Easa Part 66 - Module 11.04 - Air Conditioning and Cabin Pressurisation

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JAR 66 CATEGORY B1 MODULE 11.04 AIR CONDITIONING AND CABIN PRESSURISATION

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1.7.1 Control And Indication ... 1-25 1.7.2 The Un-Pressurised Mode ... 1-25 1.7.3 The Isobaric Mode ... 1-26 1.7.4 The Constant-Differential Pressure Mode ... 1-26 1.7.5 Cabin Air Pressure Regulator ... 1-26 1.7.6 Isobaric Control System ... 1-27 1.7.7 Differential Control System ... 1-28 1.7.8 Safety Valves ... 1-30 1.7.9 Cabin Pressure Controllers ... 1-30

1.8 SAFETY AND WARNING DEVICES ... 1-32

1.8.1 Overheating ... 1-32 1.8.2 Duct Hot Air Leakage ... 1-32 1.8.3 Excess Cabin Altitude ... 1-33

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Page 2 B1 Mod 11.04 .doc Issue 3 Feb 2002

JAR 66 CATEGORY B MODULE 11.04 AIR CONDITIONING AND CABIN PRESSURISATION

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1 AIR CONDITIONING AND CABIN PRESSURISATION

1.1 INTRODUCTION

Air conditioning systems control both the temperature and humidity of the air within the cabin, cockpit and freight areas as well as heating or cooling it as necessary. It should also supply adequate movement of air through the aircraft for ventilation as well as provide a means of removing smoke (if permitted) and odours. A typical system comprises of five principle sections:

a. Air supply b. Heating c. Cooling d. Temperature control e. Distribution 1.2 AIR SUPPLY

The source of air supply and arrangement of the system components depend on the aircraft type and system employed but in general one of the following methods may be used:

1.2.1 Ram Air

This is used in some unpressurised aircraft using either combustion heating or warm air heating from an exhaust gas heat exchanger. The ram air supply is from an intake directly in the airflow either on the nose, wing or at the base of the tail fin. The air after circulating through the cabin is exhausted to atmosphere.

1.2.2 Engine Bleed Air

This is used in turbo jet aircraft in which hot air is bled of from the engine compressors to the cabin. Before the air enters the cabin it is passed through a temperature control system which reduces its pressure and temperature and is then mixed with ram air.

1.2.3 Compressors or Blowers.

This is used by some turbo jet, turbo prop or piston engine aircraft, the compressors or blowers being either engine driven via an accessory drive, by bleed air or electric or hydraulic motors.

1.2.4 Auxillary Power Unit (APU)

This provides an independent source of pressurised air. It is basically a small gas turbine engine that provides air and other service whilst the aircraft is on the ground with its main engines stopped. It is usually a self contained unit located in the tail section of the aircraft where it can be run safely.

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1.2.5 Ground Power Trolley

For use in extreme climates on aircraft which do not have an APU or the APU is unserviceable. This is a self contained air conditioning unit and can be connected to the aircrafts cabin, to either heat or cool it depending on the climate. This unit will run until the aircraft is independent of the trolley.

1.3 AIR CONDITIONING SYSTEMS

The method of air conditioning depends on the type of aircraft and the air supply system used. Each system uses different methods for heating and cooling. In general there are 3 types of heating systems used.

1.3.1 Combustion Heating

A typical layout is shown in Figure 1. This is usually associated with a ram air supply and depends for its operation on the combustion of a fuel air mixture within a cylindrical combustion chamber. Ram air is augmented with an air blower and fuel is metered from the aircraft fuel system through a solenoid valve. The fuel air mixture is ignited in the combustion chamber and the burnt gases swirl through the transfer passages of the cylinder before being exhausted to atmosphere.

This gas swirl not only aids combustion but ensures that the gases impart against the chamber and passage walls to allow maximum heat transfer. The ram air flows over the outside of the combustion chamber where it is absorbs the heat before it enters the cabin.

Typical Combustion Heater System Figure 1

FUEL SOLENOID VALVE FUEL SUPPLY

OFF

OFF O NO N

WARM AIR OUTLETS

COLD AIR OUTLETS

RAM AIR

EXHAUST

COMBUSTION CHAMBER DEMISTER

FLOW CONTROL VALVE ENGINE DRIVEN AIR BLOWER

AIR SUPPLY

O N OFF

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The temperature is controlled manually by setting a control valve which is located downstream of the combustion chamber. This controls the amount of air flow over the combustion chamber. The slower the flow the hotter the air becomes and vice versa. The blower operation, fuel supply and ignition is normally controlled by a single on/off switch.

1.3.2 Engine Exhaust Heating

A typical exhaust heater is shown in Figure 2. This also is used with a ram air ventilation system but the heating of the supply air is much more simple and direct. A heater muff surrounds the exhaust pipe of a piston engine aircraft. The ram air enters this muff and extracts the heat from the hot exhaust. This heated air is then passed into a chamber where it is mixed with a separate cold air supply. Mechanically operated valves are provided to control the flow of air supplied and therefore regulate the cabin temperature. Carbon monoxide detectors may be used within the cabin to check for levels of the gas. These are usually indicators filled with bright coloured crystals which turn black when exposed to dangerous carbon dioxide levels. They are sited in view of the pilots.

