Air Conditioning and Pressurization

Air Conditioning & Pressurization Systems:


Table of Contents -


Normal Operations:
  • Using Ground Preconditioned Air
  • Two Pack Operation
  • Pack Valve Half Flow Operation (Later Airplanes)

Alternate Operations:
  • For Pack Trip Procedures - Chapter 2
  • One Pack Operation
  • Zone or Upper Deck Overheat Light Illuminated
  • Trim Air Valve (Master) Closed
  • Equipment Cooling Smoke Light Illuminated
  • Manual Pack Operation
  • Bypass, Inlet or Exit Doors Not In Preposition
  • Pressurization System - Man L or Man R Mode Operation
  • Pressurization System - Man Mode Operation
  • Unscheduled Cabin Pressure Change
  • Automatic Pressurization Electrical Power Interruption

Controls and Indicators
:
  • Pack Inlet Control - Passenger
  • Pack Inlet Control - Special Freighter
  • Pack Temperature Control - Passenger
  • Pack Temperature Control - Special Freighter
  • Zone Temperature Control - Passenger
  • Zone Temperature Control - Special Freighter
  • Equipment Cooling & Fwd/Aft Cargo Heat
  • Pressurization Control
  • Pressurization Monitors
  • Galley / Lavatory Exhaust Fan
  • Cockpit Air Distribution Controls
  • Zone 4 Air Flow Restrictor Valve - Combi

Schematics:
  • Pack Temperature Control
  • Zone Temperature Control
  • Equipment Cooling
  • Aft Cargo Compartment Heat
  • Pressurization Control
  • Pressurization Control (Later Airplanes)


Supplementary Information:
  • Pressurization System Description
  • Pressurization Relief
  • Air Conditioning System Description
  • Bypass Valve, Inlet and Exit Door Indicators


Air Conditioning & Pressurization - Flight Engineer's Panel






Air Cycle Machine (ACM) Inlet







ACM Outlet








One of the two outflow valves






Normal Operations:


Using Ground Preconditioned Air:

The airplane can be air conditioned by attaching a preconditioned air source to ground service connections for distribution through the conditioned air manifold.


Two Pack Operation:

Two pack operation may be used to reduce bleed air extraction from the engines when desired to reduced fuel consumption.
Perform the procedure after the airplane has reached cruise altitude and cabin temperature has stabilized to desired setting.
It may be desired to return to three pack operation for short intervals, if required to increase circulation.


Pack Valve Half Flow Operation (Later Airplanes):

The pack valve half flow system reduces bleed air extraction from the engines to reduce fuel consumption.
Stabilize cabin temperature at the desired setting before accomplishing this procedure.
Any combination of packs may be used from three at full flow to two at half flow.
Two packs at half flow and one pack shut down can be used for maximum fuel economy while still providing adequate ventilation.
Less than equivalent of two full packs required supplemental vent fan operation.

Warning:  When transporting dry ice in the lower cargo compartment, use of supplemental vent fans may be restricted to prevent excessive CO2 concentration.

Allow cabin pressure to stabilise after each configuration change before continuing the procedures.




Alternate Operations:


For Pack Trip Procedures - Chapter 2

One Pack Operation:
Galley / Lavatory exhaust fan                Auto / On
Recirculating fans                                  On
Gasper fan                                              On
Supplemental vent fans                         On


Zone or Upper Deck Overheat Light Illuminated:

Trim air indicator                            Check



Trim Air Valve (Master) Closed
Equipment Cooling Smoke Light Illuminated
Manual Pack Operation
Bypass, Inlet or Exit Doors Not In Preposition
Pressurization System - Man L or Man R Mode Operation
Pressurization System - Man Mode Operation
Unscheduled Cabin Pressure Change
Automatic Pressurization Electrical Power Interruption

Controls and Indicators
:


Pack Inlet Control - Passenger





Pack Inlet Control - Special Freighter






Pack Temperature Control - Passenger



Pack Temperature Control - Special Freighter



Zone Temperature Control - Passenger

Zone Temperature Control - Special Freighter




Equipment Cooling & Fwd/Aft Cargo Heat:


Smoke Detector Light (Amber):
Illuminates during smoke test or when smoke is detected in the equipment cooling ducting.
Cooling air is automatically discharged overboard.  (System will go to smoke mode).

Valve Control Switch:
NORM (On Ground) - Air is discharged overboard as well as ducted into the forward cargo compartment for heating.
NORM (In Flight) - During initial climb, increasing differential pressure will close the flow control valve and all air will be routed into the forward cargo compartment.
SMOKE - System will go to smoke mode.  Air dumped overboard.
DITCH - Closes flow control valve to prevent water entry upon ditching.

