11.16 Pneumatic and Vacuum

• Pneumatic systems use air pressure to transfer energy for high-power aircraft systems.
• Compressed air is used in aircraft for pressurization, air conditioning, de-icing, engine starting, hydraulic power, jet pump operation, instrument operation, landing gear, flaps and slats, and brakes.
• Power sources for pneumatic systems include engines, Auxiliary Power Units (APU), and Ground Air Power Units.

Pneumatic System Layout Components

• Pressure regulating shut-off valve regulates pneumatic supply pressure.
• Fan air modulating valve regulates pneumatic supply temperature.
• Pre-cooler cools pneumatic air with fan air.
• Isolation/shut-off valve splits or connects pneumatic systems.
• Pressure sensor measures pressure in the pneumatic duct system.
• Temperature sensors control the temperature of air going to user systems.
• Overpressure valve protects users against overpressure if the pressure regulating valve malfunctions.
• Pneumatic system controller controls the main components and provides the correct amount of air to users.
• Categories include vacuum, low-pressure, medium-pressure, and high-pressure systems.

Compressed Air Advantages

• Universally available
• Simple and lightweight components
• Lighter than hydraulic fluid
• No return lines are required
• Free from temperature problems
• Minimal fire hazard

Vacuum System

• Some aircraft use vacuum systems to power gyroscopic instruments.

Low-pressure System

• Essential for anti-icing, supplying warm air at 1-10 psi from the engine or APU.
• Valves and sensors control temperature and pressure.

Medium-pressure System

• Typically operates at 50-150 psi.
• Draws air from the compressor section of a turbine engine, and it is used to pressurize hydraulic reservoirs.

High-pressure Systems

• Air stored in metal bottles at 1000-3000 psi.
• Contains a charging valve for ground compressor connection.

Aircraft Cross-bleed System

• Engines supply the pneumatic system, feeding left or right-wing ducts.
• The cross-bleed system links these ducts, allowing air to reach all users.
• Consists of a crossover duct and one or more cross-bleed valves.
• The pneumatic duct system includes left-wing ducts, right-wing ducts, and the crossover duct.

Safety Precautions

• Depressurize before working on the pneumatic system.
• The engine components remain hot after shutdown.
• Wait one hour after shutdown before working on engine components.

Air Sources

• Include engines, APUs, and ground power units.

Engine Bleed Air

• Turbine engines use pressurized air from the compressor section (low or high).
• Bleed ports include Low-Pressure (LP), Immediate-Pressure (IP), and High-Pressure (HP) ports.
• Bleed port selection depends on engine speed and compressor stage.
• LP or IP air is for take-off/climb/cruise.
• HP bleed air is for low-speed flight/descent/low engine power.

Contamination Risk

• Bleed air is not free power and reduces engine performance
• It can be contaminated by lubricants or fuel

APU Bleed Air

• Can supply air directly or with a load compressor.
• Inlet Guide Vanes (IGV) regulate airflow to the load compressor.
• The surge control valve maintains a stable air supply.
• Bleed air is available when the APU runs above 95%.
• APU bleed air extraction in flight is limited to specific altitudes (22,000 ft or 6706 m).

Ground Air Supply

• Fixed or portable units with engine-driven air supply compressors.
• Connects to the pneumatic manifold to power the system without running engines.
• A ground pneumatic air supply adapter with a check valve is located directly in the pneumatic manifold.
• Closing the engine and APU isolation valves isolates the ground air supply.

Compressors

• The choice of air supply source is determined by the system’s air pressure requirements.
• Can be bleed air from a turbine engine compressor.
• Some aircraft have permanently installed compressors in each engine nacelle.
• Some have two stages of compression while others can have three or four.

Independent cabin compressors

• Help address the disadvantages associated with bleed air sources.
• Driven through accessory drive gearing or powered by bleed air from an engine compressor.

Compressor Groups

• Positive displacement compressors
• Multi-stage compressors
• Centrifugal compressors

Positive Displacement Compressors

• Roots-type blower takes a volume of air, compresses it, and delivers it to cabin ducts.
• Rotors rotate at the same speed in an airtight casing.
• Air passes around the outside of the case and is dumped into the plenum at the exit.
• Each rotation adds more air and the pressure increases.

Multi-stage Compressors

• Have check valves that allow air to flow in one direction.
• Pistons in cylinders driven by a shaft that is powered via electric motor or aircraft engine
• The first stage of compression occurs in the largest cylinder, with subsequent stages in smaller cylinders, increasing pressure.

Centrifugal Compressors

• Driven by the accessory gearbox or engine bleed air.
• Engine-driven compressors are typically mounted in the engine nacelle.
• Air is compressed by the high-speed impeller and delivered to a distribution system.
• Compressors driven by engine bleed air use hot air to drive a turbine, which drives a compressor.
• Compressed outside air mixes with cooled bleed air to achieve the correct temperature and pressure.