Exhaust System Heater Figure 2

1.3.3 Compression Heating

A typical compression heating sytem is shown in Figure 3. This system relies on the principle whereby the air temperature is increased during compression and is used by air supply sytems utilising either engine driven compressors and blowers and engine bleed air. Hot air is drawn in from either an engine bleed or air blower where it is then split. Some air goes directly to the distribution mixer control valves and the remainder goes to a primary heat exchanger where ram air passes through the exchanger matrix to cool the air.

CONTROL VALVE

EXHAUST MANIFOLD

HEATER MUFF

CONTROL

LEVER

CLOSED OPEN

RAM

AIR

TO CABIN

OVERBOARD DUMP

FLAP

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From the primary heat exchanger the cool air then goes to a compressor where it is compressed and heated before going through a secondary heat exchanger, again being cooled by the ram air. This cold air then passes through the turbine where it “does work” driving the compressor becoming even colder, before going to the mixer control valve where it is mixed with the hot air before being distributed to the cabin. Adjustable flow control and temperature control valves control the cabin temperature.

Typical (Compression) Bleed Air System Figure 3

1.4 AIR CYCLE AND VAPOUR CYCLE MACHINES

1.4.1 Air Cycle Cooling System

This system works on the principle of the air dissipating or absorbing heat by doing or receiving work. If it does work (expanded) its temperature will fall if it receives work (compressed) its temperature will rise. The primary component in an air cycle system is the cold air unit. There are a number of types in use:

ECU

NRV

AUXILLARY POWER UNIT NON RETURN VALVE

SHUT OFF VALVES

FLOW CONTROLLER

TEMPERATURE CONTROL VALVE

MIXER UNIT TO CABIN NRV

W ATER SEPARATOR COUPLED COMPRESSOR TURBINE

RAM AIR

PRIMARY HEAT EXCHANGER SECONDARY HEAT EXCHANGER

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1.4.2 The Turbo Compressor

In a typical system the turbine drives a coupled compressor (Figure 4). A secondary heat exchanger is located in line between the compressor outlet and the turbine inlet. A ram air supply is provided to the primary and secondary heat exchangers. For operation of the system on the ground an induction fan or blower may be used to augment the air supply and there may also be a ground air conditioning unit connection.

Turbo Compressor Figure 4

The hot air supply initially air passes through a primary heat exchanger where it is pre-cooled before entry to the compressor. It is then compressed and heated by the compressor before being cooled again as it passes through the secondary heat exchanger.

The cooled air then drives the coupled turbine where it does work and becomes even colder. As this air cools, moisture condenses out of it and is collected in a water separator. The water is centifuged out in the seperator where it collects on the outer case and is then allowed to drain overboard. To prevent this water from freezing warm air is mixed with it via a temperature control valve when it reaches a certain temperature. A typical turbo compressor is shown in Figure 5.

TEMPERATURE CONTROL VALVE

COMPRESSOR TURBINE

SECONDARY HEAT EXCHANGER

RAM AIR TO CABIN MIXER UNIT PRIMARY HEAT EXCHANGER

HOT AIR INLET

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The cold air from the turbine then enters the mixer unit where it is mixed with the pre-cooled air supply via the temperature control valve to allow a variable warm air supply to the air distribution system.

Turbo Compressor Cold Air Unit Figure 5

1.4.3 The Brake Turbine

A typical brake turbine is shown at Figure 6. When cold air is selected the bleed air is directed to the turbine of the cold air unit. As the air drives the turbine the gas expands as work is being done resulting in a drop in pressure and temperature. BLEED AIR TO INTERCOOLER FROM INTERCOOLER TO DISTRIBUTION SYSTEM COMPRESSOR DIFFUSER NOZZLE BLADES

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Brake Turbine Cold Air Unit Figure 6

To prevent the turbine from rotating too quickly and affecting the cooling efficiency, the turbine is coupled to the compressor. As the compressor rotates ambient air is used as a braking medium to slow the turbine. This system is an improvement on the turbo compressor system as only one heat exchanger is required

1.4.4 The Turbo Fan

The turbo fan is mechanically similar to the brake turbine cold air unit. In the turbo fan the turbines drives a coupled centrifugal compressor which induces a capacity of air, large enough to create a cooling flow of ram air through a heat exchanger, cooling the bleed air. It also acts as acting as a braking fan to control the turbine speed. A typical turbo fan is shown in Figure 7.

The major advantage of this system is that the air conditioning system can be operated on the ground with engines running without the need for ram air.

RAM AIR TO CABIN MIXER UNIT HEAT EXCHANGER CONTROL VALVE AMBIENT AIR INLET

COMPRESSOR TURBINE

BLEED AIR

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Turbo Fan Cold Air Unit Figure 7

1.4.5 Vapour Cycle Cooling System

The vapour cycle cooling system is used to control and reduce the temperatures generated by electronic equipment used in modern aircraft.

