Aircraft Air Conditioning System
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Questions and Answers

What is the primary advantage of a load compressor over a single compressor type?

  • Enhanced system reliability
  • Improved fuel efficiency (correct)
  • Increased air pressure
  • Reduced maintenance costs

    • What is the typical limitation of bleed air extraction in APUs designed for in-flight operation?

  • Airspeed
  • Altitude (correct)
  • Engine power
  • Fuel consumption

    • What is the primary purpose of a ground pneumatic supply?

  • Cabin pressurization
  • Main engine starting and short duration maintenance checks (correct)
  • Air conditioning system operation
  • Hydraulic system operation

    • What determines the type of unit used to provide air for the pneumatic system?

    <p>System's air pressure requirements</p> Signup and view all the answers

    How much fuel can an APU burn per hour?

    <p>Up to 500 kg</p> Signup and view all the answers

    What is the function of a load control valve in an APU?

    <p>To deliver air to the pneumatic system</p> Signup and view all the answers

    What is the typical design limitation of most APUs?

    <p>Bleed air extraction only on the ground</p> Signup and view all the answers

    What is the benefit of a dual compressor APU design?

    <p>Enhanced fuel efficiency</p> Signup and view all the answers

    What is the primary purpose of a Roots-type blower in an aircraft cabin system?

    <p>To provide a predetermined volume of air to the cabin</p> Signup and view all the answers

    What type of compressor is commonly used in aircraft engines?

    <p>Positive displacement compressor</p> Signup and view all the answers

    How does a Roots-type blower increase pressure in the cabin system?

    <p>By delivering more air than the system can use</p> Signup and view all the answers

    What is the purpose of the check valves in a multi-stage compressor?

    <p>To allow air to flow in only one direction</p> Signup and view all the answers

    What is the primary source of power for a Roots-type blower?

    <p>Accessory drive gearing</p> Signup and view all the answers

    What is the function of the finned blower housing in a Roots-type blower?

    <p>To provide some cooling as well as some outside air</p> Signup and view all the answers

    What is the primary function of the compressor in a turbo-compressor system?

    <p>To compress the outside air, increasing its temperature</p> Signup and view all the answers

    What is the purpose of the jet pump in an aircraft?

    <p>To increase the amount of air taken into the cabin</p> Signup and view all the answers

    What is the advantage of using an independent cabin compressor over a bleed air source?

    <p>It eliminates the disadvantages associated with bleed air sources</p> Signup and view all the answers

    What is the result of compressing the outside air in a turbo-compressor system?

    <p>The air is heated significantly</p> Signup and view all the answers

    How is pressure controlled in a cabin system using a Roots-type blower?

    <p>By outflow valves</p> Signup and view all the answers

    What is the primary purpose of the reservoir in a pneumatic system?

    <p>To provide a storage of compressed air for the pneumatic services</p> Signup and view all the answers

    What is the function of the turbine in a turbo-compressor system?

    <p>To drive the compressor and cool the air significantly</p> Signup and view all the answers

    What is the material used to make the storage bottles in a pneumatic system?

    <p>Steel</p> Signup and view all the answers

    What is the purpose of the venturi in a jet pump system?

    <p>To create a low-pressure region that draws air in from the outside</p> Signup and view all the answers

    What is the result of the bleed air passing through the turbine in a turbo-compressor system?

    <p>The air is cooled dramatically</p> Signup and view all the answers

    What is the primary focus of the learning objective 11.16.1 in pneumatic and vacuum systems?

    <p>Explain the layout requirements of pneumatic and vacuum systems</p> Signup and view all the answers

    What is the main purpose of the Venturi System in pneumatic and vacuum systems?

    <p>To provide a vacuum supply</p> Signup and view all the answers

    What type of system is described as having a combination of vacuum and low-pressure systems?

    <p>Combined Vacuum and Low-Pressure System</p> Signup and view all the answers

    What is the main function of the pressure control system in pneumatic and vacuum systems?

    <p>To regulate the air pressure</p> Signup and view all the answers

    What is the purpose of the sensors and indicators in pneumatic and vacuum systems?