Pneumatic Reservoirs / Storage Bottles

• Provide a reservoir of compressed air to operate all services.
• Compressors build up system pressure when it falls below the normal level.
• Made of steel, potentially with wire-wound construction for maximum strength.
• Mounted upright, with connections for supply, pressure gauge, and drain valve at the bottom.
• High-pressure systems store air in metal bottles at 1000-3000 psi.
• The reservoir should include manufacturing date, safe working pressure, and hydrostatic test date.
• Inspected and removed periodically for hydrostatic checks.

Bottle Charging

• Bottles can be charged by an onboard compressor or a ground source.
• Contains a charging valve plumbed to the compressor outlet.
• A control valve acts as a shut-off valve.
• A pressure switch is used for flight deck warnings.

Pressure and Vacuum Pumps

• Air pressure pumps (air compressors) compress air and raise its pressure.
• The compressor absorbs air, compresses it, and sends it to a user system or storage vessel.

Air Compressor Types

• Reciprocating air compressor
• Rotary air compressor
• Centrifugal air compressor
• Axial air compressor
• Roots Type Blower

Reciprocating Air Compressor

• Positive displacement compressor using multi-stage pistons.
• Pistons driven by a crankshaft to transfer high-pressure gases in and out of cylinders.
• Air enters from a suction manifold and flows through multiple cylinders.
• Up to four cylinders can be used to achieve output pressures up to 3000 psi.

Rotary Air Compressor

• Operates by trapping air between meshed rotors.
• As the rotors rotate, they draw air between them. As the internal volume decreases the air is compressed.

Reciprocating Air Compressor characteristics

• High delivery pressure (up to 1000 kg/cm2)
• Maximum air discharge around 300 m3/min
• Suitable for low discharges at high pressure
• Low speed
• Intermittent air supply
• Large size for a given discharge
• Balancing is a major problem
• Air can be dirty due to contact with lubricating oil
• Complicated lubrication
• Isothermal efficiency

Rotary Air Compressor Characteristics

• Lower delivery pressure (up to 10 kg/cm2)
• High maximum air discharge (up to 3000 m3/min)
• Suitable for high discharges at low pressure
• High speed
• Continuous air supply
• Small size for a given discharge
• Balancing is not a problem
• Clean air delivered, no contact with lubricating oil
• Simple lubrication
• Isentropic efficiency

Centrifugal Air Compressor

• Uses a rotor with curved vanes in an airtight casing.
• A high-speed rotating suction fan pulls in air that is expelled radially outward.
• Air travels through a diffuser, slows, increases in pressure, and is delivered to the user system.

Axial Air Compressor

• Rotating blades attached to a revolving drum within an airtight casing with stator blades.
• Air flows through stators and rotors, undergoing successive compression.

Centrifugal Air Compressor characteristics

• Air movement perpendicular to the axis
• Low manufacturing and running costs
• Low starting torque
• Not suitable for multi-staging
• Requires a big frontal area for a given rate of flow

Axial Air Compressor characteristics

• Air movement parallel to the axis
• High manufacturing and running costs
• High starting torque
• Suitable for multi-staging
• Needs a small frontal area for a given rate of flow, making it suitable for aircraft.

Roots Type Blower

• Used on small turbine-powered aircraft.
• Takes a volume of air, compresses it, and delivers it to the cabin duct.
• Two-lobed rotors rotate in an airtight casing.
• Pressure is controlled by outflow valves.
• Finned housing provides cooling.

Vacuum Pumps

• Wet type
• Dry type
• Driven from engine accessory drives.

Wet Pumps

• Early vacuum pumps had steel vanes lubricated from the engine's low-pressure oil system.
• Oil flows through the pump and is captured by a separator.

Dry Pumps

• Incorporate carbon vanes and rotor for self-lubrication.
• Requires perfectly clean air to minimize problems.
• Filter maintenance is very important.

Venturi System Vacuum Supply

• Used in aircraft without engine-driven vacuum pumps.
• Inefficient and limited in capacity.
• Air is forced into the tube, accelerating through the narrowing section, producing lower pressure.
• Common problems include ice formation, damage to the tube, blockage, and leaking plumbing.

Pressure Control

• The function of a pneumatic pressure control regulator is to reduce the high-pressure air to a lower, more workable air pressure.

Pressure Relief Valves (PRV)

• Prevent damage by limiting pressure.
• A spring holds the valve closed under normal pressure.
• If pressure is too high, the valve opens to bleed off excess.

Control Valves

• A spring holds the left poppet valve in the closed position
• Compressed air is unable to flow to the brake port
• One of the lobes on the control lever holds the left poppet open and a spring closes the right poppet.
• Compressed air can flow around the open left poppet
• Air flows to the brake port and apply braking pressure.
• The vent valve bleeds off air that escapes through the left poppet valve.

