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±0.1 psid. As differential pressure reaches 9.55 psid, an amber CABIN DFRN 9.6
caution message is displayed on the Crew Advisory System (CAS) and the
pressurization system begins limiting outflow valve closure.
If the pressurization system malfunctions and cannot limit maximum cabin
pressure differential to 9.55 ±0.1 psid, a safety valve limits pressure differential to
9.7 ±0.1 psid. As differential pressure reaches 9.8 psid, a red CABIN DFRN 9.8
warning message is displayed on CAS.
The pressurization system receives information from the two Air Data Computers
(ADC #1 and ADC #2). It also uses the Weight-On-Wheels (WOW) system
(commonly referred to as the nutcracker system) to control various system
operating modes.
Under typical flight conditions, the flight crew programs the system prior to takeoff.
Apart from selection of the FLIGHT/LANDING mode switch or adjustment of LFE,
system operation is virtually automatic.
Major components of the pressurization system are:
• Cabin Pressurization Transducer
• Cabin Pressure Outflow Valve
• CABIN PRESSURE CONTROL Panel
• Cabin Pressurization Selector Panel
• Cabin Pressure Safety Valve
• Cabin Rate Pressure Switch
• Cabin Pressure Warning Switch
• Cabin Differential Pressure / Altimeter / Rate-of-Climb Indicator
2. Description of Subsystems, Units and Components:
(See Figure 1.)
OPERATING MANUAL
2A-21-00 PRODUCTION AIRCRAFT SYSTEMS
Page 2
May 30/02
Revision 7
A. Cabin Pressurization Transducer:
The solid-state cabin pressurization transducer is the heart of the
pressurization system. Located in the electronics area of the entrance
compartment, produces the output signal to the outflow valve motor based
on the following input data:
• Inputs from the cabin pressurization selector panel
• Corrected static pressure data from the ADCs (or Digital Air Data
Computers [DADCs])
• FLIGHT or GROUND mode from the nutcracker system
• Sensed cabin pressure from inside the pressure vessel
To prevent undesired outflow valve movement from transient power
surges, the transducer obtains a clean, stable supply of power from an
Electromagnetic Interference (EMI) filter installed in the radio rack.
Within the transducer are circuits that limit cabin differential pressure in
flight to 6,550 feet with an airplane altitude of 45,000 feet.
B. Cabin Pressure Outflow Valve:
A single pressurization outflow valve is installed under the lower shelf of the
radio rack. It is an electrically-controlled, motor-driven valve that
determines the amount of cabin air exhausted overboard, thus controlling
cabin pressurization. It is capable of moving from fully open to fully closed
in approximately ten (10) seconds.
The outflow valve is a butterfly type valve with an electro-mechanical
actuator and a potentiometer that provides valve position information to the
position indicator on the CABIN PRESSURE CONTROL panel. Within the
actuator are a DC motor with DC brakes, an AC motor with AC brakes and
a motor-generator.
The AC motor, which operates the outflow valve in the Automatic (AUTO)
mode, responds to signals from the cabin pressurization transducer. The
DC motor operates the outflow valve in the MANUAL mode by responding
to signals from the CABIN PRESSURE CONTROL panel manual control
knob. Both motors are capable of extremely slow or fast operation, or at
any intermediate speed required by the controlling device.
During normal system operation (AUTO mode), the cabin pressurization
transducer opens and closes the outflow valve using the AC motor. While
the AC motor is in operation, the DC motor brake engages to prevent DC
motor movement. The motor-generator in turn provides a rate-of-change
signal back to the cabin pressurization transducer.
With the system in MANUAL mode, the manual control knob provides a DC
signal to position the outflow valve. The AC motor brake engages to
prevent AC motor movement. Although the motor-generator is inactive, the
potentiometer still provides valve position information to the valve position
indicator.
In the unlikely event that both Essential AC and Essential DC bus power
sources are lost, the outflow valve will cease operation and remain at the
last position commanded.
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-21-00
Page 3
May 30/02
Revision 7
C. CABIN PRESSURE CONTROL Panel:
(See Figure 3.)
The CABIN PRESSURE CONTROL panel, located on the cockpit
overhead panel, has controls and indicators for:
• Manual control of the outflow valve
　
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