Aircraft cabin pressurization (also known as aircraft cabin altitude) is a result of compressed air that comes from the air-conditioning system. If it is not controlled, cabin air pressure can increase to an unsatisfactory level. The function of the cabin pressure control system is to make sure that cabin pressurization stays at a satisfactory level at all times.

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There are two integrated air system (IAS) controllers in the aircraft. They are identified as IAS controller No. 1 and No. 2. They monitor and control the automatic operation of the aircraft cabin pressure control system on the ground and in flight. Although there are two IAS controllers in the aircraft, only one at a time is in control of the cabin pressure control system. The in-control IAS controller is known as the active IAS controller (usually IAS controller No. 1). The IAS controller that is not in control is known as the standby IAS controller (usually IAS controller No. 2).

IAS controller No. 1 is below the flight compartment floor, at FS315.00, on the left side.

IAS controller No. 2 is above the floor, in the lavatory, at FS600.00, on the left side.

The IAS controllers are attached to the aircraft structure with four screws and are cooled by natural convection around the box. Each IAS controller has four electrical receptacles. The receptacles have different key-ways to prevent an incorrect plug to receptacle connection. IAS controller No. 1 is supplied with 28 VDC from the L ESS BUS. IAS controller No. 2 is supplied with 28 VDC from the R MAIN BUS.

Each IAS controller is divided into two different sub-controllers identified as channel A and channel B. The cabin-pressure control system uses the IAS controllers channels as follows:

• IAS controller No. 1 channel A is related to the system's automatic (Auto 1) and manual modes
• IAS controller No. 1 channel B is related to the pressure sensor indication on the EICAS
• IAS controller No. 2 channel A is related to the system's automatic mode (Auto 2)
• IAS controller No. 2 channel B is not used

In the automatic mode of operation, the IAS controllers do the functions that follow:

• Work out a cabin pressure schedule related to the aircraft's flight profile
• Work out a control logic related to the aircraft's flight phases
• Control the operation of the system's outflow valve
• Give indications and warnings
• Do start-up, power-up and continuous built-in tests (BIT)
• Be an interface for maintenance data

In the manual mode of operation, the IAS controllers do the functions that follow:

• Control the operation of the system's outflow valve (to control the aircraft cabin-pressure rate of change)
• Give indications

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There is only one outflow valve in the aircraft, and it modulates discharge airflow to control cabin pressurization. The outflow valve is below the passenger compartment floor, at FS462.00, on the right side.

When the outflow valve is opened, a quantity of cabin air is sent overboard. This causes cabin air pressure to decrease. The quantity of air sent overboard is related to the magnitude of the opening of the outflow valve.

When the outflow valve is closed, cabin air pressure increases because no cabin air is sent overboard.

The outflow valve operates in the automatic and manual modes of operation (through a dedicated channel of its actuator). When the cabin pressure control system operates in the automatic mode, the outflow valve is controlled by the active IAS controller. When the cabin pressure control system operates in the manual mode, control of the outflow valve comes from the pressurization control panel. The outflow valve is made out of an electrical double actuator and mechanical hardware.

The mechanical hardware includes the following parts:

• Body
• Butterfly valve
• Sealing ring
• Bearings
• Shaft

The body and the butterfly valve are made out of composite materials (thermoplastics strengthened with fiberglass). The sealing ring prevents unwanted air leakage when the valve is closed. The shaft that goes through the butterfly valve is held in position by two bearings. One of the bearings is found in the body of the outflow valve. The other bearing is found in the electrical double actuator. When the butterfly valve is turned to the open position, cabin air is sent overboard. Operation of the electrical double actuator causes the butterfly valve to open and close. The electrical double actuator includes the following parts:

• Brushless DC motor
• Brush-type DC motor
• Gearbox
• Control box

The brushless DC motor controls operation of the cabin pressure control system in the automatic mode (dedicated automatic channel of the actuator). The brush-type DC motor controls operation of the cabin pressure control system in the manual mode (dedicated manual channel of the actuator).

The gearbox has two isolated channels (one automatic and one manual) connected to the same output. The automatic channel is attached to the brush less DC motor and the manual channel is attached to the brush-type DC motor. Each channel has electrical ends of travel. Worm-gears prevent the gearbox channels to move in the opposite direction and two mechanical stops limit travel of the gearbox output shaft. To make sure that the brush-type DC motor used for the manual channel moves freely, a friction torque limiter is used.

The control box has all the digital and analog hardware necessary to control the brush less DC motor when the cabin pressure system operates in the automatic mode. It also has the hardware necessary to control the brush-type DC motor used for the system manual mode of operation.

In the automatic mode of operation, the outflow valve operates as follows:

• When the control box receives a command from the active IAS controller (through a RS 422 serial link), it energizes and controls operation of the brush less DC motor.
  
  
• If the control box does not receive a command for a period of 80 ms, it stops the brush less DC motor at its current position.

In the manual mode of operation, the brush-type DC motor is directly energized by an external signal. Two end of travel switches protect the motor when the butterfly valve reaches the fully open or fully closed position. No other hardware or software is used for the manual mode of operation.

The outflow valve is fail-safe. It stops in position if it has not received a command for more than 80 ms or when a power loss occurs.

In the actuator if the gearbox output shaft breaks, the butterfly valve rotates to a safe position, approximately 15° closed. This ensures the aircraft air pressurization does not decrease too quickly. It also prevents aircraft air pressurization from increasing too much (which could occur if the butterfly valve went to the fully closed position).

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The safety valves give cabin pressure relief against too much positive cabin pressure and negative cabin pressure. There are two safety valves in the aircraft. They are at the top of the aft pressure bulkhead, FS669.76 at RBL9.00 and LBL9.00. Each valve is attached to the aircraft structure with six bolts. The bolts are safe tied with lock-wire.

