Air Conditioning
Revised 24-Mar-2001
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The pneumatics system provides compressed air at a constant flow rate to the two air conditioning packs.  In these units the air temperature is modified to keep the cabin and cockpit comfortable.  In normal operation-some of  the air from the left pack provides conditioned air to the cockpit.  The rest of the air from the left pack mixes with the air from the right pack in a distribution duct and provides conditioned air to the passenger cabin.  The air is exhausted through the pressurisation system at a flow rate that allows the cabin to be pressurised.

The air conditioning packs are located beneath the floor in the center fuselage area.  An air conditioning pack valve controls the flow of air from the pneumatics system into each pack.  The pack valves are controlled by two switches on the Flight Engineers panel.
In each pack the air is split into three paths. 
In one path the air passes through a refrigeration unit, then to a set of mixing valves.  The mixing valves mix the refrigerated air with air from the other two paths.  This allows the air to be delivered to the cabin at the proper temperature. 
The second path to the mixing valves delivers hot air directly. 
The third path is through only a portion of the refrigeration unit, and It reaches the mixing valves at a moderate temperature.  The refrigeration unit is called an air cycle machine.  It operates on the same principle as any other refrigeration device, except that it uses air instead of freon for refrigeration.  The usual compression cooling and expansion seen in any refrigeration cycle is accomplished in the air cycle machine by a compressor, the secondary heat exchanger and an expansion turbine.  The work extracted by the turbine is transmitted by a shaft to the compressor.  A primary heat exchanger cools the air before it reaches the compressor, and thus increases the efficiency of the air cycle machine.

The primary and secondary heat exchangers are normally cooled by air picked up by two inlets on the bottom of the fuselage.  The air passes through the primary and secondary heat exchangers and out through a set of louvers at each heat exchanger.  Doors at the inlets control the airflow through the heat exchangers.  The cooling door and louvers on each pack are interconnected and driven by a single motor.
Pack temperature is most vitally affected by the position of the cooling doors. The pack cooling doors are controlled switches on the Flight Engineer's panel.  On some aircraft the cooling door switches have positions to open and close, and are spring loaded to a center off position.  Some are on open, off and close with no spring loading.  Others are equipped with automatic operated pack cooling doors which will modulate to keep the pack at the proper temperature.  These doors have the open and close positions, but the center position is auto.  The center position is not spring loaded.
When the cooling door switch is left in the auto position the cooling doors will remain open while the airplane is on the ground or the flaps are not up.  Once flaps are retracted, the associated pack temperature will be automatically regulated to a temperature schedule bias altitude.  Below 10,000 feet the temperature is kept at 125 degrees C.  From 10.000 feet to 30,000 feet the temperature decreases linearly to 45 degrees C, and remains at 45 degrees C as altitude increases further.  This schedule should be used if the doors must be controlled manually.
To provide additional cooling for low speed flight and ground operation, an electric fan for each pack is used to force air through the heat exchangers.  This fan will operate when the pack is on and the inboard flaps are not fully retracted, or when a pack is on and the airplane is on the ground. The ground cooling fan has its own motor driven air inlet door on the side of the fuselage that remains open when the fan is in operation.  When a pack f an is started. It draws a very heavy load from the electrical system, and when stabilised, fan consumes about ten kilowatts of power.  These are the highest loads on the electrical system.
To monitor the operation of the air cycle machines, each has a temperature transmitter at the outlet off  its compressor.  The temperature sensed is displayed on a pack temperature gauge for each pack.  As more air is directed through cycle machine to provide more cooling, more compression is  required from the compressor.  This results in a higher compressor outlet temperature.  Therefore, the pack temperature gauge monitors air cycle machine work load.  To protect the compressor from excessively high temperature an over temperature sensor at the outlet of the compressor will cause the pack to shut down if the temperature reaches the limiting value.  Another temperature limiting sensor located at the inlet to the turbine.  This uses the temperature of the air as an indication of the energy in the air.  If the temperature of the air, and thus the energy entering the turbine becomes too high, the pack will shut down to prevent an overspeed.
In order to return the pack to operation after the temperature in the pack has reduced, a reset button on the pack control panel is provided.  The pack cannot be returned to operation until the button has been pressed.
If the pack fan is operating when an air conditioning pack trips off, the fan will continue to operate.  The fan will stop when the pack temperature drops, the pack switch Is turned off, and the reset button is pressed.  
To allow unattended ground operation of the air conditioning system in the 727, the pack trip off sensing and the pack valves are powered from the battery transfer bus.  Should the AC electrical power fail, the pack cooling fans will stop.  Hot air from the APU will overheat the pack and a pack trip will occur, providing the battery transfer bus is powered.  This is one reason for leaving the battery switch on.  
As air is cooled it will hold loss moisture.  To remove this condensation a water separator is installed downstream of the air cycle machine turbine.  The water separator swirls the air over an impingement surface causing the moisture to drop out.  This water can be seen coming from the lower fuselage on humid days.  The air cycle machine is capable of  lowering air temperatures below freezing, which would cause the moisture in the water separator to freeze.  To prevent ice accumulation from blocking the water separator, a sensor monitors the temperature.  If the temperature gets too low, a water separator anti-ice valve is opened which allows warm air to bypass the air cycle machine and keep the temperature above freezing. 35F.  
The air conditioning units are controlled by switches on the Flight engineer's panel.  Each switch opens and closes its pack valve at a rate that will not overload the air cycle machine.  The pack valves are powered from the battery transfer bus.  
Each air mix valve set is actually three valves ganged together, one hot, one intermediate, and one cold.  These valves operate together to provide the proper mixing of hot, cool, and cold air.  There is a set of three valves for each air conditioning pack.  
As the outside air temperature drops, the temperature of the cooling air passing through the heat exchangers is low enough to provide sufficient temperature drop in the conditioned air.  To compensate, the intermediate valve opens, allowing air to bypass the turbine and flow directly from the secondary heat exchanger into the cabin or cockpit.  The turbine slows as a result of this bypassing action causing the compressor to be driven at a slower speed.  This allows some of the compressed air to bypass the compressor, flowing directly from the primary heat exchanger to the secondary heat exchanger.  The restriction to airflow caused by the air cycle machine is reduced as a result of this bypassing, reducing the need for high-pressure bleed air.  Reducing the need for high stage bleed air improves engine efficiency, reducing the amount of fuel being used by the engine.  

