737 Flight Crew Operations Manual
Air Systems -
Pressurization System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 2.40.5
The descent mode is activated when the airplane descends 0.25 psi below the
selected FLT ALT. The cabin begins a proportional descent to slightly below the
selected LAND ALT. The controller programs the cabin to land slightly
pressurized so that rapid changes in altitude during approach result in minimum
cabin pressure changes.
While taxiing in, the controller drives the outflow valve slowly to the full open
position depressurizing the cabin.
An amber AUTO FAIL light illuminates if any of the following conditions occurs:
• Loss of DC power
• Controller fault
• Outflow valve control fault
• Excessive differential pressure (> 8.75 psi)*
• Excessive rate of cabin pressure change (±2000 sea level feet/minute)*
• High cabin altitude (above 15,800 feet).*
*If controller is not responding properly
With illumination of the AUTO FAIL light, the pressure control automatically
transfers to the other auto controller (ALTN mode).
Moving the pressurization mode selector to the ALTN position extinguishes the
AUTO FAIL light, however the ALTN light remains illuminated to indicate single
channel operation.
Selected FLT ALT Differential Pressure Limit
At or below 28,000 feet 7.45 psid
28,000 feet to 37,000 feet 7.80 psid
Above 37,000 feet 8.35 psid
September 30, 2011
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