Each safety valve is made out of the following parts:

• Body, which is attached to the aircraft structure
• Overpressure relief box

  • In the overpressure relief box, a manometric capsule operates a valve
  • Inside the manometric capsule, there is cabin pressure; outside the manometric capsule, there is static pressure

• Poppet, a diaphragm and a cover which make up the servo chamber
• Pipe which lets cabin air pressure into the servo chamber through the filter and the nozzle
• Spring that keeps the poppet on its seat

When there is too much positive pressure in the cabin area of the aircraft (the maximun positive differential pressure is 9.25 ± 0.15 psid [63.78 ± 1.03 kPa]), the safety valves operate as follows:

• Bellows that are in the manometric capsule sense the difference in pressure between the pressurized and the non-pressurized areas of the aircraft.
  
  
• When the difference in pressure between the pressurized and the non-pressurized areas of the aircraft becomes too great, the bellows in the manometric capsule expand. This opens the valve.
  
  
• When the valve opens, the air pressure inside the servo chamber decreases. This causes the poppet to compress the spring.
  
  
• While the poppet compresses the spring, air flows from the pressurized area to the not pressurized area of the aircraft.

When negative pressure is sensed in the cabin area of the aircraft (the maximun negative differential pressure is 0.5 psid [3.45 kPa]), the safety valves operate as follows:

• Poppet is kept on its seat by the force of the spring.
  
  
• When the air pressure in the non-pressurized area of the aircraft becomes more than the one in the pressurized area, the pressure force on the diaphragm becomes larger than the force of the spring.
  
  
• When the pressure force on the diaphragm overrides the force of the spring, air flows from the non-pressurized area to the pressurized area of the aircraft.

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The pressurization control panel is in the flight compartment on the center pedestal. It contains all the switches necessary to control the cabin pressure control system in the automatic and manual modes of operation.

The pressurization control panel has the following switches:

MANUAL PBA to select either automatic or manual mode of operation.

• When the PBA is pushed in, the cabin-pressure control system goes in the manual mode of operation and the PBA lamp comes on.
• Changes between the automatic and the manual mode of operation are smooth during all phases of aircraft operation.
• The PBA is also used as a reset switch to restore the system to its normal configuration after a resettable fault condition has occurred. When a change from the manual mode to the automatic mode of operation is done, resettable fault conditions in the two IAS controllers are reset.

A MAN RATE button to manually set the cabin-pressure rate of change. This function operates only when the cabin-pressure control system is in the manual mode of operation.

An EMER DEPRESS PBA to quickly release aircraft cabin air pressure. Emergency depressurization is a function that operates in the manual and automatic modes of operation. The lamp that is part of the PBA comes on to show an emergency depressurization on status.

• The EMER DEPRESS PBA has a protective cover that has to be lifted to get access to the switch. This helps prevent an accidental emergency depressurization of the aircraft.

A DITCHING PBA to start an automatic ditching sequence when the cabin-pressure control system is in the automatic mode of operation. The lamp that is part of the PBA comes on to show a ditching on status.

• The DITCHING PBA has a protective cover that has to be lifted to get access to the switch. This helps prevent an accidental start of the automatic ditching sequence.

Note:
It is also possible to prepare for ditching when the cabin pressure control system is in the manual mode. A specified ditching procedure must then be done manually.

A LNDG ALT rotary switch. Landing altitude data can be automatically received from the flight management system (FMS) with the selection of FMS or with the selection of FMS1 or FMS2 (on aircraft with the second control display unit (CDU) option installed). In the MAN position, landing altitude data can be manually changed from the LNDG ALT rotary switch.

• Landing altitude can be manually changed at all times
  
  
• When landing altitude is manually changed, the adjustment starts from the present value of landing altitude (the present value of landing altitude is shown on the EICAS)
  
  
• The landing altitude range is from -1,500 ft to +14,000 ft (-457 m to +4,267 m)
  
  
• The minimum change through the LNDG ALT rotary switch is 20 ft (6 m)
  
  
• If landing elevation has not been received from the FMS or manually set from the LNDG ALT rotary switch, the landing elevation default values are as follows:
  
  

  • 0 ft (0 m) on ground
  • 7,850 ft (2,393 m) in flight

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The ditching flapper valve is installed in the lower fuselage, at FS462.00, below the outflow valve.

The function of the ditching flapper valve is to decrease the flow of water that gets into the fuselage if the aircraft makes an emergency landing on water. The ditching valve closes approximately 80% of the opening of the outflow valve adapter.

In usual operation of the cabin-pressure control system, the ditching flapper valve stays open by gravity. If the lower fuselage gets in the water, the pressure of the water closes the ditching flapper valve.

The ditching flapper valve installation includes the components that follow:

• A flapper valve assembly
• A mounting bracket

The flapper valve assembly is made of a plate, a panel, and a hinge assembly. The plate and hinge assembly are attached to the panel with rivets. The flapper valve assembly is attached to the mounting bracket with bolts. The mounting bracket is attached to the lower fuselage structure.

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Usual control of the aircraft cabin pressurization on ground and in flight is done by airflow leakage through the system outflow valve. The cabin pressure control system has the following two modes of operation:

• Automatic — Each IAS controller automatically controls the system outflow valve
• Manual — System outflow valve controlled by the pressurization control panel

Automatic or manual mode of operation is selected through the pressurization control panel. Smooth changes between the automatic and manual mode of operation is possible at any time.

In the automatic mode, control of the cabin pressurization comes from one of the two IAS controllers. While one of the IAS controllers is in control of the cabin pressure system, the other one is in standby mode. The in-control IAS controller is known as the active IAS controller (usually IAS controller No. 1). The IAS controller that is not in-control is known as the standby IAS controller (usually IAS controller No. 2). To control cabin pressurization, the active IAS controller opens or closes the system outflow valve.

In automatic mode, the active IAS controller regulates cabin pressurization to the scheduled cabin altitude.

In the manual mode, control of the cabin pressurization is via the pressurization control panel. In this mode, selections on the control panel open or close the system outflow valve.

