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Pressurization System Description Section 30
Copyright © The Boeing Company. See title page for details.
D6-30151-425 2.30.1
Introduction
Cabin pressurization is controlled by regulating the discharge of conditioned cabin
air through the outflow valves.
Two outflow valves are installed at the rear of the cabin. The valves normally
operate in parallel. Cabin altitude and full ventilation rates can be maintained by
either valve.
There are two cabin altitude controllers, A and B. Each controller controls both
outflow valves.
Positive and negative pressure relief valves protect the fuselage against excessive
pressure differential.
The pressurization system has automatic and manual operating modes.
Pressurization SystemAutomatic Operation
The cabin pressure controllers use ambient pressure and flight plan data from the
FMC to calculate a cabin pressurization schedule. The schedule provides a
comfortable cabin climb to cruise altitude.
For takeoff, the system provides a small positive pressurization prior to rotation to
cause a smooth transition to the cabin altitude climb schedule. During climb, cabin
altitude increases on a schedule related to airplane climb rate and flight plan cruise
altitude. When the FMC climb path has a planned level segment, it is included in
the total time required for the airplane to reach the top of climb and cabin altitude
continues to increase during the level segment. If the airplane climb flight path is
above the FMC climb path and maximum cabin pressure differential is reached
during the climb, cabin rate then becomes a function of airplane climb rate so
maximum cabin differential pressure is not exceeded.
In cruise, maximum cabin altitude is 8,000 feet. During descent, cabin altitude
decreases to slightly below the FMC planned landing altitude to ensure the
airplane is slightly pressurized on landing. Landing altitude barometric pressure
correction comes from the captain’s altimeter setting. At touchdown, the outflow
valves open to depressurize the cabin.
The cabin altitude limiter closes both outflow valves if cabin altitude exceeds
11,000 feet.
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
2.30.2 D6-30151-425
Air Systems -
Pressurization System
Description
Full automatic operation of cabin altitude is possible with one outflow valve
operating automatically and the other outflow valve not operating. For this
configuration, one pack is selected off to ensure cabin doors may be opened
regardless of the position of the outflow valves if an emergency evacuation is
required immediately after landing.
If landing altitude is set manually using the landing altitude selector and cabin
differential pressure is less than maximum cabin differential pressure, cabin
altitude begins to decrease or increase to the altitude set by the landing altitude
selector. FMC altitude information is bypassed and the cabin altitude controller
uses internal rate schedules to control cabin altitude. Landing elevation limits are
1,000 feet below sea level to 14,000 feet above sea level. The maximum cabin
altitude in cruise is 8,000 feet, regardless of the manual selected landing altitude.
Pressurization System Manual Operation
If both Outflow Manual switches are ON, all automatic cabin altitude control
functions are bypassed. For this configuration, one pack is selected off to ensure
cabin doors may be opened regardless of the position of the outflow valves if an
emergency evacuation is required immediately after landing.
Pressurization Relief
Two mechanical positive pressure relief valves prevent overpressurization of the
airplane. One or both valves open if cabin pressure becomes excessive, and close
when cabin pressure is no longer excessive. Pack two shuts down to assist in
relieving excess cabin pressure. Pack two resets when both cabin pressure relief
valves close.
Negative pressure relief valves in the forward and aft cargo doors open when the
airplane cabin pressure is slightly less than outside air pressure. The valves also
open to relieve any existing differential pressure when a cargo door is unlatched.
December 01, 1999
747 Operations Manual
Air Systems Chapter 2
Bleed Air System Description Section 40
Copyright © The Boeing Company. See title page for details.
D6-30151-425 2.40.1
Introduction
Bleed air can be supplied by the engines, APU, or a ground air source.
Bleed air is used for:
Engine Bleed Air Supply
Engine bleed air is supplied fromeither intermediate pressure (IP) or high pressure
(HP) engine sections. IP air is used during high power setting operations. HP air
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