This system works on the principle of the ability of a refrigerant to absorb heat through a heat exchanger in the process of changing from a liquid into a vapour. A refrigerant is a substance that absorbs heat through expansion or vaporisation. For example if you drop some methylated spirit onto your hand it feels cold. This is because the volatile liquid starts to evaporate as it draws the heat away from your hand.

Liquids with low boiling points have a stronger tendancy to evaporate at normal temperatures than those with higher boiling points. Furthermore pressure affects the state of a liquid substance. A reduction in pressure will cause a liquid to change state into a gas or vapour.

A typical vapour cycle system operates with 2 distinct integrated systems, a sealed recirculating refrigerant system and an air system. A typical system is shown at Figure 8. MIXER UNIT BLEED AIR RAM AIR HEAT EXCHANGER CONTROL VALVE COMPRESSOR TURBINE RAM AIR OUTLET

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Schematic Vapour Cycle System Figure 8

• Refrigerant System

The system has 2 sides a high pressure side and a low pressure side. Mixed with the refrigerant is a specified amount of lubrication oil which lubricates and seals the compressor.

The liquid refrigerant passes from the receiver to the thermostatic expansion valve for controlled release into the matrix of the evaporator. Heated air from the main supply passes over the evaporator matrix and by induction transfers heat into the liquid refrigerant which on heating becomes a low pressure vapour. From the evaporator the LP vapour feeds into a compressor which pressurises the refrigerant to a high pressure. This HP refrigerant then enters the condensor where it is cooled to a liquid by ram air (or by induction fan air) passing through the matrix where it then returns as a liquid to the liquid receiver, to repeat the cycle. THERMOSTATIC EXPANSION VALVE RECIEVER DRYER CONDENSER EVAPORATOR TURBO COMPRESSOR TEMPERATURE CONTROL VALVES AIR SUPPLY RAM AIR AIR DISTRIBUTION TEMPERATURE SENSOR CAPILLARY TUBE

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• Air System

The main hot air supply drives the turbine, which is directly coupled to the compressor. The air is also fed directly downstream of the system to the temperature control valves. As the air passes through the turbine it does work which reduces the airs temperature which is then fed to the evaporator. This air then passes through the evaporator where it is further cooled (as the refrigerant absorbs the heat) and is then fed to the temperature control valves. These valves controls the air temperature being fed to the air distribution system.

All components of this type of system are usually mounted on a single removable quick release panel (Figure 9) to allow complete pack changes when a fault arises, instead of changing individual components. Some aircraft use this type of system to air condition avionics bays as well as the cabin.

Typical Vapour Cycle System Figure 9

1.4.6 The Compressor

The compressor pulls the low pressure refrigerant vapour from the evaporator and compresses it. When the vapour is compressed its pressure and temperature both rise. FILTER RAM AIR COOLANT IN COOLANT OUT GROUND SERVICE POINT RECEIVER DRIER TEMPERATURE BULB EVAPORATOR CONDENSER QUICK RELEASE PANEL THERMAL EXPANSION VALVE COMPRESSOR

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Some compressors are engine driven by a v belt (Figure 10) through an electromagnetic clutch assembly (Figure 11) which can be engaged or disengaged as required. The clutch drive plate is keyed to the compressor shaft and when the clutch is disengaged there is clearance between the drive plate and the engine driven pulley. The pulley rotates but the compressor is at rest. When the system calls for cooling the electromagnet energises and pulls and locks the drive plate to the drive pulley and therefore drives the compressor.

Engine Driven Compressor Figure 10

Electromagnetic Clutch Assembly Figure 11 V DRIVE BELT DRIVE PLATE PULLEY ELECTROMAGNETIC CLUTCH COIL COMPRESSOR OUTLET TO CONDENSER INLET FROM EVAPORATOR

DRIVE PLATE PULLEY ELECTROMAGNETIC COIL

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Some compressors are driven by a hydraulic motor whose pressure is supplied form an engine driven hydraulic pump. A solenoid valve is fitted to the hydraulic manifold. When no cooling is required the solenoid valve is de-energised allowing fluid to bypass the motor and return to the reservoir. When cooling is required the solenoid valve energises, closing off the bypass, allowing the hydraulic fluid to drive the compressor

1.4.7 The Receiver Dryer

High pressure high temperature refrigerant leaves the condenser and flows into the receiver dryer (Figure 12). This acts as a reservoir to hold the supply of refrigerant until it is needed by the evaporator. As the hot liquid enters the receiver dryer it first passes through a filter which removes any solid contaminants. It then passes through a layer of silica gel or activated alumina which removes any water moisture from the liquid. It also acts as a separator as some traces of vapour may be in the liquid.

The moisture is removed to prevent the system form freezing and becoming blocked and to prevent the moisture from acting with the refrigerant which would form hydrochloric acid which would corrode the pipelines and galleries internally. The liquid falls to the bottom of the receiver dryer where it is picked up via the pick up tube.

Some receivers include a sight glass that allows the checking of the refrigerant. If bubbles are seen then the system requires re-charging.