The cabin pressure control system has two safety valves that operate independently of the system automatic and manual modes of operation. The safety valves give cabin pressure relief when either of the following conditions occur:

• Too much positive cabin pressure
• Negative cabin pressure

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Usually, when the cabin pressure system operates in automatic mode and immediately after the startup and power-up built-in tests (BIT), IAS controller No. 1 becomes the active controller and IAS controller No. 2 becomes the standby controller.

If a major fault condition is sensed in IAS controller No. 1, it becomes the standby controller and IAS controller No. 2, if possible, becomes the active controller. If a major fault condition is sensed in the two IAS controllers, the two of them become standby. At that time, an AUTO PRESS FAIL caution message will show on the EICAS.

To manage the active/standby logic, the IAS controllers are interlinked to each other. The interlink is made up of one bi-directional ARINC 429 link and two discrete signals. If there is an interlink loss (ARINC and discrete signals), the two IAS controllers become standby and the system manual mode of operation must be used.

The theory of operation is as follows:

A pressure sensor found in each IAS controller measures the quantity of cabin air pressure found in the aircraft. In the active IAS controller (usually IAS controller No. 1), this pressure measurement is then compared to a value known as the target cabin pressure.

Note:
Target cabin pressure is a value that tends to become the same as scheduled cabin altitude, but takes the cabin altitude rate limit of change into consideration.

Scheduled cabin altitude represents the quantity of cabin air pressure that should be in the aircraft for a given aircraft configuration and flight sequence.

If there is a difference between the pressure measurement and the target cabin pressure, the active IAS controller opens or closes the system outflow valve until cabin pressurization is the same as target cabin pressure.

Control of the outflow valve by the active IAS controller is done at a small rate of change. This is to let cabin pressurization change gradually and prevent sudden changes in cabin pressurization.

The active IAS controller uses data from the data concentrator unit (DCU) and the pressurization control panel to calculate the scheduled cabin altitude.

System Inputs

The principle of operation of the cabin pressure control system is to control cabin altitude and cabin altitude rate of change on the ground and in flight. To do so, the active IAS controller uses external data to calculate the scheduled cabin altitude. The external data comes from the EICAS DCU and the pressurization control panel.

The following data comes from the DCU:

• Aircraft altitude
• Aircraft vertical speed and barometric correction
• Landing elevation
• Weight on wheels (WOW)
• Engine throttle position
• Door switch position

The following data comes from the pressurization control panel:

• Automatic/manual mode selection
• Landing altitude manual selection
• Ditching selection
• Emergency depressurization selection

Aircraft Configurations and Flight Sequences

Aircraft configurations and flight sequences can be as follows:

• Automatic pre-pressurization sequence on the ground
• Automatic depressurization sequence on the ground
• Takeoff and return to base sequence (abort capability)
• Flight sequences (climb, cruise, and descent)

Automatic Pre-pressurization Sequence on the Ground

The function of the automatic pre-pressurization sequence on ground is to prevent a cabin bump effect caused by air overpressure that occurs outside the aircraft, under the fuselage, during aircraft rotation at take-off.

Note:
When the aircraft takes-off, the air pressure under the fuselage suddenly and momentarily increases. If no action is taken to prevent that sudden air pressure increase to enter the cabin area through the outflow valve (reverse airflow), it will be felt by the persons aboard.

A cabin bump effect also occurs during aircraft landing.

To prevent a cabin bump effect, cabin air pressure is automatically increased so that it becomes equal to the cabin air pressure measured at throttle transient from idle to takeoff power plus 0.16 psi (1.10 kPa) of differential pressure.

Because cabin air pressure is adjusted to be more than the amount of increased air pressure (under the fuselage) that would otherwise enter the cabin area through the outflow valve (reverse airflow), the effect of the cabin bump is canceled.

During the automatic pre-pressurization sequence on ground, cabin pressurization is controlled with a pressure rate limit of change of +0.16 psi/min (+1.10 kPa/min). This pressure rate limit of change is equivalent to 300 ft/min (91 m/min) at sea level.

The automatic pre-pressurization sequence on the ground is started when the following conditions occur:

• WOW signals indicate aircraft on ground, and
• Both throttle positions are higher than the takeoff position (for example, more than 43.5degrees)

In the case of a takeoff without air-conditioning supply, the outflow valve is set to the closed position.

When the WOW switch indicates aircraft in flight, the takeoff sequence is started.

Automatic Depressurization Sequence On Ground

During landing, to prevent a cabin bump effect, cabin air pressure is automatically increased so that it becomes equal to the landing altitude pressure +0.16 psi (+1.10 kPa) of differential pressure.

The function of the automatic depressurization sequence on ground is to remove the differential pressure that was put into the aircraft to prevent the cabin bump effect during landing. It is started when the conditions that follow occur.

• WOW signals indicate aircraft on ground
• One (left or right) throttle signal is not on take-off power position (for example, less than 43.5 degrees)

After landing, when the WOW signals are received by the active IAS controller, cabin depressurization is controlled as follows:

• Cabin pressurization is removed at a pressure rate limit of change of -0.27 psi/min (-1.86 kPa/min) during 45 sec. This pressure rate limit of change is equivalent to +500 ft/min (+152 m/min), at sea level.
  
  
• After 45 seconds, cabin pressurization is removed at a pressure rate limit of change of -1.17 psi/min (-8.07 kPa/min) until the outflow valve is fully opened. This pressure rate limit of change is equivalent to +2 000 ft/min (+610 m/min) at sea level.

Note:
Normal depressurization time is less than 45 sec.

Take-Off And Return To Base Sequence (Abort Capability)

The function of the take-off and return to base sequence is to prevent the aircrew to set a landing altitude again in the case of an aborted flight and emergency return to the take-off field.

The take-off sequence is started when the WOW switch goes from the aircraft on ground position to the aircraft in flight position.

During this sequence, as long as 10 minutes since take-off are not elapsed and aircraft altitude is lower than the take-off altitude +6 000 ft (+1 829 m), cabin pressurization is controlled as follows:

• Target cabin pressure moves towards scheduled cabin pressure and the cabin pressure rate of change is the same as the cabin pressure rate of change used for the climb sequence.
  