The temperatures in the cabin and cockpit are normally controlled by automatic temperature regulators.  Each regulator provides signals to a motor which drives the associated air mix valve.  Each temperature regulator receives inputs from a temperature sensor in the cockpit or cabin and a temperature selector on the flight engineer's panel.  The temperature sensor in the forward cabin provides temperature signals to the automatic temperature regulator for the right pack, and the cockpit temperature and left temperature selector position are sent to the temperature regulator for the left pack.  The position of each air mix valve is shown on an indicator next to the associated temperature selector on the Flight Engineer's panel.  The air mix valve moves to the full cold position automatically when the associated pack valve is closed.

 Conditioned air flowing from the air mix valves enters a common distribution duct. From this ducting a small portion of the air is directed into the cockpit. The remainder going to the passenger cabin. Both packs supply the distribution ducting, therefore the same distribution ratio of air to the cabin and cockpit would result whether one or both packs are in operation.  
The air to the passenger cabin flows through risers between the windows to keep the cabin walls warm. The air in the cabin eventually flows out through a grills along the floor line into the lower fuselage where it is exhausted through the pressurisation valves (outflow).  
Duct temperature is automatically restricted when the temperature control is operating in the automatic range.  A temperature sensor in the duct downstream of each air mix valve signals the associated automatic temperature regulator if the temperature reaches a limiting value.  When this limiting temperature is reached, a circuits called the topping circuit, prevents the mixing valve from moving toward a higher temperature position.  
If an automatic temperature regulator fails to control the temperature of the air satisfactorily, the associated air mix valve can be controlled manually.- To operate the air mix valve manually, spring tension must be overcome and the selector rotated to the manual position.  In this position the automatic temperature regulator is cut out.  Holding the selector lightly against spring tension to the cool position will cause the air mix valve to move towards cold.  In the warm position, the valve will move towards hot.  
To prevent the air from a pack getting too hot, should the automatic temperature regulator fail, a second temperature sensor is installed downstream of each mixing valve.  When the limiting temperature is reached, the associated air mix valve will move to the full cold position, and the duct overheat light next to the associated temperature controller will illuminate.  
If the automatic temperature regulator and the duct overheat protection both fail, to prevent the duct temperature from rising, a third temperature sensor will cause the pack to trip.  If the overheat and pack trip are on the left portion of the systems the location of the supply duct temperature transmitter near the right air mix valve will prevent the temperature indication from reaching the trip off temperature.  
To regain control of the temperature regulating networks and turn off the trip lights after an overheat has occurred, a reset button is installed on the temperature control panel.  Once the temperature has reduced, pressing the button will return the temperature control system to normal operation.  
A temperature gauge on the flight engineers panel is used to monitor the temperature of the air being supplied to the cabin at two locations.  The air temperature selector can be used to select the temperature in the forward and aft supply ducts, the main supply distribution duct, and in the forward and aft cabins.  
Air is tapped off at the cold side of the left air conditioning pack and delivered to the individually controlled outlets above the passengers, the lavatories, and the cockpit.  This in referred to as the gasper system.  To increase the flow of gasper air, a fan is installed in the gasper ducting.  A switch on the flight engineers air conditioning control panel turns the gasper fan on or off.  
If the left air conditioning pack is not operating when the gasper fan is on, cabin air is recirculated through the gasper system.  
Conditioned air flows through the airplane and exhausts through three principal exit systems.  First of these is the normal pressurisation outflow valve.  Operation of this valve will be covered under pressurisation.  
