Air Conditioning & Pressurization
GENERAL AIRCONDITIONING
Airconditioning and pressurization is accomplished using compressed air routed either directly from the pneumatic manifold or heating or through air cycle machines or heat exchangers for cooling. The conditioned air is distributed through ducting tot wo temperature controlled areas.
Passenger cabin / Cargo area
Control cabin (cockpit)
Pressurization control is achieved by varying the flow of exhaust air, using two controllable pneumatic outflow valves or one controllable electric outflow valve.
AIR DISTRIBUTION
Air from the pneumatic manifold is conditioned by the two airconditioning packs and collected in the main distribution manifold. The manifold has a spring-loaded pressure relief door to protect the ductin from overpressure.
PASSENGER CABIN / MAIN CARGO DECK
Conditioned air tot he cabin is ducted from the right end of the main distribution manifold through either (or a combination of) the sidewall or overhead system. The sidewall system delivers air through the cove outlets along both sidewalls under the hatracks. The overhead system delivers air through the overhead duct along the centerline of the cabin. On most airplanes, airflow can be selected from either system or a combination of both.
COCKPIT
Conditioned air tot he cockpit is ducted from the left end of the main distribution manifold through a fixed airflow restrictor. There are ssix outlets provided; one behind each pilo’s instrument panel, one at at floor level behind the flight engineer’s instrument panel, one at floor level by the first observer’s seat and two in the aft ceiling. The ceiling and sidewall outlets have controllable baffle shutoff valves.
GASPER AIR
In addition to normal air distribution, individual (eye-ball) outlets with variable openings are provided for each seat. These outlets are supplied with the coldest air from the left air mix valve. If the left pack is OFF, air from the airconditioning distribution bay is drawn through a check valve when the gasper fan is on.
Airflow to these outlets is augmented by a single electric gasper air fan located in the airconditioning distribution bay.
EXHAUST
Air leaves the main cabin through floor-level grills, and spills behind the sidewall liners of the cargo compartments to aid in keeping the cargo areas above freezing. The air moves rearward, behind the liners, and is exhausted overboard through outflow valve(s) located behin the aft cargo compartment and the cargo heat outflow valve, located aft of the forward cargo compartment.
Galley and lavatory air is vented overboard through fied venturis, while air from the cockpit leaves through the electronic equipment cooling system.
AIRCONDITIONING PACKS
Hot air from the pneumatic manifold is cooled by two identical airconditioning packs. The hot air enters each pack through a pack valve. Initial cooling takes place in the primary heat exchanger (PHX). If additional cooling is required, the AIR MIX VALVE is positioned towards COOL to direct airflow through an air cycle machine (ACM). The air passes through a water separator, and an air mix valve, then enterst he main distribution manifold.
(1) Cooling Doors Switch Controls pack coolant air inlet door and heat exchanger exhaust doors simultaneously.
(2) Airconditioning Pack Switch On: Opens pack valve, allows associated flow control and shutoff valve to open, and arms temperature selector. If APU is operating, both flow control and shutoff valves remain closed and when the second pack is turned on, the APU flow multiplier operates. Off: Closes pack valve, allows flow control and shutoff valve to close, disarms the temperature selector, and the mix valve runs full cold. If APU is operating, the flow multiplier is bypassed when the first pack is turned off.
(3) Comperssor Discharge Temp Gage Indicates temperature at exit of ACM compressor and therefore, operating condition of ACM and PHX.
(4) Pack Trip Off Light Off: Pack operating normally. On: Indicates respective pack valve has closed and locked out pack because of one or more of the following:
ACM compressor discharge temperature over 200 degrees Celsius.
ACM turbine inlet temperature over limits.
Main distribution manifold temperature is over 250 degrees Farenheit.
(5) Reset Switch When pressed, resets auto pack trip or resets either pack TRIP OFF lockout if:
ACM compressor discharge temperature is below 200 degrees Celcius.
ACM turbine inlet temperature has decreased below its limit.
Main distribution manifold temperature is below 250 degrees Farenheit.