    <p>To monitor the air pressure</p> Signup and view all the answers

    What is the main purpose of the distribution system in pneumatic and vacuum systems?

    <p>To distribute the air supply</p> Signup and view all the answers

    What is the main function of the overheat detectors in pneumatic and vacuum systems?

    <p>To detect overheating</p> Signup and view all the answers

    What is the primary purpose of the interface with other systems in pneumatic and vacuum systems?

    <p>To integrate with other systems</p> Signup and view all the answers

    What is the primary purpose of the venturi tube in an aircraft?

    <p>To produce suction to drive flight instruments</p> Signup and view all the answers

    What is the primary advantage of using filtered air in gyro instruments?

    <p>It reduces the risk of oil contamination</p> Signup and view all the answers

    What is the purpose of the oil separator in a wet-type vacuum pump?

    <p>To capture oil and return it to the sump</p> Signup and view all the answers

    What is the primary improvement in wet type vacuum pumps with the incorporation of carbon into the vanes and rotor?

    <p>Self-lubricating vanes and rotor</p> Signup and view all the answers

    What is the primary characteristic of a dry-type vacuum pump?

    <p>It is not lubricated by engine oil</p> Signup and view all the answers

    What is the effect of oil particles on the operation of gyro instruments?

    <p>They have a detrimental effect on the instrument's operation</p> Signup and view all the answers

    What is essential for the proper functioning of dry type vacuum pumps?

    <p>Clean air flowing through the pump</p> Signup and view all the answers

    What is the primary function of an engine-driven vacuum pump in an aircraft?

    <p>To produce suction for flight instruments</p> Signup and view all the answers

    What type of instruments use low-pressure, air-driven gyro systems in some aircraft?

    <p>Backup gyro instruments</p> Signup and view all the answers

    What is the common problem with venturi tubes in aircraft?

    <p>Formation of ice in the throat</p> Signup and view all the answers

    What is the purpose of regulating and filtering the air in low-pressure systems?

    <p>To prepare the air for the gyros</p> Signup and view all the answers

    What type of pumps are used to provide low-pressure air on aircraft fitted with reciprocating engines?

    <p>Vane-type air pumps</p> Signup and view all the answers

    What is the purpose of filtering the air in air-driven gyro instruments?

    <p>To prevent oil contamination from the engine</p> Signup and view all the answers

    What is the source of air for low-pressure systems in turbine-powered aircraft?

    <p>Engine compressor bleed air</p> Signup and view all the answers

    What is the purpose of filtering the air in low-pressure systems?

    <p>To remove contaminants and moisture</p> Signup and view all the answers

    What is the advantage of using a dry type vacuum pump over a wet type?

    <p>No oil supply required to the vane contact area</p> Signup and view all the answers

    What is the primary function of a variable orifice in a pneumatic system?

    <p>To enable the variable pneumatic restrictor to be set for a wide range of airflow</p> Signup and view all the answers

    What is the purpose of the bleed valve in a pneumatic system?

    <p>To relieve compressor load when the engine stops or is shut down</p> Signup and view all the answers

    At what system pressure does the internal relief valve in the pressure-regulating (unloading) valve dump pump output pressure overboard?

    <p>3800 psi</p> Signup and view all the answers

    What is the function of the spring in a relief valve?

    <p>To hold the valve closed at normal pressures</p> Signup and view all the answers

    What is the primary purpose of a relief valve in a pneumatic system?

    <p>To prevent damage to the system</p> Signup and view all the answers

    What happens when the system pressure drops to 2900 psi in a pressure-regulating (unloading) valve?

    <p>The output of the pump is directed back into the system</p> Signup and view all the answers

    What is the normal state of the bleed valve in a pneumatic system?

    <p>Normally closed</p> Signup and view all the answers

    What is the purpose of the pressure-regulating (unloading) valve in a pneumatic system?

    <p>To maintain a system pressure between 2900 psi and 3300 psi</p> Signup and view all the answers

    Under what condition does the amber dual bleed light come on?

    <p>When the APU bleed valve is open and either the number 1 engine bleed switch or the number 2 engine bleed switch and isolation valve are on</p> Signup and view all the answers

    What is the function of the pneumatic duct pressure indicator?