  
• If the aircraft starts to descend (aircraft vertical speed less than -500 ft/min (-152 m/min) during 10 seconds), the return to base sequence is started. The scheduled cabin pressure is then reset to pre-pressurization value (cabin pressure measured at throttle transient from idle to take-off power +0.16 psi (+1.10 kPa) of differential pressure).
  
  
• During the return to base sequence, the cabin pressure rate limit of change is limited within the range that follows: -0.37 psi/min (-2.55 kPa/min) to +0.37 psi/min (-2.55 kPa/min). This pressure rate limit of change is equivalent to +700 ft/min (+213 m/min) to -700 ft/min (-213 m/min) at sea level.

In the case of a throttle switch fault condition in idle position, the pre-pressurization sequence will not be done. The system will instead go directly from the depressurization sequence to the take-off sequence as soon as the WOW switch indicates aircraft in flight.

A flight sequence is started 10 minutes after take-off time or when the aircraft altitude is more than the take-off altitude +6 000 ft (+1 829 m).

Flight Sequences (Climb, Cruise, Descent)

When the WOW switch indicates aircraft in flight and the take-off sequence is completed, the applicable flight sequence starts.

The flight sequences are divided in three categories. They are as follows:

• The climb sequence
• The cruise sequence
• The descent sequence

The climb sequence occurs when the aircraft climb rate is more than 500 ft/min (152 m/min) during a period of more than 10 seconds.

The cruise sequence occurs when one of the conditions that follow are met:

• The aircraft does not climb or descend by more than 500 ft/min (152 m/min) during a period of more than 10 seconds.
• The aircraft descent rate is less than 500 ft/min (152 m/min) during a period of 3 minutes and the aircraft altitude is more than 25 000 ft (7 620 m).

The descent sequence occurs when the aircraft descent rate is more than 500 ft/min (152 m/min) during a period of more than 10 seconds.

During the flight sequences, the active IAS controller uses the data that follow to control cabin pressurization (to calculate cabin-pressure (scheduled cabin altitude) and cabin-pressure rate of change).

• The landing elevation
• The aircraft altitude
• The maximum climb performance of the aircraft (vertical speed)
• The usual differential pressure of 8.84 psi (60.95 kPa) which gives a cabin altitude of 7,850 ft (2393 m) for maximum flight altitude of 45,000 ft (13,716 m).
  
  
• For zoom climb, the need for more differential pressure margin when aircraft altitude is between 28,000 ft (9,144 m) and 45,000 ft (13,716 m).

Flight Sequences With Landing Field Altitude That Is More Than 7,850 ft (2,393 m)

Flight sequences that have a landing field altitude that is more than 7,850 ft (2,393 m) do not need more crew intervention than usual. The crew only needs to input the landing field altitude before take-off.

During flight, the scheduled cabin altitude is automatically limited to the lowest value between 7,850 ft (2,393 m) and a theoretical cabin-pressure value related to the aircraft altitude and maximum aircraft performance.

When the aircraft starts to descend (descent rate is more than 500 ft/min (152 m/min) during a period of more than 10 seconds) and the altitude of the aircraft is less than 41,000 ft (12,497 m), the scheduled cabin altitude is automatically reset to the landing field altitude.

Software Safety Functions

Maximum Positive Differential Pressure Relief

In some cases, it is possible that a usual flight sequence will cause cabin differential pressure to be more than its maximum. This can occur in cases of extreme zoom climb. It can also occur when the cabin-pressure control system automatically goes into a climb flight sequence after the aircraft was flown at maximum differential pressure in the manual mode of operation, etc. Such situations could cause the CABIN DELTA P warning message to show on the EICAS.

To prevent the CABIN DELTA P warning message to show on the EICAS during the situations described above, the IAS controllers' software have a special positive differential pressure control-loop that overrides the usual control loop.

This software safety function does not operate when the cabin pressure control system is in the manual mode of operation.

Cabin Altitude and Cabin Altitude Rate of Change Limitation

If an outflow valve malfunction caused it to open at full speed, the cabin altitude and/or cabin altitude rate of change could become too much very quickly.

To prevent that to occur, the IAS controllers have a software function that monitors the cabin altitude and cabin altitude rate of change. That software function overrides the usual control loop.

If the cabin altitude or the cabin altitude rate of change becomes too much, a hardware not enable signal is sent to the outflow valve hardware driver. This stops the movement of the outflow valve and prevents it to become fully open. This also gives the aircrew time to take corrective action through the system's manual mode of operation or to do an emergency descent if necessary.

A hardware-not-enable signal is sent to the outflow valve when either of the following conditions are met:

• When the cabin altitude is more than 14,500 ft (4,420 m)
• When the cabin altitude rate of change is more than 1.59 psi/min (10.96 kPa/min). This pressure rate change is equivalent to 3,000 ft/min (914 m/min) at sea level.

This function operates only when IAS controller No. 1 is the active controller. It does not operate when IAS controller No. 2 is the active controller or when the cabin pressure control system is in the manual mode of operation.

Outflow Valve Opening Speed Limitation

After the cabin altitude and cabin altitude rate of change limitation has been in function, the outflow valve opening speed is limited. This limitation occurs for a period of 30 seconds. During this period, the opening speed of the outflow valve is limited to 1.8° per second.

This software function operates only when the cabin pressure control system is in the automatic mode of operation. It does not operate when the cabin pressure control system is in the manual mode of operation.

Emergency Depressurization

Automatic

When the cabin-pressure control system operates in the automatic mode and the EMER DEPRESS PBA on the PRESSURIZATION control panel is pushed in, cabin depressurization starts at a controlled rate. The rate of depressurization is +3,500 ft/min (+1,067 m/min) up to a cabin altitude of 14,500±500 ft (4,420±152 m). When cabin altitude becomes 14,500±500 ft (4,420±152 m), it is kept to that level.

