurization Indicating and Warning System provides visual and aural warning if the pressurization control system fails (Ref 21-31-00).
 (1)  
A high cabin altitude (low cabin pressure) can cause injury to passengers and damage the airplane structure. The cabin altitude is sensed by an aneroid switch. When the cabin altitude gets too high, the aneroid switch actuates an aural warning siren, a level A EICAS message CABIN ALTITUDE, and visual warnings in the flight deck.

 (2)  
Three pressure indicators in the flight deck, show cabin-to-ambient differential pressure, cabin altitude, and cabin pressure rate of climb or descent.

Component Details
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 A.  Altitude Switches
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 (1)  
GUN 001-003, 008-999; The altitude switch is an aneroid type switch installed on the left miscellaneous electrical equipment panel, P36. As cabin altitude increases the aneroid expands. At 10,000 feet cabin altitude the aneroid closes a warning switch completing the warning circuit.

 (2)  
GUN 004-007; Two altitude switches installed in the left miscellaneous electrical equipment panel, P36, are identical aneroid type switches. As the cabin altitude increases, the aneroid expands. One switch closes when the cabin altitude gets above 10,000 ft. The other switch closes when the cabin altitude gets above 14,500 ft.

 B.  
Differential Pressure Sensor

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 (1)  
The differential pressure sensor is located on the left side of the forward equipment bay. A pitot static system provides ambient pressure signals to the sensor. The cabin pressure is sensed directly by the sensor. The sensor compares the ambient pressure to the cabin pressure, converts the pressure difference to an electrical signal and sends it to the CABIN DIFF gage.

 C.  
Pressurization and Indication Warning Module

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 (1)  The pressurization and indication warning module is located on the pilots overhead panel (P5). It consists of three pressure gages which provide the cabin pressure indication. The CABIN DIFF gage receives signals from the differential pressure sensor and displays the cabin-to-outside differential pressure. The CABIN ALT gage receives cabin pressure signals through an open port in the indicator case and displays the pressure as the equivalent altitude. The indicator shows positive cabin altitude from 0 to 25,000 ft. and airplane altitudes below sea level, reaching a lower limit at the gage's OFF peg. The CABIN RATE gage receives cabin pressure signals through an open port in the indicator case. The gage displays the rate of cabin pressure change as a function of time for airplane altitudes of -24,000 ft. and above.
Operation
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757 MAINTENANCE MANUAL
P5 PANEL SEE A P1-3
CAPTAINS' MAIN INSTRUMENT PANEL SEE B
FLIGHT COMPARTMENT
P36 PANEL
PRESSURIZATION SEECAND D
AND INDICATING
WARNING MODULE
MAIN EQUIPMENT CENTER
CABIN
HIGH ALT LDG
0  12
1
102  2502 6
204 0

VIDEO ON 15 6
b 84 6 1
ON
6 108 2 w
INOP
FT X FPM X aDIFF PSI ALT 1000 RATE 1000
R ALTITUDE CABIN
PRESS DIFF LIMIT: TO/LDG
.125 PSI
OVERHEAD PANEL
A
CONFIGFIRE
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