    <p>To indicate the pressure of the left- and right-hand pneumatic ducts in the pneumatic manifold</p> Signup and view all the answers

    What is the purpose of the wing and body overheat detection system?

    <p>To detect overheat conditions in the pneumatic distribution system ducts</p> Signup and view all the answers

    What happens when the wing and body overheat detection system detects an overheat condition?

    <p>The left and right wing-body overheat light comes on and is displayed on the air conditioning panel</p> Signup and view all the answers

    What is the purpose of the sensing elements in the wing and body overheat detection system?

    <p>To detect overheat conditions in the pneumatic distribution system ducts</p> Signup and view all the answers

    What is the location of the sensing elements in the wing and body overheat detection system?

    <p>Adjacent to the pneumatic ducts</p> Signup and view all the answers

    What is the color of the wing and body overheat light when an overheat condition is detected?

    <p>Amber</p> Signup and view all the answers

    What is displayed on the air conditioning panel when the wing and body overheat detection system detects an overheat condition?

    <p>The left and right wing-body overheat light (amber)</p> Signup and view all the answers

    What is the primary purpose of using a venturi system in aircraft?

    <p>To produce suction to drive the turn-and-bank indicator and the directional and attitude gyros</p> Signup and view all the answers

    What is the primary difference between a wet-type vacuum pump and a dry-type vacuum pump?

    <p>The material used to make the vanes and rotor</p> Signup and view all the answers

    What is the primary problem with using oil-lubricated vacuum pumps in aircraft?

    <p>The oil particles have a detrimental effect on the operation of the gyro instruments</p> Signup and view all the answers

    What is the primary function of the filter in a vacuum system?

    <p>To remove impurities from the air</p> Signup and view all the answers

    What is the primary advantage of using air-driven gyro instruments in aircraft?

    <p>They are more reliable and less prone to failure</p> Signup and view all the answers

    What is the primary reason for using compressed air over hydraulic or electrical systems in aircraft?

    <p>Compressed air is universally available in an inexhaustible supply and system components are reasonably simple and lightweight</p> Signup and view all the answers

    What is the typical operating range of medium-pressure systems in aircraft?

    <p>50 psi to 150 psi</p> Signup and view all the answers

    What is the primary purpose of vacuum systems in early aircraft?

    <p>To drive gyro instruments</p> Signup and view all the answers

    What is the primary advantage of using air-driven systems over hydraulic systems in aircraft?

    <p>Air is lighter than hydraulic fluid and requires no return plumbing system</p> Signup and view all the answers

    What is the primary function of high-pressure systems in aircraft?

    <p>To generate hydraulic power and operate mechanical systems</p> Signup and view all the answers

    What is the primary reason why bleed air from a turbine engine is not used for cabin air during certain conditions?

    <p>Because of contamination from lubricants or fuel</p> Signup and view all the answers

    What is the purpose of the pressure regulator or pressure-regulating valve in a pneumatic system?

    <p>To control air pressure</p> Signup and view all the answers

    What is the advantage of using a dual compressor APU design over a single compressor type?

    <p>All of the above</p> Signup and view all the answers

    What is the primary source of pressurised air in modern aircraft with turbine engines?

    <p>Engine compressors</p> Signup and view all the answers

    What is the result of bleeding air from the engine compressor?

    <p>Reduced engine power output</p> Signup and view all the answers

    What is the primary purpose of the air control valve in the leading edge de-icing system?

    <p>To apply vacuum to the air tubes when the de-icing system is not in use</p> Signup and view all the answers

    What is the primary function of the pressure-controlling unit in medium-pressure pneumatic systems?

    <p>To reduce the pressure to about 40 psi</p> Signup and view all the answers

    What is the primary disadvantage of high-pressure systems using metal bottles?

    <p>They are not suitable for continuous operation</p> Signup and view all the answers

    What is the primary function of the sliding vanes in vane-type air pumps?

    <p>To rotate the driveshaft and create airflow</p> Signup and view all the answers

    What is the primary purpose of using filtered air in low-pressure systems?