The emergency depressurization function overrides the following functions:

• Usual automatic mode of operation of the cabin pressure control system
• Maximum positive differential pressure relief function
• Cabin altitude and cabin altitude rate of change limitation function
• Software ditching function

Note:
The emergency depressurization function does not override the outflow valve opening speed limitation function. The IAS controllers receive the emergency depressurization command through discrete inputs.

Manual

When the EMER DEPRESS PBA on the pressurization control panel is pushed, the outflow valve is moved to the fully open position. Movement of the valve is done through the brush-type DC motor (dedicated manual channel of the outflow valve's electrical double actuator).

A hardware cabin altitude limitation overrides the manual emergency depressurization selection, but the cabin altitude rate of change limitation does not operate. Consequently, the cabin can be de-pressurized quickly but a cabin altitude overshoot between 15,000 to 20,000 ft (4,572 to 6,096 m) during a few seconds, can occur.

Note:
It is also possible to do an emergency depressurization of the aircraft cabin with the MAN RATE button found on the pressurization control panel. To do so, the MAN RATE button must be set to its fully UP position (maximum climb rate of 2,500 ft/min [762 m/min]).

Ditching Sequence

When the DITCHING PBA on the pressurization control panel is pushed, an automatic ditching sequence begins. The following actions are completed by the active IAS controller:

• Air-conditioning system is turned off
• Outflow valve is opened to de-pressurized the cabin
• Outflow valve is closed when cabin depressurization is complete (when cabin differential pressure is less than 0.16 psi [1.10 kPa] during 10 seconds or after a maximum of 100 seconds after the end of the outflow valve opening)

Note:
The automatic ditching sequence described above does not occur if the aircraft altitude is more than 15,000 ft (4,572 m). The IAS controllers receive the ditching command through discrete inputs.

It is possible to prepare for ditching even when the automatic ditching sequence is not available. To do so, the following procedure can be done manually:

1. Aircraft air-conditioning system must be de-energized.
2. EMER DEPRESS PBA on the pressurization control panel must then be pushed to de-pressurize the cabin.
3. When the cabin is de-pressurized, the EMER DEPRESS PBA must be pushed in again.
4. MANUAL PBA on the pressurization control panel must be pushed in.
5. The MAN RATE button on the pressurization control panel must be turned to the fully DN (down) position to close the outflow valve.

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The cabin pressure control system manual mode of operation is set when the MANUAL PBA on the pressurization control panel is pushed. When selection of the manual mode is made, the automatic mode is disengaged. The IAS controllers receive the manual command through discrete inputs.

The manual mode operates independently of the IAS controller software. To do so, it uses the following hardware items:

• Switches and potentiometer circuit from the pressurization control panel
• Analog circuit and a pressure sensor in IAS controller No. 1
• Brush-type DC motor (dedicated manual channel of the outflow valve electrical double actuator) and an isolated gear in the outflow valve assembly

Cabin Pressure Rate Of Change Control

To manually set the cabin pressure rate of change, the MAN RATE button on the pressurization control panel must be used. The position of the MAN RATE button gives the following selections:

• The 12 o’clock position is the neutral (0 ft/min, 0 m/min) position. There is a small dead angle around the 12 o’clock position to make it easier to find.
  
  
• Selections from –1,000 to 1,000 ft/min (–305 to 305 m/min) are found approximately at the two thirds of the MAN RATE button range.
• Cabin rate of change is limited to ±2,500 ft/min (±762 m/min). These maximums are set when the MAN RATE button is at its fully UP or DN (down) position.

When the cabin pressure control system is in the manual mode of operation, control of the cabin pressure rate of change is done by IAS controller No. 1. IAS controller No. 1 then operates the outflow valve through the brush-type DC motor (dedicated manual channel of the outflow valve electrical double actuator). Selections made with the MAN RATE button are sent to IAS controller No. 1 as analog inputs.

Emergency Depressurization

When the EMER DEPRESS PBA on the PRESSURIZATION control panel is pushed in, the outflow valve is moved to the full open position. Movement of the valve is done through the brush-type dc motor (dedicated manual channel of the outflow valve's electrical double actuator).

A hardware cabin altitude limitation overrides the manual emergency depressurization selection, but the cabin altitude rate of change limitation will not operate. The consequence of this is that the cabin can be de-pressurized quickly, but a cabin altitude overshoot above 15,000 ft (4,572 m) (up to 20,000 ft (6096 m) during a few seconds) can occur.

Note:
It is also possible to do an emergency depressurization of the aircraft cabin with the MAN RATE button found on the PRESSURIZATION control panel. To do so, the MAN RATE button must be set to its full UP position (maximum climb rate of 2,500 ft/min (762 m/min).

Ditching Sequence

It is possible to prepare for ditching even when the automatic ditching sequence is not available. To do so, the procedure that follows can be done manually.

• The aircraft air conditioning system must be de-energized.
• The EMER DEPRESS PBA on the PRESSURIZATION control panel must then be pushed in to de-pressurize the cabin.
• When the cabin is de-pressurized, the EMER DEPRESS PBA must be pushed in again.
• After that, the MANUAL PBA on the PRESSURIZATION control panel must be pushed in.
• The MAN RATE button on the PRESSURIZATION control panel must be turned to the full DN (down) position to close the outflow valve.

System Safeties

Safety Valve

The two safety valves that are part of the cabin pressure control system operate independently from the other system components. They override the system’s automatic and manual mode of operation.

They give cabin pressure relief against too much positive pressure and negative pressure.

Positive Pressure Relief

When a maximum positive cabin differential pressure of 9.25 ± 0.15 psid (63.78 ± 1.03 kPa) is reached, the safety valves open and send cabin air into the aft equipment compartment. The maximum positive cabin differential pressure stays controlled even if one of the two safety valves becomes unserviceable.

Negative Pressure Relief

Although negative cabin differential pressure is unusual, it may sometimes occur. For instance, during an emergency aircraft descent with the air-conditioning system de-energized, external air pressure can become more than cabin air pressure.

When a negative cabin differential pressure of 0.5 psid (3.45 kPa) is reached, the safety valve opens. This lets external air into the cabin.

