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the doors.
The flight crew initially sets 45,000 feet on the FLIGHT scale of the cabin
pressurization selector panel. This results in the adjacent CABIN scale
reading of 6,550 feet, the isobaric cabin altitude that corresponds to the
maximum differential pressure of 9.41 psid. The cabin RATE dial is set at
500 FPM UP, resulting in the adjacent DOWN scale reading 300 FPM. The
average rate of climb to 45,000 feet is approximately 2,700 FPM, with a
cabin rate of climb of 500 FPM. The departure runway field elevation
altitude is then programmed into the LDG (Landing) dial. Destination
runway field elevation altitude (4,000 feet in this example) is set during
descent. At this point, the pressurization system is programmed for
automatic operation and, assuming the AUTO/MANUAL switch is in AUTO,
the system will operate in automatic mode when electrical power is applied
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-21-00
Page 5
May 30/02
Revision 7
to the aircraft.
With the airplane on the ground, the AUTO/MANUAL switch in AUTO and
the FLIGHT/LANDING switch in LANDING, the outflow valve will cycle
open as soon as electrical power is applied. This is due to the absence of
airflow and the cabin pressurization transducer sensing an on-ground
condition from the nutcracker.
With the main entrance door still open at this point, there is no pressure
buildup. A negligible pressure increase (approximately 0.05 psid, equalling
pressure drop across the outflow valve) occurs after closing the main
entrance door, remaining negligible through engine start, APU shutdown
and transfer of air source to the engines. During taxi, the pressure remains
low until lineup in preparation for takeoff. At this point, the flight crew
selects the FLIGHT position on the FLIGHT/LANDING switch on the
CABIN PRESSURE CONTROL panel.
With the FLIGHT position selected, the outflow valve immediately begins to
close under rate control, “holding” at approximately 0.25 psid of pressure
buildup. This “holding”, known as ground differential pressure control,
serves to minimize the possibility of “pressure bumps” during takeoff.
As soon as the aircraft becomes airborne, the nutcracker system sends an
inflight signal to the cabin pressurization transducer. The transducer in turn
commands the outflow valve to close, thus pressurizing the cabin at the
programmed rate of 500 FPM UP until the maximum differential pressure of
9.55 ±0.10 psid is reached. The cabin altitude remains stable within ±25
feet of the final cabin altitude, in this case 6,550 feet at an aircraft altitude
of 45,000 feet. Stability is maintained provided the aircraft does not climb
above 45,000 feet, the maximum allowable altitude of the aircraft.
In preparation for descent, the flight crew enters the destination runway
field elevation altitude (4,000 feet in this example) in the LDG window. A
discriminator circuit is incorporated within the cabin pressurization
transducer to prevent inadvertent programming of an isobaric cabin altitude
(CABIN value) lower than the landing field altitude (LDG value). This
feature prevents landing with excessive cabin differential pressure. If the
discriminator circuit detects a LDG value higher than the CABIN value, it
allows the LDG value to override the CABIN value and control that outflow
valve. This is the only instance in which the LDG value can override the
FLIGHT position on the FLIGHT/LANDING switch while in the AUTO mode.
When ready to descend, the FLIGHT/LANDING switch is positioned to
LANDING.
Noting that the cabin rate of climb was preprogrammed to be 300 FPM
DOWN prior to takeoff, the flight crew adjusts the BARO CORR (barometric
correction), if necessary. This completes the descent programming. As the
aircraft descends, the system automatically begins to open the outflow
valve until a descent rate of 300 FPM is reached. Cabin pressure is
maintained at approximately 400 to 500 feet below the actual LFE, or at
approximately 0.25 psid.
Upon touchdown, the nutcracker system signals the cabin pressurization
transducer of an on-ground condition, causing the outflow valve to be
driven fully open. With the outflow valve fully open, cabin pressure drops to
a negligible pressure of approximately 0.05 psid until the air supply is shut
off.
OPERATING MANUAL
2A-21-00 PRODUCTION AIRCRAFT SYSTEMS
Page 6
May 30/02
Revision 7
B. Manual Operation Mode:
To operate the pressurization system manually, the AUTO/MANUAL switch,
located on the CABIN PRESSURE CONTROL panel, is selected to
MANUAL. This causes the MANUAL switch legend to illuminate amber, the
　
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