    <p>To prevent contamination of the instruments</p> Signup and view all the answers

    What is the primary purpose of a turbocharger in an aircraft?

    <p>To drive the compressor stage and provide high-pressure air</p> Signup and view all the answers

    How does a jet pump system increase the airflow into the cabin?

    <p>By using a venturi to create a low-pressure region</p> Signup and view all the answers

    What is the purpose of the reservoir in a pneumatic system?

    <p>To store compressed air for emergency situations</p> Signup and view all the answers

    What is the function of the regulator valve in a vacuum system?

    <p>To relieve excessive vacuum by allowing atmospheric air into the system</p> Signup and view all the answers

    What is the purpose of the stack pipes in a pneumatic system?

    <p>To prevent contamination from entering the system</p> Signup and view all the answers

    What is the primary purpose of a ground pneumatic supply in an aircraft without a serviceable APU?

    <p>To start the main engine and for short duration maintenance checks</p> Signup and view all the answers

    What type of compressor is commonly used to provide air for the pneumatic system in some aircraft?

    <p>Roots-type blower</p> Signup and view all the answers

    What is the primary function of the check valves in a multi-stage compressor?

    <p>To allow air to flow in only one direction</p> Signup and view all the answers

    How does a Roots-type blower increase pressure in the cabin system?

    <p>By adding more air to the system with each rotation</p> Signup and view all the answers

    What is the primary purpose of the drive mechanism in a centrifugal cabin compressor?

    <p>To automatically adjust to compensate for changes in engine rpm</p> Signup and view all the answers

    What is the primary function of the overpressure switch in the Pressure-Regulating and Shutoff Valve (PRSOV)?

    <p>To close and cause a bleed trip off condition if supply air to the bar gets to 220 psi</p> Signup and view all the answers

    What determines the pressure range maintained by the unloading valve in a pneumatic system?

    <p>The system pressure and pump output pressure</p> Signup and view all the answers

    What is the purpose of the inlet filter fitted to the turn-and-slip indicator when using a specific reducer?

    <p>To filter out impurities in the air</p> Signup and view all the answers

    What is the primary function of the surge control valve in the load compressor?

    <p>To discharge the excess of air in the exhaust</p> Signup and view all the answers

    What is the result of the spring tension being overcome in a relief valve?

    <p>The valve opens and relieves pressure</p> Signup and view all the answers

    What is the purpose of the gold coating on some ducts in the pneumatic manifold?

    <p>To protect the ducts from the effects of hydrocarbon contamination</p> Signup and view all the answers

    What is the primary function of the valves in the pneumatic manifold?

    <p>To control the flow of bleed air into the manifold and isolate the manifold into left and right sides</p> Signup and view all the answers

    What is the primary function of the variable orifice in a pneumatic system?

    <p>To enable the setting of the variable pneumatic restrictor</p> Signup and view all the answers

    What is the purpose of the insulation blankets on some ducts in the pneumatic manifold?

    <p>To reduce the heat transfer</p> Signup and view all the answers

    What is the primary limitation of the high-pressure storage cylinder system?

    <p>It cannot be recharged during flight, limiting its operation</p> Signup and view all the answers

    Study Notes

    Air Cycle Machine

    • The turbine cools dramatically due to work performed and de-compression of bleed air, resulting in cold air at the output.
    • Compressor compresses outside air, heating it significantly.
    • Heated compressed air is blended with cooled, decompressed bleed air to achieve correct temperature and pressure.

    Jet Pump

    • Jet pump flow multiplier increases air taken into the cabin.
    • Nozzle blows high-velocity compressor bleed air into a venturi, producing a low-pressure region that draws in outside air.

    Reservoirs

    • Storage bottles provide reservoir of compressed air for pneumatic systems.
    • Bottles are made of steel and may be wire-wound for maximum strength.
    • Compressors build up system pressure when it falls below normal level.

    Compressor Types

    • Independent cabin compressors are designed to overcome disadvantages of bleed air sources.
    • Compressors can be driven through accessory drive gearing or powered by bleed air from an engine compressor.
    • Two main types of compressors: positive displacement and centrifugal.