Cabin Altitude Limitation

The cabin altitude limitation function overrides the cabin pressure control system automatic and manual modes of operation. It is started when one of the following conditions occur:

• Cabin altitude reaches 14,500 ± 500 ft (4,420 ± 152 m)
• Cabin altitude rate of change reaches 1.59 psi/min (10.96 kPa/min). This pressure rate of change is equivalent to 3,000 ft/min (914 m/min) at sea level.

Cabin pressure is sensed by the pressure sensors in the IAS controllers (pressure sensor in IAS controller No. 1 when the cabin pressure control system operates in the manual mode, and pressure sensor in IAS controller No. 2 when the cabin pressure control system operates in the automatic mode). When one of the conditions given above occurs, the applicable IAS controller supplies a cabin altitude limitation signal to the system outflow valve.

When the outflow valve receives the cabin altitude limitation signal, the following actions occur:

• Brushless DC motor (dedicated automatic channel of the outflow valve electrical double actuator) is de-energized.
• Brush-type DC motor (dedicated manual channel of the outflow valve electrical double actuator) is energized and the outflow valve is closed completely.

The cabin altitude limitation signal stays active until the following conditions occur:

• Cabin altitude becomes less than 14,500 ± 500 ft (4,420 ± 152 m)
• Cabin altitude rate of change becomes less than 1.59 psi/min (10.96 kPa/min). This pressure rate of change is equivalent to 3,000 ft/min (914 m/min) at sea level.

The cabin altitude limitation function has the beneficial effect to prevent a full opening of the outflow valve. It also ensures cabin altitude does not become more than 15,000 ft.

Displays

The CABIN ALTITUDE warning message shows on the EICAS when cabin altitude is more than the cabin-altitude warning threshold.

The usual cabin-altitude warning threshold is 9 400 ft (2 875 m). When the cabin-pressure control system operates in the automatic mode, this threshold can be changed. The threshold can be changed if there is a high altitude landing and/or take-off at more than 7 850 ft (2 393 m).

The CABIN ALTITUDE warning threshold is calculated as follows:

• The cabin-altitude warning threshold is equal to the cabin-altitude caution threshold +900 ft (+274 m), from 9,400 ft (2,865 m) to 14,300 ft (4,359 m).

The CABIN DELTA P warning message shows on the EICAS with one of the conditions that follows:

• The cabin positive differential-pressure is more than 9.20 psid (63.43 kPa)
• The cabin negative differential-pressure is more than 0.50 psid (3.45 kPa)

The AUTO PRESS FAIL caution message shows on the EICAS with one of the conditions that follows:

• The EICAS does not receive pressure data from the IAS controller No. 1 and/or the IAS controller No. 2 during three seconds.
• The EICAS receives an AUTO PRESS FAIL caution signal from the IAS controller No. 1 and/or the IAS controller No. 2 during three seconds. 

The CABIN ALTITUDE caution message shows on the EICAS when cabin altitude is more than the cabin-altitude caution threshold.

The usual cabin-altitude caution threshold is 8,500 ft (2,591 m). This threshold is equal to 7,850 ft (2,393 m) +650 ft (+198 m). When the cabin-pressure control system operates in the automatic mode, this threshold can be changed. The threshold can be changed if the altitude of the take-off and/or landing is more than 7,850 ft (2,393 m). In this case, the cabin caution threshold is increased to more than 8,500 ft (2,591 m). This prevents an unwanted CABIN ALTITUDE caution message when the aircraft does a high altitude take-off or landing.

When the take-off or landing is more than 7,850 ft (2,393 m), the initial value of the cabin-altitude caution threshold becomes equal to the cabin altitude +650 ft (+198 m). This occurs from 8,500 ft (2,591 m) to 14,300 ft (4,359 m) and at a maximum aircraft altitude of 41,000 ft (12,497 m). When the cabin altitude is more than 41,000 ft (12,497 m), the cabin caution threshold is 8,500 ft (2,591 m).

The cabin-altitude caution threshold is calculated with the parameters that follow: 

• Aircraft altitude
• Take-off altitude
• Landing altitude
• Aircraft vertical speed
• Flight sequence (pre-pressurization, take-off, return-to-base, climb, cruise, descent, or depressurization)

The cabin altitude caution threshold for high airport altitudes (more than 7,850 ft (2,393 m) is calculated as follows:

• In the case of the take-off and return-to-base sequence, with airport altitude of more than 7,850 ft (2,393 m): The cabin-altitude caution threshold stays set at take-off altitude +650 ft (+198 m) and is limited to 14,300 ft (4,359 m).
  
  
• In the case of the take-off sequence, from an airport altitude of more than 7,850 ft (2,393 m) and to an airport altitude of less than 7,850 ft (2,393 m): The cabin-altitude caution threshold (initial value of take-off altitude +650 ft (+198 m)) gradually decreases in relation to the aircraft altitude and rate of climb (minimum rate of -300 ft/min (-91 m/min)), until it becomes 8,500 ft (2,591 m).
  
  
• In the case of a climb sequence, after a take-off from an airport altitude of more than 7,850 ft (2,393 m) and to an airport altitude of more than 7,850 ft (2,393 m): The cabin-altitude caution threshold (initial value of take-off altitude +650 ft (+198 m)) gradually decreases in relation to the aircraft altitude and rate of climb (minimum rate of -500 ft/min (-152 m/min)), before the aircraft altitude of 41,000 ft (12,497 m).
  
  
• In the case of a climb sequence, after a take-off from an airport altitude of less than 7,850 ft (2,393 m) and to an airport altitude of more than 7,850 ft (2,393 m): The cabin altitude caution threshold stays at 8,500 ft (2,591 m).
  
  
• In the case of a cruise sequence, after a take-off from an airport altitude of less than 7 850 ft (2 393 m) and to an airport altitude of more than 7 850 ft (2 393 m): The cabin altitude caution threshold continues to decrease in relation to the aircraft altitude and rate of climb (minimum rate of -300 ft/min (-91 m/min)), until it becomes 8,500 ft (2,591 m).
  