Receiver Dryer Figure 12 DESICCANT FILTER PADS SIGHT GLASS PICK UP TUBE

FROM CONDENSER TO THERMOSTATIC

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1.4.8 Thermostatic Expansion Valve

The thermostatic expansion valve (TEV) is a metering device that controls the amount of refrigerant that is allowed to flow into the evaporator by measuring the temperature of the evaporator discharge. All of the refrigerant should evaporate by the time it exits through the evaporator coils.

• The Term Superheat

Superheat, is heat energy that is added to a refrigerant to change it from a liquid into a vapour. Superheated refrigerant is very cold, not hot.

A TEV is shown at Figure 13. The TEV outlet attaches to the evaporator inlet. The TEV inlet comes from the receiver dryer. A diaphragm situated on top of the valve locates against push rods that act against a superheat spring. This action controls the position of the needle valve. The superheat spring tension is factory pre-set.

A temperature sensing bulb connects above the diaphragm via a capillary tube. The bulb is located in the vapour flow at the evaporator discharge outlet. It is insulated to allow only the outlet temperature to be sensed. The bulb and capillary tube is filled with a highly volatile fluid which reacts readily with temperature changes. When the bulb senses a rise in temperature the bulb liquid expands and exerts a force against the diaphragm, the superheat spring and the evaporator inlet pressure acting underneath the diaphragm. The amount of force exerted is directly related to the temperature of the vapour at the evaporator discharge.

A needle valve is located between the inlet and outlet of the TEV and its position is determined by the balance of the forces acting above and below the diaphragm including the pre set tension of the superheat spring.

When the system is started the evaporator is relatively warm and the bulb pressure above the diaphragm is high. This acts against the push rods and overcomes the superheat spring tension, to open the needle valve to allow maximum flow to the evaporator. As the refrigerant evaporates the evaporator outlet temperature decreases and the pressure above the diaphragm also decreases. The superheat spring overcomes this drop in pressure and closes the needle valve to a new position which restricts the amount of refrigerant that flows into the evaporator to ensure that it all evaporates by the time it reaches the evaporator outlet.

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Thermostatic Expansion valve Figure 13

1.5 DISTRIBUTION SYSTEMS

The air distribution system on most aircraft takes cold air from the air conditioning packs and hot air bleed from the engines and mixes the 2 in a mixer unit to the required temperature. The air is then distributed to side wall and overhead cabin vents. On some aircraft the cabin air is then drawn back into the mixing unit by re-circulating fans where it is mixed with new air and then re-distributed.

All major components are usually located together in a designated bay for ease of maintenance. ( Figure 14).

A gasper fan provides cold air to the individual overhead air outlets for the aircrew and passengers. This air can be drawn direct from outside or from the cooling packs. Each passenger or crew can control the amount of air received by controlling the position of the air outlet. This outlet could be a rotary nozzle or a louvre.

VALVE BODY

SUPERHEAT SPRING

TEMPERATURE SENSING BULB DIAPHRAGM INLET OUTLET CAPILLARY TUBE PUSH RODS NEEDLE VALVE

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Air Conditioning Distribution Manifold Figure 14

Conditioned air systems dispense temperature controlled air evenly throughout the cabin and crew areas. One duct system supplies the cockpit (Figure 17) while another supplies the cabin. The cabin ducting is then divided into 2 systems, the overhead (Figure 15) and the sidewall systems (Figure 16). The overhead system releases air into the cabin from outlets in ducting running fore and aft in the cabin ceiling. The sidewall duct system takes air through ducting between the sidewall and cabin interior linings and releases it through cove light grills and louvres. A cockpit controlled selector valve located on the main distribution manifold allows all overhead, side wall or any combination of the two systems to be used and varies the flow between the two.

WATER SEPARATOR GASPER FAN

MANIFOLD RELIEF VALVE

MIXER VALVES TO OVERHEAD DUCTS TO SIDEWALL DUCTS TO GASPER OUTLETS TO SIDEWALL DUCTS TO COCKPIT CONTROL VALVE SELECTOR LINKAGE CONTROL VALVES

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Overhead Panel Figure 15

Duct sections throughout both the cabin and cockpit are joined together with clamps or clips. Means of equalising the duct pressures and balancing the air flows are designed into each system. The systems are protected from excess pressures by use of a spring loaded pressure relief valve usually located in the main distribution manifold. The main manifold is located immediately downstream from the mixing units in the air conditioning bay.

On large aircraft a cockpit controlled dual selector valves divides the air between cockpit and cabin areas. These butterfly valves are interlinked. When one is fully open the other is fully closed and vice versa.

Air is exhausted from the passenger cabin through grills and outflow valves in the sidewalls above the floor. This air can then be directed around the cargo compartment walls where it assists in compartment temperature control. Some air then flows to the cargo heat distribution duct under the compartment floor and is then discharged overboard through the outflow valves.

GASPER FAN FLOOR EXHAUST DUCT

ADJUSTABLE AIR OUTLETS DUCTING

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Sidewall Ducting Figure 16

Below each floor air exhaust outlet is a flotation check valve. This valve is a plastic ball held in a cage. If the cargo compartments become flooded the balls float up the cage and seals off the floor to help prevent water from entering the cabin.