    Positive Displacement Type

    • Roots-type blower takes a predetermined volume of air, compresses it, and delivers it to the cabin duct.
    • Rotors mounted on parallel shafts rotate at the same speed, building up pressure.

    Multi-Stage Compressor

    • Check valves allow air to flow in only one direction.
    • Three check valves are used in the diagram.

    Load Compressor

    • Load compressor can react to pneumatic demands of the aircraft more efficiently.
    • Ability to offload the load compressor when full demand is not required makes it fuel-efficient.

    Ground Supply

    • Ground pneumatic supply is not designed to cope with demands of the air conditioning system.
    • Use is normally restricted to main engine starting and short duration maintenance checks.

    Ground Supply Cart

    • Provides a source of pneumatics for ground operations when an APU is not serviceable.

    Pneumatic and Vacuum Systems

    • Pneumatic and vacuum systems are used to power aircraft instruments and systems
    • Venturi systems are used to produce suction to drive certain instruments
    • Venturi tubes are rated by the amount of vacuum they will produce at 120 mph or 104 knots
    • Common problems with venturi systems include:
      • Formation of ice in the throat
      • Damage or deformation to the tube assembly
      • Foreign object blockage
      • Blocked filter cartridge
      • Leaking plumbing

    Vacuum Systems and Pumps

    • Air-driven gyro instruments use an engine-driven vacuum pump to evacuate the instrument case
    • There are two main types of positive-displacement, vane-type vacuum pumps:
      • Wet-type pump
      • Dry-type pump
    • Wet pumps use oil lubrication, which can contaminate the instrument
    • Dry pumps use self-lubricating vanes and rotors, eliminating the need for oil lubrication
    • Filter maintenance and plumbing condition are crucial for dry pumps

    Low-Pressure Systems

    • Some aircraft use low-pressure, air-driven gyro instruments as primary or backup instruments
    • Turbine-powered aircraft use engine compressor bleed air, while reciprocating engines use vane-type air pumps
    • The air is regulated and filtered before being directed to the instruments

    Vane-Type Air Pump

    • Vane-type air pumps are used on aircraft with reciprocating engines to provide low-pressure air
    • A variable orifice enables the variable pneumatic restrictor to be set for a wide range of airflow

    Bleed Valves and Pressure Regulating Valves

    • Bleed valves are used to relieve compressor load in the event of engine lubricating oil pressure loss
    • Pressure regulating (unloading) valves maintain a system pressure of between 2900 psi and 3300 psi
    • Relief valves are used to prevent damage from excessive pressures

    Indications and Warnings

    • Pneumatic duct pressure indicators display left- and right-hand pneumatic duct pressure
    • Overheat detectors monitor the pneumatic distribution system ducts for overheat conditions
    • Warnings are displayed in the flight compartment in the event of an overheat condition

    Aircraft Pressure and Vacuum Systems

    • Many aircraft use compressed air or pneumatic systems for various purposes, including:
      • Pressurization
      • Air conditioning
      • De-icing
      • Anti-icing
      • Engine-starting
      • Hydraulic power generation
      • Jet pump operation
      • Water and hydraulic system pressurization
      • Instrument operation
      • Landing gear
      • Flaps
      • Brakes
      • Other forms of mechanical actuation

    Pneumatic Systems Categories

    • Pneumatic systems fall into four basic categories:
      • Vacuum systems
      • Low-pressure systems (1-10 psi)
      • Medium-pressure systems (50-150 psi)
      • High-pressure systems (1000-3000 psi)

    Advantages of Pneumatic Systems

    • Advantages of using compressed air over hydraulic or electrical systems:
      • Universally available in an inexhaustible supply
      • System components are reasonably simple and lightweight
      • Compressed air is lighter than hydraulic fluid
      • No return plumbing system is required
      • Minimised fire hazard
      • Temperature problems are minimised

    Venturi System Vacuum Supply

    • Used in early light aircraft and some simple home-built aircraft
    • System is extremely inefficient and limited in its capacity to drive instruments
    • Works by accelerating air through a narrowing section, producing a lower pressure at the throat
    • Common problems with venturi systems:
      • Ice formation in the throat
      • Damage or deformation to the tube assembly
      • Foreign object blockage
      • Blocked filter cartridge
      • Leaking plumbing