Smoke Detector Test Switch:
TEST - Checks the smoke detection system.  SMOKE light illuminates.

No Air Flow Reset Switch:
Resets the equipment cooling system.  NO AIR FLOW light will extinguish approx 30 seconds after air flow has been restored.

Blower Selector Switch:
Selects either of two main blowers to provide cooling airflow in the lower electronics compartment.
TEST position turns off blowers.  Flight deck blower continues to operate during TEST.
Note:  Both main blowers operate on ground when NORM selected.

No Air Flow Light (Amber):
Illuminates to indicate less than normal cooling air flow.
Ground crew call horn will sound continuously if light illuminates.
Until reset, the system will go to smoke mode and cooling air flow maintained by cabin differential pressure.


Lower Aft Cargo Overheat Light (Amber):

Indicates compartment temperature above normal.  (Override valve closes automatically).

Cargo Heat On Light (Green):
Illuminates when heat supplied to compartment.  (Override and control valve open).

Cargo Heat Switch:
NORMAL - When heat is required, valves open and bleed air heats compartment.
OFF - Closes override and control valves.
TEST - Valves open reardless of compartment temperature.


Pressurization Control

Pressurization Mode Selector:
CHK - Maintenance use only. Not to be used by flight crew.
AUTO - Prevents pressurization on the ground, outflow valves open.  In flight outflow valves will modulate to control cabin climb or descent to the desired altitude.
MAN - Both outflow valves controlled manually.
MAN L - Left outflow valve controlled manually, right outflow valve and remained of system controlled automatically.
MAN R -  Right
outflow valve controlled manually, left outflow valve and remained of system controlled automatically.



Outflow Valve Position Indicators:

Indicates outflow valve position.



Pressurization Rate Selector:

Index mark provides for 500 fpm climb and 300 fpm descent.
Range of control from full decrease to full increase is: 150 to 2500 fpm climb, 100 to 1500 fpm descent.



Rate Limit Test Swi
tch:
Pressing switch initiates an excessive pressurization rate of change signal
With AUTO pressurization mode selected:
- AUTO Fail light will illuminate
- Outflow valves will initially move in the direction to reduce cabin vertical speed.





Pressurization Monitors




Cabin Vertical Speed Indicator.
Indicates climb or descent rate of cabin.

Cabin Altitude Indicator:
Digital counter displays atitude in thousands of feet.  Pointer make one revolution each 1000 feet.

Cabin Differential Pressure Indicator:
Displays differential pressure between cabin and ambient..

Pressure Relief Valve Open Lights (Amber):
Illuminates when corresponding pressure relief valve opens.

Rate Limit Warning Light (Amber):
- Illuminates to indicate AUTO pressurization control failure resulting in an excessive cabin vertical speed.
- Moving the pressurization mode switch to MAN will extinguish the light and reset the pressurization controller.

As Installed:
Auto Fail Warning Light (Amber):
- Illuminates to indicate AUTO pressurization control failure resulting in an excessive cabin vertical speed.
- Moving the pressurization mode switch to MAN will extinguish the light and reset the pressurization controller.




Galley / Lavatory Exhaust Fan:

Flight or Ground:
ON:  Fan operates at all times, overboard discharge valve (#4 Entry Door area) will close.  Venting will be inside pressure hull in vicinity of right outflow valve.
AUTO: Fan operates up to 6.8psi differential pressure, overboard discharge closed.  Venting will be inside pressure hull.  Above 6.8 psi fan shuts off, overboard discharge valve opens, venting will be through the overboard discharge valve.  When Supplemental Vent Fan (later airplanes) is ON the GALLEY/LAVATORY EXHAUST FAN operates in the ON mode regardless of switch position.
OFF:  Fan not operating.  Venting will ne at the overboard discharge valve.


Cockpit Air Distribution Controls:

Adjustable overhead outlets
Fixed overhead outlet
Window 2 & 3 ducts
Control knob - Off or On
Windshield / Foot air controls



Zone 4 Air Flow Restrictor Valve - Combi

Vent control No. 4 Right entry door - Restrictor Air Valve Control Handle.



Schematics:

Pack Temperature Control
Zone Temperature Control
Equipment Cooling
Aft Cargo Compartment Heat
Pressurization Control
Pressurization Control (Later Airplanes)


Supplementary Information:

Pressurization System Description:

The pneumatic system supplies bleed air to the air conditioning and pressurization systems.
Pressurization is maintained and controlled by governing the rate of airflow through two outflow valves.
Each outflow valve can be actuated by either an AC or DC motor.  AUTO mode utilizes 115 VAC and full travel time of the outflow valves is five seconds.  MAN mode utilizes 28 VDC power and full travel time of the outflow valves is twenty-five seconds.