(6) Cargo Heat Outflow Switch Close: Valve closed to prevent airflow overboard. Normal: Valve open, to allow a portion of cabin air to flow around forward cargo compartment and exhaust overboard.
(7) Gasper Fan Switch On: Applies power to gasper fan to increase airflow through various "eyeball" outlets.
PACK VALVE
Each pack has a motor-driven pack valve that is controlled by a switch on the flight engineer’s panel. When closed, the pack valve isolates the pack rom the pneumatic manifold. When the valve is open, it routes pneumatic manifold air into the pack.
The pack valve can be overridden closed by excessive pack or main dristribution duct temperature or the auto pack trip system.
FLOW CONTROL AND SHUTOFF VALVE
APU Bleed Air
Both flow control and shutoff valves remain closed when either pack is turned on; duct pressure may indicate zero. When using APU bleed air for engine starting, both flow control and shutoff valves are overridden open.
Engine Bleed Air
When a pack is turned on, the associated flow control and shutoff valve opens slowly for ACM protection, and then automatically modulates to control the airflow to the pack.
PACK COOLING AIR SYSTEM
The pack cooling air system controls fan or cooling ram airflow through the heat exchangers during ground and flight operation of the airconditioning system, by modulating the inlet, PHX and SHX exit doors. Manual control may be used to reduce airflow through the heat exchangers in flight to reduce drag.
Pack Cooling Fan
Each cooling air sysem incorporates an electric fan to ensure adequate air flow across the heat exchangers. Whenever the respective A/C PACK switch is ON, the fan operates automatically when:
Airplane on the ground
Airplane in flight with flaps not up.
Heat Exchangers
Cooling of the hot air takes place in the air-to-air primary (PHX) and secondary (SHX) heat exchangers. A check valve in the ram air duct prevents the fan air from reversing direction to flow out the inlet.
Inlet and Exit Doors
Airflow through the heat exchangers is regulated by doors at the ram air inlets and heat exchanger exits. The door for each pack are modulated by a single electric motor which can be manually controlled by operation of the respective pack COOLING DOORS switch. The doors will be driven open (if they are not already open) by an ACM compressor discharge temperature above 115 degrees celsius, overriding manual switch operation. Whe nthe temperature drops below 115 degrees celsius, the doors stop moving open and manual control is restored.
AIR CYCLE MACHINE (ACM)
When additional cooling is required, air leaving the primary heat exchanger is directed through the ACM by the air mix valve. The ACM is a cooling device which incorporates a compressor that is driven by an expansion turbine. Both units are installed on a common shaft. Semiconditioned air exiting the PHX enterst he compressor section fot he ACM, where it is compressed and heated (as a result of compression). Flow through a secondary heat exchanger removes the compression heat before the high-pressure air enterst he expansion turbine, where it is cooled by expansion as it drives the turbine at high speed. Both the expansion process and the load imposed by the compressor on the turbine cool the airflow tot he lowerst temperature in the cycle.
PACK INDICATORS
Pack operation is monitored by:
The position of the cooling doors.
ACM compressor discharge temperature.
Supply duct temperature
PACK TRIP PROTECTION
A pack trip automatically closes the pack valve and illuminates the TRIP OFF light. Three conditions can cause a pack to trip:
An ACM compressor discharge temperature of 200 degrees celsius or higher. The pack trip prevents excessive temperatures in the ACM compressor.
An excessive ACM turbine inlet temperature. The trip prevents ACM overspeed.
A main distribution manifold temperature of 250 degrees farenheit or higher, adjacent to either air mix valve outlet. The trip prevents excessive temperature in the distribution ducts.
The 250 degrees farenheit protection feature will function on battery power alone, if the BATTERY switch is ON.
Reset of a pack trip can only be done after the temperature is reduced. A single RESET switch is used for both packs.
AIRCONDITIONING CONTROL
Each airconditioning pack is equipped with an automatic and a manual control system. The left temperature selector controls the control cabin air mix valve (left pack) while the right temperature selector controls the passenger cabin/main cargo deck air mix valve (right pack). Both air mi valves discharge conditioned air from their respective pack into the main distribution manifold.