    Vacuum Systems and Pumps

    • Air-driven gyro instruments use an engine-driven vacuum pump to evacuate the instrument case
    • Two main types of positive-displacement, vane-type vacuum pumps:
      • Wet-type pump
      • Dry-type pump
    • Wet-type pump:
      • Lubricated from the engine low-pressure oil system
      • Oil has a one-way passage through the pump and is captured by the oil separator
    • Dry-type pump:
      • Incorporates carbon into the vanes and rotor
      • Self-lubricating, no oil is needed
      • Filter maintenance and plumbing condition is crucial

    Low-Pressure Systems

    • Some aircraft use low-pressure, air-driven gyro instruments as either primary or backup instruments
    • Turbine-powered aircraft use bleed air from the engine compressor section
    • Aircraft with reciprocating engines use vane-type air pumps driven by electric motors or the aircraft engine
    • Air is regulated and filtered before it reaches the instrument

    Vane-Type Air Pump

    • Used to provide low-pressure air on aircraft with reciprocating engines
    • Pump is driven by electric motors or the aircraft engine
    • Air is pulled into the pump and moved out as the chambers decrease in size

    Combined Vacuum and Low-Pressure Systems

    • Used in aircraft with rubber de-icer boots on the leading edges of the aerofoils
    • Inflatable boots are constructed with several integrated air tubes
    • Tubes are attached to plumbing from an air control valve, enabling some tubes to be inflated while others are deflated

    Medium-Pressure Systems

    • Usually does not include an air bottle
    • Compressed air is taken from the turbine engine compressor section or a separate compressor device
    • Air is routed to a pressure-controlling unit to reduce the pressure to about 40 psi

    High-Pressure Systems

    • Air is usually stored in metal bottles at pressures ranging from 1000 to 3000 psi
    • Bottle has two valves:
      • Charging valve
      • Control valve
    • System is limited by the small supply of bottled air
    • Cannot be used for continuous operation of systems like landing gear or brakes

    Pneumatic Sources

    • In flight:
      • Engine compressors
      • Airborne Auxiliary Power Unit (APU) (if approved for bleed air supply in flight)
    • On the ground:
      • Mobile starter units or ground pneumatic networks
      • APUs
      • Engine compressors

    Engine Bleed Air Supply

    • Pressurised air is taken from the engine compressor
    • Bled off the compressor in one or two places
    • Air from the fifth stage is considered low-pressure air
    • Air from the tenth stage is considered high-pressure air
    • Both bleed ports are never open at the same time
    • Changeover is automatic

    APU Supply

    • Two basic types of APUs:
      • Single compressor
      • Dual compressor
    • Compressed air is delivered to the aircraft pneumatic system via a load control valve
    • Most APUs are designed for bleed air extraction on the ground only

    Ground Supply (External Supply)

    • Used for main engine starting and short duration maintenance checks
    • Not designed to cope with the demands of the air conditioning system

    Compressors

    • Type of unit used to provide air for the pneumatic system is determined by the system's air pressure requirements
    • Independent cabin compressors have been designed to overcome the disadvantages of bleed air sources
    • Compressor types:
      • Positive displacement type (Roots-type blower)
      • Centrifugal compressor
    • Roots-type blower:
      • Takes a predetermined volume of air, compresses it, and then delivers it to the cabin duct
      • Pressure is built up because the blower can deliver more air than the system can use
    • Centrifugal compressor:
      • Has a centrifugal impeller similar to a turbo supercharger
      • Outside air is admitted to the supercharger and is then compressed by the high-speed impeller
      • Delivered to a distribution system

    Turbochargers

    • Used to provide air for the pneumatic system
    • Driven by a smaller amount of high-pressure air drawn from the engine compressor stage (bleed air)
    • Bleed air drives a turbine, which directly drives a compressor
    • Hot compressed bleed air driving the turbine cools dramatically
    • Act of compressing the air heats it significantly
    • Heated, compressed outside air is blended with the cooled, decompressed bleed air to achieve the correct temperature and pressure