Cockpit Air Distribution Figure 17

Aircraft may be separated into zones each with its own air conditioning system and controls for that zone located in a distribution bay. Some areas may have a remote heat exchanger and fan assembly in the vapour cycle system, to allow cooling to specific areas such as avionics bays, fed from one of the zone packs.

SILENCER

FAN ASSY

FAN ASSY PRESSURE SWITCH COOLING FANS

FLIGHT DECK

TEMPERATURE SENSOR AIR VENT

CABIN TEMPERATURE SENSOR

WINDOW DEMISTER

FLOOR EXHAUST VENTS WALL FEEDER DUCTS

DISTRIBUTION BOXES

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1.5.1 Recirculation Air System

To improve cabin ventilation and supplement airflow the cabin air is recirculated back to the main distribution manifold where it is mixed with conditioned air form the cooling packs. The use of re-circulated air improves airflow and offloads the air supply system. This off loading of the air conditioning packs is converted into a fuel saving.

The re-circulation fan will draw air from the cabin area, through a check valve and filter assembly to remove any smoke and noxious odours before passing it to the mixer unit for re-distribution. The check valve prevents any reverse flow through the fan and ducting when the fan is not in use.

1.6 FLOW,TEMPERATURE AND HUMIDITY CONTROL SYSTEMS

Humidity control is the means of ensuring that the correct amount of water moisture is in the air conditioning air within the cabin (Figure 18). This is to ensure that passengers do not suffer from the low humidity levels at higher altitudes and that excessive moisture is removed at lower altitudes.

Typical Humidity Control System Figure 18

CABIN HUMIDITY SENSOR

OVERFILL DRAIN WATER SEPARATOR

DRAIN COLLECTOR TANK

WATER PUMP AND CONTROLLER

SPRAY NOZZLE

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Humidity can be controlled in 2 ways: • Water Separation

This is the removal of excessive moisture from the conditioned air normally using a water extractor or separator.

• Water Infiltration

This is the addition of water into the conditioned air as it enters the cabin using a water pump and spray nozzle.

Water Extraction

Water extraction is carried out by an extractor or separator and there are differing designs, but its function is the same, to remove moisture from the conditioned air. Water is produced into the air conditioning system due to the cooling and heating effects of the air in the air cycle system. The extractor is located in the air conditioning ducting prior to entry into the cabin. There are 3 main types of water separator in use:

1.6.1 Coalescer Type Water Extractor

Coalescer Water Extractor Figure 19 PRESSURE RELIEF VALVE DRAIN DIFFUSER COALESCER COLLECTOR SHELL CONDENSER TUBES

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The coalescer (Figure 19) consists of layers of monel gauze and glass fibre cloth sandwiched between layers of stainless steel gauze. It is supported by the diffuser cone and held in place by the relief valve. As the conditioned air is passed through the coalscer the moisture in the air is converted into water droplets. These droplets then enter the collector shell and deposited into the collector tubes where they drain down into the collector box.

This water is either drained overboard or passed to a water tank where it can be stored and used to infiltrate the system if required. The purpose of the relief valve is to open if the coalescer becomes blocked to allow conditioned air into the cabin.

1.6.2 Bag Type Coalescer

The bag is fitted over a support shell within the extractor. A swirl is imparted into the conditioned air as it passes the support shell. The fabric bag converts the moisture to water droplets and the centrifugal effect of the swirl on the droplets forces the droplets onto the outer shell where it collects and then drains from the component. There is usually a bag indicator which protrudes when the coalescer becomes dirty or blocked. A relief valve is fitted in case the coalescer becomes totally blocked. A typical bag coalescer is shown at Figure 20.

Bag Type Water Extractor Figure 20

BLOCKAGE INDICATOR BAG

PRESSURE RELIEF VALVE

WATER DRAIN

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1.6.3 Swirl Vane Type Water Separator

This extractor (Figure 21) uses either a rotating or fixed vane within the conditioned airflow. The vane rotates at high speed or rotates the airflow at high speed as the air passes through it and imparts a centrifugal force on the air impinging it against the exit shell. This impact converts the moisture into water droplets where it collects and falls into the sump area where it is then drained away.

Swirl Vane Type Water Separator Figure 21

1.6.4 Water Infiltration

As aircraft increase in altitude the moisture content of the outside air reduces to a level that may cause discomfort to passengers. To counteract this, water must be added to the conditioned air. This is done by pumping water through a spray nozzle into the ducting downstream of the extractor.

The action of the spray nozzle and velocity of the conditioned air converts the water droplets into a moisture. The water used in this sytem is usually the water that is collected and stored in a tank from the water extraction systems. This tank can also be replenished from ground services if required. The tank has an overboard drain in case it becomes overfull.Humidity sensors located in the cabin automatically turn on the humidity controller water pump to maintain cabin humidity at a certain level.

DRIAN

SWIRL VANE

_{WATER SUMP}

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1.7 PRESSURISATION SYSTEMS

As aircraft became capable of obtaining altitudes above that at which flight crews could operate efficiently, a need developed for complete environmental systems to allow these aircraft to carry passengers. Air conditioning could provide the proper temperature and supplemental oxygen could provide sufficient breathable air.