(1) Duct Overheat Lights When illuminated, indicates that respective supply temperature from pack is over 190 degrees Farenheit in the main distribution duct.
(2) Reset Switch When pressed, resets either DUCT OVERHEAT lockout if SUPPLY temperature has decreased to lass than 190 degrees Farenheit.
(3) Air Temp Selector Indicates temperature in the area selected. FWD SUPPLY: Temperature in duct to farward half of cabin. AFT SUPPLY: Temperature in duct to aft half of cabin. MAIN SUPPLY: Temperature in duct to farward and aft supply ducts. AFT CAB/FWD CAB: Temperature in cabin at hat rack level.
(4) Air Mix Valve Position Indicators Indicates position of respective air mix valve. COLD: Coldest air available from the pack. HOT: Hottest air available from the pack.
(5) Temperature Selectors Control respective control cabin air mix valve to adjust temperature in respective cabin. AUTO: Arms controller which automatically modulates the air mix valve to maintain selected temperature between 65 and 85 degrees Farenheit, vertical position is approximately 75 degrees Farenheit. OFF: Disarms controller and maintains position of air mix valve. MANUAL WARM: While held, drives air mix valve toward HOT to increase temperature. MANUAL COOL: While held, drives air mix valve toward COLD to decrease temperature.
(6) Temperature Gage Indicates air temperature of area selected by AIR TEMP selector.
TEMPERATURE SELECTOR
A temperature selector is provided to control the output temperature of each pack. Each temperature selector has an AUTO and a MANUAL operating position. The AUTO position has a variable control range of 65 to 85 degrees farenheit. The vertical position of the selector is approximately 75 degrees farenheit.
In automatic operation, each controller compares the setting of the temperature selector tot he existing CONTROL CABIN or PASSENGER CABIN temperature and modulates the respective air mix valve to achieve the selected temperature.
In MANUAL, each temperature selector controls the respective air mix valve directl; the automatic controller is de-energized and bypassed. The selector is springloaded to OFF when it is in the MANUAL sector.
AIR MIX VALVE
Each air mix valve has internal butterfly valves to control the air temperature from the respective pack. The butterfly valves are connected by linkage to a reversible electric motor, and modulate to obtain a gradual temperature change of the outlet air when the motor is operated. No alternate means of operation of the mix valves is provided.
The air mix valve has three inlet ports; cold, cool and hot. In operation it functions as follows:
Full cold. The cold butterfly is open, the hot and cool butterflies are closed, forcing the hot pneumatic air through the ACM.
Cool. The hot butterfly is closed, the cold and cool butterfly valves are open. The pneumatic air will take the path or least resistane through the PHX and SHX, and bypass the ACM.
Hot. The hot butterfly is open, the cold and cool butterfly valves are closed allowing the hot pneumatic air to bypass the PHX, SHX and ACM.
All intermediate positions, the valve mixes hot, cool and cold air to obtain the selected temperature.
OVERHEAT PROTECTION
Two 190 degrees Farenheit thermal switches are located adjacent to each air mix valve in the main distribution manifold. A duct temperature of over 190 degrees farenheit:
Illuminates the repsective DUCT OVERHEAT light.
Drives the respective air mix valve to full cold.
AUTO PACK TRIP
The auto pack trip system increases the thrust on the operating engines by closing both pack valves if an engine fails during takeoff or initial climb.
To arm the system:
The airplane must be on the ground.
The trailing edge flaps must not be up.
The AUTO PACK TRIP switch must be set to NORMAL.
All engines must be above approximately 1.50 EPR.
When the system arms, the green AUTO PACK TRIP ARMED light on the flight engineer’s panel illuminates.
When the system is armed (green light on) and the thrust on any engine falls below a preset value, the auto pack trip system will:
Illuminate both pilot’s ENGINE FAIL lights.
Illuminate both pack TRIP OFF light.
Close both pack valves.
Shut off both pack cooling fans.
The system is automatically dis-armed (AUTO PACK TRIP ARMED light out) when the trailing edge flaps are up; it cannot be re-armed in flight. The system is deactivated by placing the AUTO PACK TRIP switch to CUTOUT.