    Jet Pump

    • Used to increase the amount of air taken into the cabin
    • Essentially a special venturi inside a line from the outside of the aircraft
    • Nozzle blows a stream of high-velocity compressor bleed air into the throat of the venturi
    • Produces a low-pressure region that draws air in from the outside

    Reservoirs

    • Storage bottles provide the reservoir of compressed air which operates all services

    • Compressors are used to build up system pressure when it falls below the normal level

    • Volume of the actuators and pipelines determines the size of the bottles required for normal and emergency operation of the pneumatic services### High-Pressure Storage Bottles

    • Bottles are mounted upright and have a fitting at the bottom with a supply connection, pressure gauge connection, and a drain valve to remove moisture or sediment.

    • Stack pipes are provided at the connections to prevent contamination.

    • Pressure testing is required at specified periods, and the date of testing is stamped on the neck of the bottle.

    Pressure Control and Regulators

    • A regulator valve is used to maintain a stable airflow of about four inches of mercury in a vacuum system.
    • The valve is adjustable and relieves excessive vacuum by allowing atmospheric air into the system through its own filter assembly.
    • Some vacuum systems have a suction reducer or restrictor to operate at different vacuum levels.
    • The suction reducer has a variable orifice to set a wide range of airflow.

    Bleed Valves

    • Bleed valves are used in pneumatic systems and are normally kept closed by engine oil pressure.
    • When the engine stops or is shut down, the valve opens and relieves compressor load to prevent air stored in the system from leaking into the compressor case.

    Pressure Regulating (Unloading) Valve

    • The unloading valve maintains a system pressure of between 2900 psi and 3300 psi.
    • When the system pressure rises to 3800 psi, an internal relief valve dumps pump output pressure overboard.
    • When the system pressure drops to 2900 psi, the output of the pump is directed back into the system.

    Relief Valves

    • Relief valves are used in pneumatic systems to prevent damage from excessive pressure.
    • They act as pressure-limiting units and prevent excessive pressures from bursting lines and blowing out seals.
    • At normal pressures, a spring holds the valve closed, and air remains in the pressure line.
    • If pressure grows too high, the force it creates on the disk overcomes spring tension and opens the relief valve.

    Engine Bleed Air Pressure Control

    • The Pressure-Regulating and Shutoff Valve (PRSOV) regulates bleed air pressure and contains a latching solenoid valve, an overpressure switch, and a pressure relief valve.
    • The PRSOV actuator opens the valve against spring force and pressure in chamber B to regulate the downstream pressure to 42 psi (nominal).

    APU Bleed Air Pressure Control

    • The Auxiliary Power Unit (APU) load compressor supplies the APU bleed air.
    • The quantity of air changes with the different bleed air demands of the pneumatic system.
    • Bleed air supply to the aircraft pneumatic system is controlled by a load control valve.
    • The load control valve is controlled by the Electronic Control Box (ECB) and opens when the APU bleed switch is set to the on position.

    Distribution and Ducts

    • The pneumatic manifold collects compressed air from the engines, APU, or pneumatic ground cart and supplies it to the user systems.
    • Valves in the manifold control the flow of bleed air into the manifold, isolate the manifold into left and right sides, and control the flow of manifold air into the user systems.
    • Pneumatic manifold ducts are made of strong, corrosion-resistant, light-weight tubing.
    • Some ducts have a gold coating to protect them from hydrocarbon contamination and decrease heat transfer.

    High-Pressure Distribution System

    • High-pressure systems store air in metal bottles at pressures ranging from 1000 to 3000 psi.
    • The high-pressure storage cylinder has a charging valve and a control valve.
    • Although the system cannot be recharged during flight, operation is limited by the small supply of bottled air.

    Emergency Backup Distribution Systems

    • Many aircraft use a high-pressure pneumatic back-up source of power to extend the landing gear or actuate the brakes if the main hydraulic braking system fails.
    • The nitrogen is not directly used to actuate the landing gear actuators or brake units, but instead applies pressurised nitrogen to move hydraulic fluid to the actuator.

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    Description

    This quiz covers the processes involved in an aircraft's air conditioning system, including compression, decompression, and blending of air to achieve the correct temperature and pressure.

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