The problem was that not enough atmospheric pressure exists at high altitude to aid in breathing, and even at lower altitudes the body must work harder to absorb sufficient oxygen through the lungs to operate at the same level of efficiency as at sea level. This problem was solved by pressuring the cockpit/ cabin area. Cabin pressurisation is a means of adding pressure to the cabin of an aircraft to create an artificial atmosphere that when flying at high altitudes it provides gives an environment equivalent to that below 10000 feet.

Aircraft are pressurised by sealing off a strengthened portion of the fuselage. This is usually called the pressure vessel and will normally include cabin, cockpit and possibly cargo areas. Air is pumped into this pressure vessel and the pressure is controlled by an outflow valve located at the rear of the vessel.

Sealing of the pressure vessel is accomplished by the use of seals around tubing, ducting, bolts, rivets, and other hardware that pass through or pierce the pressure tight area. All panels and large structural components are assembled with sealing compounds. Access and removable doors and hatches have integral seals. Some have inflatable seals.

Pressurisation systems do not have to move large volume of air. Their function is to raise the pressure inside the vessel. Small reciprocating engine powered aircraft receive their pressurisation air from the compressor of a coupled turbocharger. Larger reciprocating engine powered aircraft receive air from engine driven compressors and turbine powered aircraft use compressor bleed air

Small Reciprocating Engine Powered Aircraft

Turbochargers are driven by the engine exhaust gases flowing through a turbine. A centrifugal compressor is coupled to the turbine. The compressors output is fed to the engine inlet manifold to increase manifold pressure which allows the engine to develop its power at altitude. Part of this compressed air is tapped off after the compressor and is used to pressurise the cabin. The air passes through a flow limiter (or sonic venturi) and then through an inter-cooler before being fed into the cabin. A typical system is shown at Figure 22.

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• Sonic Venturi

A sonic venturi is fitted in line between the engine and the pressurisation system. When the air flowing across the venturi reaches the speed of sound a shock wave is formed which limits the flow of air to the pressurisation system

Small Reciprocating Engine Aircraft Pressurisation System Figure 22

Large Reciprocating Engine Powered Aircraft

These aircraft use engine driven compressors driven through an accessory drive or by an electric or hydraulic motor. Multi engine aircraft have more than one air compressor. These are interconnected through ducting but each have a check valve or isolation valve to prevent pressure loss when one system is out of action.

OUTFLOW VALVE SAFETY VALVE

RAM AIR HEATING AIR PRESSURISED AIR EXHAUST GASES

COMBUSTION HEATER RAM AIR SHUT OFF VALVE

COUPLED TURBO COMPRESSOR

INTERCOOLER

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+3Turbine Powered Aircraft

The air supplied from a gas turbine engine compressor is contamination free and can be suitably used for cabin pressurisation (Figure 23). Some aircraft use an independent compressor driven by the engine bleed air. The bleed air drives the coupled compressor which pressurises the air and feeds it into the cabin

Turbo Compressor Figure 23

Some aircraft use a jet pump to increase the amount of air taken into the cabin (Figure 24). The jet pump is a venturi nozzle located in the flush air intake ducting. High velocity air from the engine flows through this nozzle. This produces a low pressure area around the venturi which sucks in outside air. This outside air is mixed with the high velocity air and is then passed into the cabin

BLEED AIR

ENGINE

PRESSURE VESSEL (CABIN/COCKPIT)

OUTFLOW VALVE FLUSH AIR INTAKE TURBO COMPRESSOR

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Jet Pump Figure 24

1.7.1 Control And Indication

There are 3 modes of pressurisation, un-pressurised, the isobaric mode and the constant–differential pressure mode. In the un-pressurised mode the cabin altitude remains the same as the flight altitude. In the isobaric mode the cabin altitude remains constant as the flight altitude changes and in the constant-differential pressure mode, the cabin pressure is maintained at a constant amount above the outside ambient air pressure.

The amount of differential pressure is determined by the structural strength of the aircraft. The stronger the aircraft structure the higher the differential pressure and the higher is the aircrafts operating ceiling.

1.7.2 The Un-Pressurised Mode

In this mode the outflow valve remains open and the cabin pressure is the same as the outside ambient air pressure. This mode is usually from sea level up to 5000` but does vary from aircraft to aircraft.

ENGINE FLUSH AIR INTAKE

PRESSURE VESSEL (CABIN/COCKPIT) JET PUMP

BLEED AIR

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1.7.3 The Isobaric Mode

In this mode the cabin pressure is maintained at a specific cabin altitude as flight altitude changes. The cabin pressure controller begins to close the outflow valve as the aircraft climbs to a chosen cabin altitude. The outflow valve then opens or closes (modulates) to maintain the selected cabin altitude as the flight altitude changes up or down. The controller will then maintain the selected cabin altitude up to the flight altitude that produces the maximum differential pressure for which the aircraft structure is rated. At this point the constant differential mode takes control.