GENERAL PRESSURIZATION
The airplane is pressurized by engine bleed air supplied to and distributed by the airconditioning system.
(1) Auto Fail Light Illuminates if automatic pressurization control fails. Control is automatically transferred to standby if STANDBY light also illuminates. (2) Off Schedule Descent Light Illuminates when airplane descending before reaching the planned cruise altitude set in flight altitude counter. Cabin will pressurize to the takeoff field elevation during descent. (3) Standby Light Illuminates when system is operating in standby mode.
(4) Manual Light Illuminates when system is operating in manual mode.
(5) Flight Altitude Select Window Indicates selected cruise altitude.
(6) Flight Altitude Selector Rotate to set planned cruise altitude, 0 to 40.000 ft.
(7) Landing Altitude Select Window Indicates selected altitude of destination airport
(8) Landing altitude Selector Rotate to set altitude of destination airport from -990 to +13.999 ft.
(9) Flight/Ground Switch GRD: Depressurizes the airplane when on the ground. FLT: Pressurizes the airplane when on the ground to -200 ft.
(10) Cabin Rate Selector DECR: Decreases cabin altitude rate of change. INCR: Increases cabin altitude rate of change. Index (plp): Cabin pressure rate approximately 300 ft/min.
(11) Cabin Altitude Window Indicates selected cabin altitude.
(12) Cabin Altitude Selector Manual selection of desired cabin altitude.
(13) Outflow Valve Position Indicator Indicates position of the outflow valve. Left equals full closed, right equals full open.
(14) Manual Control Switch Directly controls outflow valve by selected ac or dc motor.
(15) Mode Selector AUTO: Pressurization automatically controlled. CHECK: Tests excessive cabin rate of climb protection system; AUTO FAIL and STANDBY lights illuminate. STBY: Pressurization controlled by standby system. MAN AC: Pressurization controlled manually using the ac motor. MAN DC: Pressurization controlled manually using the dc motor.
CONTROL
The pressurization system controls cabin pressure by controlling the rate at which air is exhausted overboard. The xhaust rate is regulated by controllable outflow valve(s). A contiuous exhaust is provided for each lavatory and galley, to carry odors overboard.
CABIN PRESSURE AND ALTITUDE INDICATORS
A cabin rate of climb indicator and a combined cabin altimeter/differential pressure indicator are located on the flight engineer’s upper panel. Some airplanes have an additional separate cabin altimeter.
CABIN ALTITUDE WARNING
An intermittent warning horn sounds when cabin altitude reaches 10.000 ft. An ALTITUDE HORN CUTOUT switch is provided.
PNEUMATIC CONTROL
The pressurization system is controlled and operated pneumatically using low pressure air created by a venturi or jet pumps.
AUTOMATIC CONTROL
Automatic control is accomplished by the automatic cabin pressure controller on the flight engineer’s panel. It maintains the selected cabin altitude and rate of pressure change by the adjustment of the controls; CABIN ALT and RATE. Adjustment of the CABIN ALT control also sets the FLIGHT altitude in the controller.
MANUAL CONTROL
Manual control of the pressurization system is accomplished by use of a manual controller on the flight engineer’s panel. Use of the manual controller overrides the auto controller when the pointer is not in AUTO. Cabin altitude is manually increased by moving the controller pointer into the DECREASE PRESSURE sector. Conversely, the cabin altitude may be lowered by moving the pointer into the INCREASE PRESSURE sector.
In either case:
The rate of change is prpoortional tot he amount of pointer movement from AUTO.
The rte of change is stopped by moving the pointer toward AUTO
The manual controller will not automatically maintain a cabin altitude.
GROUND VENTURI
The fan blows air through the venturi to create a low pressure source that holds the outflow valves open on the ground until the packs are on.
OUTFLOW VALVES
The outflow valves, used to exhaust cabin air overboard, are pneumatically controlled by the automatic or manual cabin pressure controller.