The outflow valves may be controlled by any one of the following modes of operation:

AUTO - The normal operating mode.  Both outflow valves are controlled automatically by the pressurization controller to maintain selected cabin altitude.  Both valves modulate together and maintain approximately the same relative position.

MAN L / MAN R - The selected valve, L or R is controlled by its respective outflow valves manual control switch.  The other valve remains in automatic mode of operation, controlled by the pressurization controller.

MAN - Each outflow valve is controlled by its respective outflow valve control switch.

CHK - Provided for maintainance and is not used by the flight crew.


Automatic Operation:

Cruise cabin altitude is normally set on the flight/cabin altitude selector prior to takeoff.
After takeoff the cabin will climb to the altitude corresponding to the cruise altitude setting at a controlled rate as selected in the pressurization rate selector.
At top of descent, airport elevation and baro setting are selected.
Cabin altitude will descend to the altitude corresponding to the airport altitude setting at a controlled rate as selected.


Differential Pressure Limiter:

There is an automatic differential pressure limiter set to 8.9 psi which is operative in AUTO, MAN L and MAN R modes.
Setting a climb altitude on the flight/cabin altitude selector will result in a differential pressure of 8.9 psi when the airplane reaches the corresponding flight altitude shown on the flight/cabin altitude selector.

When the flight/cabin altitude selector is set above the actual airplane cruise altitude, the cabin altitude will remain constant when minor variations in airplane altitude occur.
If the airplane climbs above the flight altitude selected on the flight/cabin altitude selector, the cabin altitude will increase at the selected rate until the normal maximum differential pressure of 8.9 psi is attained.  This differential will then be maintained as the airplane continues to climb.

If the differential pressure exceeds approximately 8.9 psi, switching from MAN to AUTO of operation drives the outflow valves toward open.
As the differential pressure decreases toward 8.9 psi, the system modulates the outflow valves to maintain 8.9 psi.


Rate Limit Control:

In AUTO, MAN L and MAN R modes, the system has a protective feature called rate limit control.
This feature is armed when cabin altitude is below 10,000 feet and is activated by cabin vertical speed rates above 3,100 feet per minute ascending or 2000 fpm descending.
Activation of this feature is indicated by illumination of the AUTO FAIL (RATE LIMIT) light.
The rate limit control feature is deactivated and AUTO FAIL (RATE LIMIT) light is extinguished if cabin altitude exceeds 10,000 feet.

The rate limit control, using 28 VDC power (low speed), commands the outlfow valves to stop the cabin altitude change within 1,500 feet of the altitude at the time of AUTO FAIL (RATE LIMIT) light illumination.
This mode of operation is designed for interim control only until procedural steps are initiated.
It provides a holding operation which results in small cabin altitude oscillation near the arrested cabin altitude.
Remaining in rate limit control, the cabin altitude gradually drifts at a rate up to +/- 500 sea level feet per minute until eventually a cabin pressure relief valve opens or until cabin altitude exceeds 10,000 feet.

A rate limit test switch is provided for test of the rate limit control circuit.  This test is intended for ground use only.  Pressing this switch in flight activates the rate limit control system and illuminates the AUTO FAIL (RATE LIMIT) light.

Power interruptions of 15 seconds or more while in any auto mode may cause the rate limit control to be activated with AUTO FAIL light illuminated upon power restoration.

Note:  On some airplanes, HF transmissions may activate the rate limit control system and illuminate the AUTO FAIL light.


Maximum Cabin Altitude Override:

There is an additional protective feature called maximum cabin altitude override which is armed in AUTO, MAN L or MAN R modes and is activated when cabin altitude exceeds 10,000 feet.  This feature using 115 VAC power (high speed) drives the outflow valves toward closed position limiting the cabin altitude between 10250 and 14000 feet.

Changing the pressurization mode selector from AUTO to MAN deactivates the maximium differential pressure limiter, rate limit control and maximum cabin override protective features.

When the mode selector is switched from MAN to AUTO there is no outflow vavle movement for approximately four to six seconds while the pressurization control tracks and schedules a signal to the outflow valves.


Pressurization Relief:

Two cabin pressurization relief valves are installed to prevent excessive pressure within the airplane.

If cabin differential pressure reaches 9.25 psi (or 9.7 psi as a back-up) one or both valves will modulate OPEN.