  
• In the case of a cruise sequence, after a take-off from an airport altitude of less than 7,850 ft (2,393 m) and to an airport altitude of more than 7,850 ft (2,393 m): The cabin altitude caution threshold stays at 8,500 ft (2,591 m).
  
  
• In the case of a decent sequence, to an airport altitude of more than 7,850 ft (2,393 m): The cabin altitude caution threshold stays at 8,500 ft (2,591 m) until the aircraft altitude is 41,000 ft (12,497 m). When the aircraft altitude becomes less than 41,000 ft (12,497 m), the cabin threshold gradually increases in relation to the aircraft altitude and rate of decent, and becomes equal to the landing altitude +650 ft (+198 m) and is limited to 14,300 ft (4,359 m).

The CABIN PRESS FAULT caution message shows on the EICAS with the conditions that follow:

• The IAS controller No. 1 or No. 2 (auto channel) senses a major fault condition
• The IAS controller No. 1 or No. 2 (auto channel) senses the altitude limiter to be defective
• The EICAS senses that all data that comes from the IAS controller No. 1 or No. 2 (auto channel) is lost

The DITCHING NOT AVAIL caution message shows on the EICAS when the DITCHING PBA is pushed in and the system is in manual mode.

The CABIN ALT WARN HIGH advisory message shows on the EICAS when a high airport take-off or landing altitude has been initiated and the pressurization system is in the automatic mode. The cabin altitude warning threshold is set above 9 400 ft (2 865 m).

The MANUAL PRESS FAIL advisory message shows on the EICAS when the system's manual mode of operation is sensed to be unserviceable.

The DITCHING ON status message shows on the EICAS when the DITCHING PBA is pushed in (ditching sequence active) and the system is in automatic mode.

The EMER DEPRESS ON status message shows on the EICAS when EMER DEPRESS PBA is pushed in (emergency depressurization active).

The MAN PRESS ON status message shows on the EICAS when the cabin-pressure control system operates in the manual mode.

The EICAS messages that follow are related to the cabin-pressure control system:

EICAS MESSAGES	LEVEL (COLOR)
CABIN ALTITUDE	WARNING (red)
CABIN DELTA P	WARNING (red)
AUTO PRESS FAIL	CAUTION (amber)
CABIN ALTITUDE	CAUTION (amber)
CABIN PRESS FAULT	CAUTION (amber)
DITCHING NOT AVAIL	CAUTION (amber)
CABIN ALT WARN HIGH	ADVISORY (cyan)
MANUAL PRESS FAIL	ADVISORY (cyan)
DITCHING ON	STATUS (white)
EMER DEPRESS ON	STATUS (white)
MAN PRESS ON	STATUS (white)

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The cabin-pressure control system has interfaces with the aircraft systems/components that follow:

• Wing Cross-Bleed Valve
• Engine Indication and Crew Alerting System (EICAS)
• Flight Management System (FMS)
• Cross-Bleed Valve
• Maintenance Diagnostic Computer (MDC)

The integrated air system (IAS) controllers interface with many aircraft systems. Those systems, and a brief description about their relationship with the IAS controllers follow:

Avionics Cooling System

Through discrete inputs, discrete outputs, and 28 VDC torque motor power outputs, the IAS controllers monitor and control operation of the avionics cooling fan and avionics cooling valve.

Through analog inputs, the IAS controllers get data from the equipment racks vent temperature sensors to control operation of the avionics cooling fan and avionics cooling valve.

As applicable, avionics cooling system fault conditions sensed by the IAS controllers are shown on the EICAS.

Filtering and Flow Control System

Through 28 VDC torque motor power outputs, each IAS controller controls the operation of its related flow control valve (FCV). IAS controller No. 1 controls FCV No. 1 and IAS controller No. 2 controls FCV No. 2.

Through a discrete input, each IAS controller monitors the fully closed position of its related FCV. IAS controller No. 1 monitors the fully closed position of FCV No. 1 and IAS controller No. 2 monitors the fully closed position of FCV No. 2.

Through analog inputs, each IAS controller gets data from its related differential pressure sensor. IAS controller No. 1 gets data from differential pressure sensor No. 1, and IAS controller No. 2 gets data from differential pressure sensor No. 2. Data from the differential pressure sensors calculates true mass airflow measurement of the hot bleed air that comes from the pneumatic system and to calculate airflow schedules for each FCV.

As applicable, filtering and flow control system fault conditions sensed by the IAS controllers are shown on the EICAS.

Air Conditioning System

Through analog inputs, the IAS controllers use data from the pack temperature sensor (PTS) to control operation of the air-conditioning system and to prevent ice from forming in the water extractor.

Through analog inputs, the IAS controllers use data from the pack discharge temperature sensor (PDTS) to monitor temperature in the pack discharge duct. The measurements from the PDTS prevent ice formation from occurring and preventing too much heat at the output of the air conditioning system. The measurements from the PDTS are also used for the manual control of the air conditioning system.

Through analog inputs, the IAS controllers use data from the compressor discharge temperature sensor (CDTS) to find out if the compressor temperature gets too high.

Through 28 VDC torque motor power outputs, IAS controller No. 1 controls operation of the temperature control valve (TCV).

Through discrete inputs, IAS controller No. 1 monitors the fully closed and fully opened position of the TCV.

As applicable, air-conditioning system fault conditions sensed by the IAS controllers are shown on the EICAS.

Ram Air System

Through 28 VDC torque motor power outputs, IAS controller No. 2 controls the operation of the ram air regulating valve.

Through discrete inputs, IAS controller No. 2 monitors the fully closed and fully open position of the ram air regulating valve As applicable, ram air system fault conditions sensed by the IAS controllers are shown on the EICAS.

Temperature Control and Indication System

Through 28 VDC torque motor power outputs, IAS controller No. 2 controls operation of the hot-air regulating and shutoff valves (HARSOV).

Through discrete inputs, IAS controller No. 2 monitors the fully closed and fully opened position of the HARSOV.