PROTECTION
System protection consists of:
The autocontroller, which maintains 8.6 +- .3 psi maximum differential pressure.
The outflow valves which limit maximum differential to 9.42 +- .35 psi maximum cabin altitude to 13.000 +- 1500 ft, and negative differential pressure to -0.36 psi.
ELECTRONIC CONTROL
The pressurization system is controlled electronically. This system utilizes electric power to energize either an ac motor or a dc motor to modulate the single outflow valve. Pneumatic pressure is not used.
In the electronic control system, control of the outflow valve can be accomplished by selection of any one of the four modes of operation noted below.
AUTO. Automatic; the normal mode of operation using an ac motor.
STANDBY. Semi-automatic; a standby system that uses the dc motor in the event of AUTO failure.
MAN AC. Manual control of the outflow valve using the ac motor.
MAN DC. Manual control of the outflow valve using the dc motor. The power source is the batter bus.
AUTOMATIC MODE
In the automatic mode of operation, cabin rate of climb and descent are automatically determined by the electronic controller. This mode of operation also utilizes selecte FLT ALT and LAND ALT values to calculate the cabin cruise altitude without exceeding an 8.5 psi differential pressure.
Any time the airplane climbs above the selected FLT ALT the controltter maintains the scheduled cabin altitude until a maximum cabin differential pressure of 8.65 psi is reached. This pressure is then maintained by allowing the cabin altitude to increase.
The cabin rate of climb and descent are proportional to airplane vertical speed, up to a maximum of approximately 500 feet per minute climb and 350 feet per minute descent.
If the airplane begins a descent before reaching the selected FLT ALT, the controller initiates an off-schdeule descent. The OFF SCHED DESCENT light will illuminate and cabin altitude will be programed to descend tot he takeoff runway elevation.
When the AUTO FAIL and STANDBY lights come on, it indicates that pressurization control has transferred tot he STANDB mode due to one of the following:
Cabin altitude above approximately 14.000 ft.
Cabin pressure change in exess of 1900 fpm.
Loss of ac power for more than 15 seconds.
STANDBY MODE
The standby mode will automatically be activated if the automatic mode fails for more than 115 seconds. The standby mode can also be manually selected if required. In flight, the rate of change of cabin altitude is determined by the position of the cabin rate selector Full DECR to full INCR provides a range of operation of approximately 50 to 2000 feet per minute.
At touchdown, if the FLT GRD switch has been set tot he GRD position, the cabin will depressurize at the selected cabin rate.
MANUAL MODE
In the manual mode of operation, either MAN AC or MAN DC may be selected. The rate of outflow valve operation is faster in MAN AC than in MAN DC.
The FLT GRD switch functions are inoperative in this mode.
Pressurization is controlled by a three-position outflow valve MANUAL control switch which is springloaded tot he center off position. The switch directly positions the ouflow valve using the selected ac or dc motor.
SYSTEM CHECK
The CHECK position of the mode selector tests the AUTO mode automatic failure protection feature by simulating an excessive cabin rate of climb. The AUTO FAIL and STANDBY lights should illuminate.
ELECTRONIC CABIN PRESSURE CONTROLLER
The electronic cabin pressure controller is used during AUTO and STANDBY operation. Inputs for the controller are received from cabin pressure, ambient pressure, ADC No. 1 and barometric correction.
FLT/GRD SWITCH
When the airplane is on the ground, the FLT GRD switch determines the submode of operation. In GRD, the controller drives the outflow valve full open. With FLT selected, the controller modulates the outflow valve to produce a cabin altitude approximately 200 ft below field elevation. The switch has no function in flight.
OUTFLOW VALVE
The single outflow valve is actuated by either an ac or dc power actuator. The ac actuator is used during AUTO and MAN AC operation; the dc actuator during STANDBY and MAN DC operation.
PRESSURE RELIEF VALVES
Positive pressure relief valves limit the differential pressure to a maximum of 9.6 psi.
NEGATIVE PRESSURE RELIEF VALVE
Interior negative pressure is prevented by a springloaded door (valve), located on the fuselage, below floor level, aft of the aft galley service door.
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