The operating valve(s) will close when the pressure returns below the activating pressure diiferential.

A PRESS RELIEF light for each relief valve will illuminate whenever its respective valve is open and will extinguish when the valve closes.

On late model three pack airplanes, the No. 2 air conditioning pack will automatically trip to assist in relieving excess cabin pressure, if either cabin pressure relief valve opens.

The pack cannot be reset until both cabin pressure relief valves are closed.




Negative pressure relief valves are installed in the forward and aft cargo doors.

These valves open to prevent external atmospheric pressure from exceeding cabin pressure.

These valves also open mechanically to relief any existing pressure when the cargo door is opened.

Whenever the negative pressure relief valves are opened mechanically, a cargo door warning light illuminates on the door annunciator panel.


The pressurization relief and negative relief valves are activates by differential pressure and are active in any mode of pressurization system operation.

At touchdown a signal from the landing gear ground safety relay causes the outflow valve to modulate towards OPEN, depressurizing the cabin at the rate set on the pressurization rate selector.  The outflow valves will remain OPEN while the airplane is on the ground.



Air Conditioning System Description:

The primary source of pneumatic air for the air conditioning system is taken from two stages of engine bleed, high stage pressure and mid stage pressure.  Air is directed from the pneumatic manifold to three air conditioning packs through individual pack valves.  Isolation valves are provided in the pneumatic system to isolate packs and bleed sources if required.
The output from the packs enter a common conditioned air manifold.
From this manifold air is mixed with hot air from the trim air system, if additional heat is required and directed to all zones of the airplane.

The APU or a ground source may be used to supply pneumatic air to the packs for normal usage of the air conditioning system.

Pack Valves:
Each pack receives its pneumatic air supply through an electrically controlled, pneumatically operated pack valve.
The pack valve will be open when the switch is in the open position and the pneumatic manifolf is pressurized.
If a loss of electrical power to the pack valve occurs the valve will automatically go to the full flow mode when the pneumatic manifold is pressurized.
Pneumatic air through the pack valve flows directly to the dual heat exchangers and the air cycle machine for cooling.

Half Flow Pack Valves (If Installed):
The half flow pack valves operate in the same manner as the full flow pack valves except they provide a 1/2 open position.
The pack valve switch has a CLOSE, 1/2 OPEN and OPEN position.
When the 1/2 open position is selected, the pack valve will position to supply approximately one-half the normal air flow.

Heat Exchangers:
Conditioned air temperature is controlled by cooling the engine pneumatic air by passing it through dual heat exchangers.
Inflight, the heat exchangers extract heat from the pneumatic bleed air by ram air flow across the heat exchangers.
The movement of air across the heat exchangers during ground operation is provided by a fan driven by the air cycle machine.
Flow across the heat exchangers on the ground and inflight is controlled by the pack temperature control system.

Pack Temperature Control:
The temperature control system contains a temperature switch for each zone of the passenger and crew compartments, a trim air system, ram air inlet door and a turbine bypass valve.

Inlet and Exit Doors:
The inlet and exit doors are sequenced to control ram air flow across the heat exchangers for each pack.
The amount of air flow through these doors will control the amount of heat extracted from the pneumatic air by the heat exchangers.

Air Cycle Machine:
Pneumatic air flow in the Air Cycle Machine (ACM) enters the compressor where its temperature and pressure are increased by compression.  The air then flows through the secondary heat exchanger, where the heat of compression is extracted and into a cooling turbine where its energy is used to drive the ACM turbine.
The expanding air cools to supply cold air for the air conditioning system.

Water Separator:
Cooled air leaving the ACM passes through a water separator to eliminate condensed moisture caused by the cooling process.

Turbine Bypass Valve:
The turbine bypass valve controls the amount of hot air from the heat exchanger that passes through the ACM for cooling requirements.
When heating is needed the turbine bypass valve allows more hot air from the heat exchanger to bypass the ACM and supply air partially cooled by only the heat exchangers.

Zone Temperature Control:
Bleed air from the manifold passes through the master trim air valve and is regulated by the zone trim air valves to provide additional heat for those zones requiring it.
Each of the trim valves is controlled by the zone temperature selector for its individual zone.
The zone controller for the zone with the coolest air requirement will determine the ACM outlet temperature for all packs and the trim air valve position indicator for that zone will indicate full cool.  The remaining trim air valve position indicators will reflect the amount of hot trim air being added.
With all zone temperature controllers positioned in MANUAL and packs operating in AUTO the ACM outlet temperature will drive to and remain at 35 degrees Fahrenheit (2 degrees Celsius).