1.7.4 The Constant-Differential Pressure Mode

Cabin pressurisation puts the aircraft structure under a tensile stress as the cabin pressure expands the pressure vessel. The cabin differential pressure is the ratio between the internal and external air pressures. At maximum constant-differential pressure as the aircraft increases in altitude the cabin altitude will increase but the internal/external pressure ratio will be maintained. There will be a maximum cabin altitude allowed and this will determine the ceiling at which the aircraft can operate.

1.7.5 Cabin Air Pressure Regulator

The pressure regulator maintains cabin altitude at a selected level in the isobaric range and limits cabin pressure to a pre-set pressure differential in the differential range by regulating the position of the outflow valve. Normal operation of the regulator requires only the selection of the desired cabin altitude and cabin rate of climb the adjustment of the barometric control.

The regulator shown in Figure 25 is a typical differential pressure type regulator that is built into the normally closed air operated outflow valve. It uses cabin altitude for its isobaric control and barometric pressure for the differential control. A cabin rate of climb controller controls the pressure change inside the cabin. There are 2 main sections to the regulator, the head and reference chamber and the base with the outflow valve and diaphragm. The balance diaphragm extends outward from the baffle plate to the outflow valve creating an air chamber between the baffle plate and the outer face of the outflow valve. Cabin air flowing into this chamber through holes in the side of the outflow valve exerts a force against the outer face of the valve which tries to open it. This force is opposed by the force of the spring around the valve pilot which tries to hold the valve closed.

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Cabin Pressure Regulator Figure 25

The actuator diaphragm extends outward from the outflow valve to the head assembly creating an air chamber between the head and the inner face of the outflow valve. Air from the head and reference chamber exert a force against the inner face of the outflow valve helping the spring to hold the valve closed.

The position of the outflow valve controls the amount of cabin air that is allowed to flow from the pressure vessel and this controls the cabin pressure. The position of the outflow valve is determined by the amount of reference chamber air pressure that presses on the inner face of the outflow valve.

1.7.6 Isobaric Control System

The isobaric control system of the pressure regulator shown in Figure 26 incorporates an evacuated capsule, a rocker arm, valve spring and a ball type metering valve. One end of the rocker arm is connected to the valve head by the evacuated capsule and the other end of the arm holds the metering valve in a closed position. A valve spring located on the metering valve body tries to move the metering valve away from its seat as far as the rocker arm allows.

ACTUATOR DIAPHRAGM OUTFLOW VALVE BAFFLE PLATE BASE REFERENCE CHAMBER HEAD PILOT DIAPHRAGM

ISOBARIC METERING VALVE ADJUSTER CONTROL

BAROMETRIC CAPSULE STATIC ATMOSHERE CONNECTION

ADJUSTER CONTROL DIFFERENTIAL METERING VALVE SOLENOID DUMP VALVE RESTRICTOR

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When the cabin air pressure increases enough for the reference chamber air pressure to compress the evacuated capsule the rocker arm pivots around its fulcrum and allows the metering valve to move away from its seat an amount proportional to the compression of the capsule. When the metering valve opens reference pressure air flows form the regulator to atmosphere through the atmospheric chamber.

Isobaric Control Operation Figure 26

When the regulator is operating in the isobaric range, cabin pressure is held constant by reducing the flow of reference chamber air through the metering valve. This prevents a further decrease in reference pressure.

The isobaric control responds to slight changes in reference pressure by modulating to maintain a constant pressure in the chamber throughout the isobaric range of operation. Whenever there is an increase in cabin pressure the isobaric metering valve opens which decreases the reference pressure and causes the outflow valve to open which then decreases the cabin pressure.

1.7.7 Differential Control System

EVACUATED BELLOWS ISOBARIC METERING VALVE

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The differential control system of the pressure regulator (Figure 27) incorporates a diaphragm a rocker arm, a valve spring and a ball type metering valve. One end of the rocker arm is attached to the head by the diaphragm which forma a pressure sensitive face between the reference chamber and the atmospheric chamber.

Differential Pressure Mode Figure 27

Atmospheric pressure acts on one side of the diaphragm and reference chamber pressure acts on the other. The opposite end of the rocker arm holds the metering valve in a closed position. A valve spring located on the metering valve body tries to move the metering valve away from its seat as far as the rocker arm allows.

When reference chamber pressure increases to the system differential pressure limit set above the decreasing atmospheric pressure it collapses the diaphragm which is set at differential pressure and opens the metering valve. Air flows from the reference chamber to atmosphere through the atmospheric chamber, which causes a reduction in the reference pressure. This reduction in reference pressure causes the outflow valve to open to reduce the cabin pressure to maintain the system pressure differential.

METERING VALVE

OUTFLOW VALVE

ATMOSPHERIC CHAMBER DIAPHRAGM

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1.7.8 Safety Valves

Cabin Air Pressure Safety Valve

The pressure relief valve prevents cabin pressure from exceeding the predetermined cabin to ambient pressure differential. A negative pressure relief valve and pressure dump valve may also be incorporated into this valve assembly.

Negative Pressure Relief Valve

A pressurised aircraft is designed to operate with the cabin pressure higher than the outside air pressure. If the cabin pressure were to become lower than the outside air pressure the cabin structure could fail. Outside air is allowed to enter the cabin to ensure that this does not happen. It is basically an inward pressure relief valve.

