Pressurisation System
Revised 21-Mar-2002
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There are two types of pressurisation installed on the B727, Pneumatic controlled and electronic controlled. 

The electronic pressurisation system 
Four independent modes of operation: Automatic, Standby, Manual AC, and Manual DC. The Standby mode is a semiautomatic system which, acts as a backup for the automatic mode of operation.  The two manual modes, acting as backups for the entire system.  The chart below the pressurisation panel is used to set or check the cabin altitude in relation to airplane altitude in the Standby mode.
The gate-type electrically controlled outflow valve is located on the right rear side of the airplane near the tailskid.  Although there are other acceptable pressure bleeds, most of the air exits through this valve.  Two safety pressure relief valves located just forward of the tailskid limit the cabin to a maximum of 9.6 psi differential pressure.  A negative relief valve prevents negative cabin pressure from exceeding 1 psi.

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Automatic warning of cabin altitude exceeding 10,000 feet is provided by a horn.  Depressing the horn cutout on the pressurisation panel raises cabin PA volume as well as silencing the warning horn.
Correct operation of the pressurisation control system requires information for the system from the air data computer. The Captain's or First Officer's altimeter setting, (Captain's altimeter setting is used in Auto mode and First Officer's setting in Standby), ambient air pressure from an external part, and the landing gear ground safety sensor.  On some airplanes the barometric pressure must be set on a separate counter on the flight engineer's panel.
When the proper information is set, the automatic mode will smoothly control the cabin from before takeoff to after landing with little or no further input from the crew.  The left side of the pressurisation section of the flight engineer's panel is used for manual inputs to the Auto mode.  Planned cruise altitude is entered in the Flight Altitude window and landing field altitude in the Land Altitude window.  The Flight/Ground switch, when moved to Flight, signals the pressurisation system to begin pressurising the airplane by moving the outflow valve from the full open position.  With the mode selector in Auto and the airplane an the ground, it will cause cabin altitude to descend to 200 feet below the present field elevation, resulting in a differential pressure of less than .125 psi.
After takeoff, the cabin climbs in proportion to airplane climb.  Cabin rate is limited to approximately 500 feet per minute during climb and 350 feet per minute during descent.  The cabin climbs automatically holding at intermediate altitudes as the airplane holds until the airplane reaches cruise altitude. During high altitude cruise the cabin enters a barometric hold phase.  The cabin will maintain a constant altitude at a nominal differential pressure of 8.5 psi.  Slight variations of airplane flight altitude will cause no change in cabin altitude, but the differential pressure will vary.  If the airplane climbs, so that the pressure differential reaches 8.65 psi, however, it will remain at that differential and the cabin will climb. Upon descent, the cabin will smoothly descend holding again as the airplane holds until the airplane reaches the landing altitude programmed.  With the barometric pressure set for the destination, the cabin descends to 300 feet below the setting in the Land Altitude window and at touchdown the cabin climbs to 200 feet below the Land Altitude setting.  Placing the Flight/Ground switch to Ground after landing depressurises the cabin slowly.  In the auto mode during low altitude cruise, the full differential pressure is not needed.  The cabin climbs or descends to a cabin altitude 300 feet below the landing altitude and stays there until touchdown.
If it becomes necessary to return to the departure airport before reaching cruise altitude, the system automatically sets the cabin for landing at the takeoff field.  If the airplane has not reached the cruise altitude set in the Cruise Altitude window before descending, the OFF SCHEDULE DESCENT sequence will occur.  If the landing is to be made at the departure airport, the system will set cabin altitude without further crew input.
If, however, the flight has reached its destination without climbing to the final cruise altitude, the OFF SCHEDULE DESCENT mode must be cancelled.  The light may be extinguished by rotating the flight altitude digital readout to the airplane's present altitude.  At this point the system will be returned to normal operation and the cabin pressure will be set for landing at the altitude in the Land Altitude window.  The correct land altitude value must be set in the Land Altitude window prior to beginning this procedure if landing at other than the original destination.