Recirculating Fans:


Recirculating fans are provided to increase ventilating rate.


These fans, located above the passenger cabin ceiling, take air from above the ceiling and return the air to the conditioned air distribution system.





Supplemental Vent Fans (If installed):
Airplanes equipped with the half flow pack valve systems have an additional supplemental vent fans system controlling three fans from a single switch.
These fans, located under the passenger cabin floor, each draw air from below the cabin floor near the forward cargo compartment through a filter and discharge back into the air distribution ducts.  This system provides additional air circulation when operating with two packs in a 1/2 flow mode.

Protection Features:
The air condition system has numerous safeguards to protect the system and airplane components.
The air conditioning pack will trip off automatically (pack valve closed) for conditions of excessive ACM outlet temperature, excessive compressor discharge temperature or improper sequencing of BYPASS VALVE and INLET DOOR and/or EXIT DOOR.
Valve and exit door positions for an out of sequence are shown in your systems manual section 06.50.07.

Zone overheat protection is provided to prevent excessive duct temperatures and is indicated by the zone OVERHEAT light.
When a zone overheat occurs the trim air valve for the overheated zone will automatically drive to the COOL (closed) position.
All protective features are operative in AUTO and MANUAL modes except the Trim Air overheat which is operative only when the Zone Temperature switch is in the AUTO mode.

Auto Pack Operation - Inflight:
The AUTO mode of operation is recommended for all flight conditions.
In the AUTO mode the pack temperature control system controls the pacl ACM outlet temperature by automatically regulating the position of the turbine BYPASS VALVE, INLET DOOR and the EXIT DOOR. 
ACM temperatures, BYPASS VALVE, INLET and EXIT DOOR position should be approximately the same for all operating packs.
Pack temperature controllers will position the BYPASS VALVE, INLET and EXIT DOORS in a fixed sequence as heating and cooling requirements change.
Airplane drag reduction on later airplanes is achieved by reducing the inflight maximum cool position (door open) of the INLET and EXIT DOORS.
Immediately after takeoff the INLET DOOR moves from full COOL to approximately 20% from COOL and the EXIT DOOR moves from full cool to approximately 70% from COOL, and hold these positions until a change in temperature requirements is sensed.
When heating requirements are sensed the BYPASS VALVE modulates towards full HEAT intil full open.
The INLET and EXIT DOORS then modulate as required to their maximum HEAT (Closed) position reducing the cooling by the heat exchanger.
When  cooling requirements are sensed the INLET and EXIT doors will modulate towards their inflight maximum COOL (Open) positions followed by the BYPASS VALVE modulating towards the COOL (Closed) position.
When switching from MAN to AUTO mode, a time delay of approximately 18 seconds will occur before any change is noticeable.
The BYPASS VALVE, INLET and EXIT DOORS will first move towards auto operation pre-position.
Then the controller will position the
BYPASS VALVE, INLET and EXIT DOORS as necessary for the existing conditions.


Auto Pack Operation - Ground:
The on-ground pack operation is identical to flight operation with the exception that the INLET and EXIT DOORS are positioned to the full COOL position by the ground sensing system.  The BYPASS VALVE will modulate as necessary to maintain the ACM outlet temperature as required by the settings of the ZONE TEMPERATURE switches.



Manual Pack Operation:
Manual control and operation of the packs is provided by selecting MAN on the pack control switch, pressing the appropriate pack selector switch to ON and then controlling the ACM outlet temperature with the manual temperature switch.

Selecting MA
N on the pack control switch positions the EXIT DOOR to the full COOL position.

The door remains in full COOL as long as manual operation is selected.

The manual temperature switch controls the positions of the INLET DOOR and BYPASS VALVE.

ACM outlet
temperature is maintained by momentarily positioning the manual temperature switch to HEAT and COOL as required.

Changes in ambient temperature, airspeed and altitude will require frequent adjustments with the manual temperature switch to maintain a constant temperature.
The ACM outlet temperature must be maintained above 2 Degrees C to prevent freezing in the water separator.
The BYPASS VALVE and INLET DOOR will remain in the position at time of shut down for any reason.


Bypass Valve, Inlet and Exit Door Indicators
:



Limitations:


Pack Trip triggers:

  • Compressor discharge temp over 218 C,
  • ACM outlet temp over 85C,
  • Doors/Bypass valve out of sequence,
  • Pressure Relief Valve open on 3 Pack jet. (Trips #2 Pack).


Zone: 57 C.

Zone UD OvrHt: 85 C. 


Pneumatic sources:

8th and 14th stage compressor (15th for P/W) bleeds,

APU or Ground carts



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