Dump Valve

This valve is normally solenoid actuated by a cockpit switch. When the solenoid is energised the valve opens dumping cabin air to atmosphere. Cabin pressure will decrease rapidly until it is the same as the outside air pressure and cabin altitude will increase until it is the same as the flight altitude.

1.7.9 Cabin Pressure Controllers

Most pressurisation systems have three basic cockpit indicators cabin altitude, cabin rate of climb and the pressure differential indicator.

The cabin altitude gauge (Figure 28) measures the actual cabin altitude. On most aircraft this altitude is controlled and maintained to around 5000`

Cabin Altitude Gauge Figure 28 0 1 2 3 4 5 6 7 8 9 10 CABIN ALTITUDE

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The cabin rate of climb indicator (Figure 29) tells the pilot the rate that the aircraft is either climbing or descending. The normal climb rate is 500` per minute and the decent rate is 400` per minute. The control can be automatic or manual depending on aircraft type

Cabin Rate Of Climb Figure 29

The differential pressure gauge (Figure 30) reads the difference in pressure between the cabin and the outside air pressures. This differential pressure is normally controlled and maintained to around 7psi. This depends on the aircraft type and the operating ceiling of the aircraft. The differential pressure gauge may be combined with the cabin altitude (Figure 31).

Differential Pressure Gauge Dual Gauge Figure 30 Figure 31 0 1 2 3 4 5 6 7 8 9 10 DIFF PX PSI UP DOWN CLIMB 1000 FT PER MIN .5 1 2 1.5 .5 1 1.5 2 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 PX D IFF P SI CABIN ALTITUDE

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1.8 SAFETY AND WARNING DEVICES

Both air conditioning and pressurisation systems use safety and warning devices to protect the aircraft from possible catastrophic failures. Some of the protection devices may be inhibited in certain stages of flight, landing or take off where the extra distractions caused by such warnings may be too much for the crews to deal with safely.

With the air conditioning system the main concerns are with overheating of the air conditioning packs and extraction and ventilation fans, as well as hot air leaks from ducting which could damage surrounding structure or components.

1.8.1 Overheating

Most packs systems are protected from overheating by a thermal switch downstream of the pack outlet. If the outlet temperature reaches a pre determined figure the switch will operate causing the pack valves to shut, preventing air from getting to the packs, as well as sending a warning signal to the cockpit central warning panel with associated caution/warning lights and aural chimes and to illuminate a fault light on the pack selector switch.

Once the system has cooled down sufficiently the crew may have an option to reselect the overheated system. The overheat may have been caused by a fault in the automatic temperature control system in which case the pilot may be able to control the system manually via a manual selector switch on the cockpit controller.

Extraction or ventilation fans will be protected in much the same way. An overheat will signal the central warning panel with associated caution/warning lights and aural chimes. The fan may be isolated automatically or manually. Once the fan has cooled down it may be possible to re-select if required.

Fans may also be protected from over or under speeding which will also have an effect on the system temperatures. Speed sensors on the fan will indicate a fault when over or under speed limits are reached and a warning signal is sent to the cockpit central warning panel with associated caution/warning lights and aural chimes.

1.8.2 Duct Hot Air Leakage

Any ducting that includes joints is liable to leak under abnormal conditions. A duct protection system will include fire-wire elements around the hot zones such as engine air bleeds, air conditioning packs and auxillary power units if fitted.

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The sensing elements will be the thermistor type. As the temperature around the wire increases the resistance decreases until an electrical circuit is made. When the circuit is made a warning signal is sent to the cockpit central warning panel with associated caution/warning lights and aural chimes. The leaking duct may be isolated automatically or may require the pilot to take action to close off the air valves. The faulty system will then remain out of use.

1.8.3 Excess Cabin Altitude

If the cabin altitude was allowed to increase unchecked the crew and passengers could unknowingly suffer the effects of hypoxia. This dangerous condition is obviously undesirable especially for the aircrew. Most aircraft give a warning on the CWP with associated audio and visual warnings when the cabin altitude reaches 10000`.

1.8.4 Smoke Detection

Smoke detectors may be fitted within the cabin, avionics bay and cargo areas to monitor systems which if become faulty may generate smoke on overheating or are may be liable to catch fire. These detectors will send a signal to the the CWP with associated lights and audio warnings. They may also automatically switch on extractor fans which will remove the smoke overboard and away form the cabin and cockpit areas. In this event, the pilot may have a switch or control lever to operate a valve to isolate the cockpit air conditioning ducting from the rest of the aircraft to prevent any smoke from getting to the cockpit.

Easa Part 66 - Module 11.04 - Air Conditioning and Cabin Pressurisation

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CONTENTS

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PAGE

INTENTIONALLY

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1

AIR CONDITIONING AND CABIN PRESSURISATION

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CONTROL VALVE

EXHAUST MANIFOLD

HEATER MUFF

CONTROL

LEVER

RAM

AIR

OVERBOARD DUMP

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DRIAN

SWIRL VANE

WATER SUMP

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_{WATER SUMP}