The Standby Mode.  The green Standby light will illuminate as a result of automatic transfer from the auto mode, or crew selection of the Standby mode.  Control of pressurisation in Standby mode is through the Cabin Altitude selection in the center of the pressurisation panel.  The desired cabin altitude is set by the flight engineer to control the pressurisation  He uses the chart next to the panel to determine the proper cabin altitude for the airplane altitude being flown.  The Standby rate knob controls the rate at which cabin altitude changes in the Standby mode.  The rate knob has 50 to 2000 feet rate of cabin climb or descent capability.  Normally the rate knob is set on the Index Mark, which results in a rate of climb or descent of 300 feet per minute.  To operate the pressurisation system in Standby, the Cabin Altitude is set to 200 feet below the takeoff airport altitude for takeoff, the barometric pressure is set in the appropriate instrument or counter, and the Flight/Ground switch is moved to Flight.  After takeoff, the proper cabin altitude is found on the chart next to the pressurisation panel and is set in.  As the airplane begins its descent for landing, the flight engineer sets the Cabin Altitude to 200 feet below the destination airport's altitude, and the barometric pressure is set.  The airplane is depressurised after landing by moving the Flight/Ground switch to Ground.
The pressure profiles that might be experienced, on a flight are.  In this example, the takeoff airport elevation is at sea level.  Since the takeoff airport elevation is sea level, the controller is set to minus 200 feet prior to takeoff.  This allows the airplane to make a smooth transition to pressurised flight.  The cruise altitude will be 35,000 feet.  After takeoff, the corresponding cabin altitude is set in the controller, 5.600 feet.  Prior to descent, the landing elevation minus 200 feet is set in the controller.  The airport elevation is 2,000 feet, so the controller is set at 1,900 feet.
Control will automatically shift from the Auto mode to the Standby mode for any of the following reasons: an excessive cabin rate of change, a power loss of more than 15 seconds to the auto portion of the pressurisation system, or if the cabin altitude exceeds 14,000 feet.  If an auto mode failure occurs, the amber AUTO FAIL light will illuminate and the green STANDBY light will also illuminate because the pressurisation system has reverted to the Standby mode. If any of the modes fail to control the cabin pressure so that the cabin altitude rises to 14,500 feet, the outflow valve will be driven fully closed by DC power from the battery transfer bus.
The green Manual light will illuminate when either MANUAL AC or MANUAL DC is selected on the mode selector.  For use in checking the outflow valve, a valve position indicator is provided.  This indicator functions in all modes.  The CLOSE/OFF/OPEN toggle switch below this indicator will move the outflow valve towards the position selected and thereby control the pressurisation system.  Electrical power to the DC Manual mode should always be available from the battery transfer bus.  Manual AC, powered by the Essential AC bus, operates much faster than Manual DC and should be used during a rapid depressurization.  Manual DC should be used during an AC power failure, as this will be the only operational mode at this time. I personally prefer to use DC when in manual as it gives a much smoother control of the pressurisation

Pneumatic Control System
It is a little long in the tooth now, but it is still around and in use on a great number of aircraft, mostly 100's. Affectionately know as "steam driven". There are two control panels, again at the flight engineers panel. One for automatic control and  one for manual mode. You set these by markings on the instrument and it is then entirely controlled by sensed pressures and venturi's. It's basic, but robust, though pressure bumps are quite a common feature of this system.

On the Auto controller you also have a rate control knob to increase or decrease the rate of change.  Cabin altitude selector, with which you select the desired altitude from the instrument markings.  Finally a barometric setting control knob with which you set the local pressure datum.
On the manual controller you have a knob with which you can control the outflow valves, either increase or decrease pressure by moving it clockwise or anti clockwise respectively.

Note that the valves are operated by sensed pressures and no physical link, unlike the manual mode on the electronic system which is controlled by electric actuators.  The manual controller will override the auto controller, also on the same panel is the  ground venturi blower switch which is used only on the ground and performs the same function as the ground / flight switch of the electronic system. On the top left of is panel is the altitude horn cutout switch which will silence the altitude warning horn.

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