Some air flows into the electronic equipment compartment and circulates through the various electronic components, it passes through electronic equipment and circuit breaker panels in the cockpit, the electronic equipment bay, and the weather radar compartment.  This air absorbs the heat generated by these units and carries it overboard through an exhaust system on the forward right side of the fuselage.  
In normal flight, cabin differential pressure provides necessary airflow through this system.  Since the electronic equipment operates continuously, a means of inducing airflow on the ground and at low cabin pressure differential pressure is required.  To provide this flow, an electric fan has been installed in the exhaust duct.  This fan comes on automatically at low cabin differential pressure.  The exhaust to this system, has a large and small outlet.  So that unrestricted flow can be achieved at low cabin differentials, both outlets are used.  As the flow rate increases, a flow rate sensitive valve closes preventing excessive loss of air at high differential pressures.  
A warning light on the lower right corner of the flight engineers panel will alert the crew to inadequate cooling of electronic equipment.  A sensor in the cooling air outlet monitors airflow through the cooling system.  If cooling airflow becomes inadequate the "no equipment cooling" light will come on.  
The cargo compartments on the 727 are class D cargo compartments which are designed to confine a fire without endangering the safety of the airplane or the occupants.  No air circulates through them although a small amount of air flows through the equalisation valves to maintain equal pressure between the cargo compartment and the surrounding cavities, should cabin pressure vary.  If a fire develops, it will smother itself as the oxygen in the compartment is consumed.  To maintain temperature in the forward cargo compartment, conditioned air from the cabin flows around an airtight inner shell then is discharged through the cargo heat outflow valve.  Approximately 30% of the air in the aircraft will exit through this valve.  
A switch on the flight engineers panel controls the cargo heat outflow valve.  In the normal position the valve is open, permitting air circulation around the forward cargo compartment.  If a pressurisation. problem should occur, closing the switch can stop the flow of air through this exit.  Without airflow around the forward cargo compartment the temperature within the compartment will drop rapidly to a much lower value.  
The air that passes from the cabin to the pressurisation. outflow valve in the aft fuselage of the airplane heats the aft cargo compartment.  
An automatic pack trip system is incorporated in the 727 200 series aircraft.  With the system armed before takeoff, loss of thrust on any engine will trip off both packs.  This allows the engines to develop somewhat higher thrust for the remainder of the takeoff and initial climb.  In addition, both pack fans will stop, thereby reducing the electrical load.  To arm the auto pack trip system, the airplane must be on the ground, the flaps must be out of the up position, the auto pack trip switch must be in the normal position, and all engines must be above 1.5 EPR.  
When the flaps reach the up position after takeoff the auto pack trip system will be deactivated.  After takeoff, and when clear of obstacles the auto pack trip switch should be returned to the coot position.  This will deactivate the auto pack trip system.  Should any engine lose power below 1.3 EPR both packs will trip off, both pack valves will close, both pack fans will stop and both pack trip lights will illuminate.  In addition, an engine fail light will illuminate on each side of the pilot's glareshield.  These engine fail lights can be extinguished by pressing on either light cap.  When a substantial power reduction is anticipated, such as a noise abatement takeoff. The flight engineers should anticipate the thrust reduction and place the auto pack trip switch to cutout prior to reducing thrust to remove the possibility of an inadvertent auto pack trip.  
The airplane is equipped with a means of controlling the temperature in the aft cabin without affecting the temperature of the forward cabin.  This is done through the aft cabin zone temperature system.  A single switch operates two valves in this system.  This allows warm air from the right air conditioning pack to enter the forward or aft cabin bringing about the requested change in aft cabin temperature.  Should the aft cabin ducting overheat an amber light on the panel will illuminate.  Both zone control valves will close and the needle will center.  The flight engineer can monitor the use of this system with the air temperature selector. 

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