Through a discrete input, IAS controller No. 1 monitors the fully closed position of HARSOV No. 1.

Through discrete inputs, the IAS controllers monitor the fully closed and fully opened position of the pre-cooler crossover valve.

Through analog inputs, the IAS controllers use data from the flight compartment duct temperature sensor to control operation of the environmental control system (ECS).

Through analog inputs, IAS controller No. 2 uses data from the cabin duct temperature sensor to control operation of the ECS.

Through analog inputs, the IAS controllers use data from the cabin ventilated temperature sensor to control operation of the ECS.

Through analog inputs, the IAS controllers use data from the flight compartment ventilated temperature sensor to control operation of the ECS.

Through analog inputs, the IAS controllers use data from the hot-air temperature sensor (HATS) to control operation of the ECS.

Through analog inputs, the IAS controllers use data from the pack inlet temperature sensor (PITS) to control operation of the ECS.

As applicable, temperature control and indication system fault conditions sensed by the IAS controllers are shown on the EICAS.

Fire Detection and Extinguishing (FIREX) System

When a fire is detected on the left or right engine, the FIREX gives a ground contact (and thus electrical continuity) to the coil of relay K11 (for the left engine) and relay K68 (for the right engine). This causes the relays to energize and remove the supply voltage that goes to the applicable side (left or right side) intermediate pressure valve (IPV) and high-pressure valve (HPV). Both valves are then released to the closed position and the applicable engine bleed-air source is isolated from the rest of the air systems.

To prevent showing an incorrect L BLEED FAIL or R BLEED FAIL caution message on the EICAS, the FIREX gives the IAS controllers L ENG FIRE or R ENG FIRE data to let them know if fire has been sensed.

L ENG FIRE or R ENG FIRE data does not go directly to the IAS controller, but goes through the following path: a discrete output from the FIREX control unit goes to the data concentrator unit (DCU) which transforms it and sends it to the IAS controllers in ARINC format (label 263, bit 22 for the left engine and label 263, bit 23 for the right engine).

Stall Protection System

Through discrete outputs, IAS controller No. 1 sends a WAI LOW TEMP 1 signal to channel A of the stall protection computer (SPC), and IAS controller No. 2 sends a WAI LOW TEMP 2 signal to channel B of the SPC. The SPC uses those signals for advanced mode purposes.

The cabin-pressure control system monitors and isolates fault conditions up to the line replaceable unit (LRU) level. EEPROM checksum, input signal verification, watchdog reset verification, and discrete signal monitoring are continuously done by the IAS controllers.

Major fault conditions for which aircrew action is necessary are shown on the EICAS. Minor fault conditions that may reduce the system's performance are also shown on the EICAS. Other fault conditions are stored in the MDC for maintenance purposes.

The parameters that follow are shown on the ECS synoptic page:

• Cabin altitude (CAB ALT), in feet, shows in one of the colors that follow:
  

  • Green, when usual
  • Amber, when the CABIN ALTITUDE caution message shows on EICAS
  • Red, when the CABIN ALTITUDE warning message shows on EICAS
  • Magenta dashes, when data is invalid, unknown or out of range

• Cabin pressure rate of change (CAB RATE), in ft/min, shows in one of the colors that follow:
  

  • Green, when usual
  • Magenta dashes, when data is invalid, unknown or out of range

• Cabin differential pressure (CAB ΔP), in psi, shows in one of the colors that follow:
  

  • Green, when usual
  • Red, when the CABIN DELTA P warning message shows on EICAS
  • Magenta dashes, when data is invalid, unknown or out of range

• Landing altitude (LNDG ALT), in feet, shows in one of the colors that follow:
  

  • Cyan, when the landing altitude data has been manually changed from the LNDG ALT rotary switch found on the PRESSURIZATION control panel.
    
    
  • Magenta, when the landing altitude data comes from the FMS
  • Magenta dashes, when data is invalid, unknown or out of range

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Start-up Built-in Test

During the cabin-pressure control system start-up, the tests that follow are done:

• The central processing unit (CPU) test
• The random-access memory (RAM) test
• The pin programming test
• The watchdog test

For the CPU test, instruction test, register test and internal RAM test are done to make sure that the CPU is serviceable.

The RAM test writes data patterns into dedicated RAM areas and then reads and compares the data.

The pin programing test does a pin programming discrete input test.

The watchdog test makes sure that the watchdog circuit is serviceable. The function of the watchdog circuit is to monitor the operation of the IAS controller's software. When the software operates correctly, the watchdog circuit does not take action. When the software does not operate correctly, the watchdog circuit resets the microprocessor found in the IAS controllers. After three resets (one after the other), the microprocessor is continuously reset.

Power-up Built-in Test

During the cabin-pressure control system start-up, a power-up built-in test is done if the conditions that follow are met:

• The cabin-pressure control system is in the automatic mode of operation
• There are no demand from the MDC
• The aircraft is on ground
• No pressurization sequence is started
• There are no emergency depressurization demand
• There are no ditch demand
• Cold start

The power-up built-in test does the tests that follow:

• The electrically erasable programmable read only memory (EEPROM) test
• The outflow valve test
• The altitude limitation test

The EEPROM is tested by a checksum test. The outflow valve test verifies the outflow valve controller RAM. The test writes data patterns into dedicated RAM areas and then reads and compares the data. The altitude limitation test energizes the outflow valve and, through different demands, makes checks related to the speed of operation of the valve. A cabin altitude limitation test is also done in the IAS controllers.

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
A55	INTEGRATED AIR SYSTEM (IAS) CONTROLLER (FWD) A55	ZONE(S) FS315	21-31-01
A56	INTEGRATED AIR SYSTEM (IAS) CONTROLLER (AFT) A56	ZONE(S) FS600	21-31-01
MPE1	OUTFLOW VALVE MPE1	ZONE(S) 142	21-31-05
-	SAFETY VALVE	ZONE(S) 241/242	21-31-09
PL18	PRESSURIZATION CONTROL PANEL PL18	ZONE(S) 211	21-31-13

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