Check Valve

• Air enters through the inlet, compresses a spring, opens a flap, and exits through the outlet.
• A one-direction flow control valve.

Restrictor Valve

• Reduces airflow volume and airspeed delivered to the user system.

Variable Restrictor

• A threaded adjustable needle valve is turned to adjust the opening.

Filters

• Protect against dirt.
• A micron filter consists of a housing with two ports, a replaceable filter cartridge, and a relief valve.
• Clogged filters force the relief valve open to allow unfiltered air to flow.

Desiccant/Moisture Separator

• Located downstream of the compressor to remove moisture.
• Consists of a reservoir, pressure switch, dump valve, and check valve.

Chemical Drier

• Absorbs any moisture that collects in the lines and other parts of the system.

Distribution

• Ensures systems get the air supply they need at the right pressure.
• Air distribution starts at pneumatic manifolds/ducts from engines, APU, or GPU.

Pneumatic Power

• Used for engine starting, air conditioning, pressurization, engine inlet cowl anti-ice, Wing Thermal Anti‑ice (WAI), water tank pressurization, Total Air Temperature (TAT) probe, and hydraulic reservoir pressurization.
• Can be controlled manually or automatically.

Pressurization with Engine Air

• Each engine feeds the left- or the right-wing duct, linked together by a cross-bleed system.
• A crossover duct connects the air systems, with at least one cross-bleed or isolation valve.

Pressurization with External Air Supply

• Ground air supply connects to the pneumatic system.
• External air supplies must be connected by trained personnel, using a checklist.
• Most aircraft pressure systems use approximately 45 psi.
• A standard commercial transport aircraft system pressure is around 45 psi.
• A window or door must be open to prevent accidental cabin pressurization.

APU Air Pressurization

• Start the APU and select the APU bleed air switch to supply pressurized air.

Pneumatic Ducts

• Made of strong, corrosion-resistant, lightweight tubing.
• Some have a gold coating for protection and reduced heat transfer.
• Send compressed air to user systems.
• An isolation or cross-feed valve divides the manifold and connects to a ground source.

Duct Temperature Regulation

• The temperature in the pneumatic system must remain between 160 °C and 200 °C (320 °F to 400 °F).
• A built-in pre-cooler cools the engine bleed air temperature using fan air.
• A Fan Air Valve (FAV) modifies the amount of fan air passing the pre-cooler.

Duct Pressure and Flow Sensors

• The manifold pressure sensor monitors the pressure in the distribution system at approximately 20 psi.

Fan Air Valve (FAV)

• A pneumatically operated butterfly-type valve with a manual override.

Leak Detection

• Overheat detection sensors are located along the ducts.
• These are found along pylon ducts, pneumatic wing ducts, ducts in the fuselage, and the APU duct.

Indications and Warnings

• The pneumatic system is controlled manually via the pneumatic control panel in the cockpit, or automatically by the pneumatic controller.
• The control panel generates advisory messages regarding operational irregularities.
• Pneumatic manifold pressure is monitored on the flight deck.
• Isolation valve control switches are on the flight crew's overhead panel.
• Over temperature and duct leak sensors provide flight deck indication.
• Green flow bars show airflow direction.
• Valve positions, duct pressures, and other parameters are displayed.
• Duct pressure is measured in PSIG.
• The pressure readout changes colour when it drops below 11 PSIG.
• If any information is not available, the indication is blank.

Interfaces with Other Systems

• Hydraulics reservoirs
• Auxiliary Power Unit (APU)
• Engine starting
• Wing Anti‑ice (WAI) systems
• Air conditioning systems
• Total Air Temperature (TAT)
• Potable water system
• Cargo areas
• Air Driven Pumps (ADP)

Interfaces - Hydraulics Reservoirs

• Use pneumatic air to provide positive pressure to the hydraulic systems.

Interfaces - Auxiliary Power Unit (APU)

• Starting the APU requires airflow from a ground air power unit or a running engine.

Interfaces - Engine Starting

• Uses pneumatic power to turn the core engine spool (N2 or N3 shaft).

Interfaces - Wing Anti-ice (WAI)

• The Wing Anti‑ice (WAI) system heats some of the wing leading edges to prevent the build-up of ice on the surface.

Interfaces - Air Conditioning Packs

• Air from the engines or APU is often used to provide a mass flow to the air conditioning packs

Interfaces - Total Air Temperature (TAT) Probe

• Air from the air supply distribution system flows through a tube, creating a vacuum which draws outside air across the temperature probe.

Interfaces - Potable Water System

• Pressurized pneumatic air is used to supply positive air pressure to the water tanks

Interfaces - Cargo Areas

• Pneumatic air is used to supply a controlled flow of warm air to the cargo areas

Interfaces - Air Driven Pumps (ADP)

• Pneumatic air is used to supply a controlled flow of pressurized air to drive a turbine in the pump