Sub-Module 6.3 Pneumatic and Air System
Sub-Module 6.3 Pneumatic and Air System
Sub-Module 6.3 Pneumatic and Air System
SUB-MODULE 6.3 :
o These systems are dependable and lightweight and because the fluid is air there is no
need for a return system.
o A system used to supply driving power, in the form of a suction of air, to several
gyro instruments, such as :
o The vacuum driven instruments are now used only on light aircraft.
Low Pressure and Vacuum System
Pressure Sources
o Aircraft that do not have a pneumatic pump to evacuate the instrument cases
can use venturi tubes mounted on the outside of the aircraft .
o Air flowing through the tube speeds up in the narrowest part, and according to
Bernoulli's principle,the pressure drops. This "suction" is fed to the instrument
case by a piece of tubing.
o Filtered air flows into the instruments through filters built into the instrument
cases. In this system, ice can clog the venturi tube and stop the instruments
when they are most needed.
o This system is not suitable for aircraft that fly above 18,000 ft.
For aircraft flying higher than 18,000 ft there is a compressor system
which provides enough air-mass through the gyro.
Fig. Ventury tube
Pressure-reducing needle valve -
is used to decrease the suction.
The two attitude instruments operate on approximately 4 inHg suction; the turn-and-
slip indicator needs only 2 inHg.
2. Engine Driven Air Pump
o Many aircraft equipped with reciprocating engines obtain a supply of low-
pressure air from vane-type pumps.
o The drive shaft and the vanes contain slots, so the vanes can slide back
and forth through the drive shaft.
o When the pump begins to operate, the drive shaft rotates and changes
positions of the vanes and sizes of the chambers. Vane No.1 then moves to
the right, separating chamber B from the supply port.
Air Pump Construction and Operation
o Near the bottom of the pump, chamber B connects to the pressure port and
sends compressed air into the pressure line.
o There are four such chambers in this pump and each goes through this same
cycle of operation. Thus, the pump delivers to the pneumatic system a
continuous supply of compressed air from 1 to 10 psi.
All duct supports and struts must not put any strain on to the duct.
Medium Pressure Pneumatic Systems
( Bleed Air Pneumatic System )
o These are used on most turbine-engined aircraft and are supplied with
compressed air tapped from the engine compressor ( Compressor Bleed Air )
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Medium Pressure Pneumatic System
Distribution and Control - Components
Main Components
3. LP Check Valve
Prevents reverse flow to the engine low stage compressor.
8. Isolation Valve
This is used to split or connect the different pneumatic systems.
o Because of the great variation of air output available from a gas turbine engine
between idle and maximum power there is a need to maintain a reasonable
supply of air during low power operation as well as restricting excessive pressure
and temperature when the engine is at maximum power.
Fig. Typical Power Source ( Engine Compressors )
Fig. Typical Engine Bleed Air control devices
Compressor Stage Selection
o The air is ducted from two different stages of the compressor :
o As the engine power increases low stage airpressure will increase and close the
valve, so in normal cruise flight bleed air will come from the low stage.
o In case of electrical power loss, the valve opens and regulates a constant pressure.
Temperature Regulation
o The Fan air valve - Regulates the amount of fan cooling airflow via the heat
exchanger to control the required pneumatic temperature.
o The controller or computer receives all the necessary input signals from the engine
pneumatic system sensors, valves and bleed air control switches to monitor the
system or shut off the system in case of dangerous conditions. It provides warning
and status signals for the indication on the Engine Warning Display.
o The controller contains also a Built-In-Test Equipment (BITE) to localize and store the
failures of the faulty bleed air components.
o For troubleshooting, the BITE system can be used via a Centralized Fault Display
System (CFDS).
APU Bleed Air Supply System
o There are different APU bleed air supply systems installed in modern aircraft.
The system varies with the respective APU Type.
o Most APUs are designed for bleed air extraction on ground only.
If an APU is designed for bleed air extraction in flight, then the bleed air
extraction is normally limited up to a specific flight altitude only.
o APU bleed air supply has normally a higher priority than engine bleed air supply.
That means the engine bleed supply is automatically inhibited if APU bleed is
selected.
APUs which extracts the bleed air from the compressor of the power section.
( Example: B-747 )
APU which extracts the bleed air from a separate load compressor driven by
the turbine of the power section ( Example: A330 )
APU with two shafts (N1 & N2) which extracts the bleed air from the N1-
Compressor driven from the N1-Turbine (Example: MD-11)
APUs which extracts the bleed air from the compressor of the power
section. ( Example: B-747 )
APU which extracts the bleed air from a separate load compressor driven by
the turbine of the power section ( Example: A330 )
APU which extracts the bleed air from a separate load compressor driven by the
turbine of the power section ( Example: A330 )
APU with two shafts (N1 & N2) which extracts the bleed air from the N1-
Compressor driven from the N1-Turbine (Example: MD-11)
APU Bleed Air Control System
o
The APU bleed air control system consists of two main systems :
o Inlet Guide Vane Control System controls the position of the load compressor
inlet guide vanes in order to avoid overtemperature (high EGT) of the power
section
o Bleed Control System controls the APU Bleed Control Valve in order to avoid
load compressor surge condition.
IGV Control System
o The APU load compressor, driven with a constant speed from the turbine of the APU
power section, supplies the APU bleed air. The quantity of APU bleed air can be
controlled with the load compressor inlet guide vanes. The inlet guide vanes are
moved by the IGV actuator.
o The IGV actuator is fuel pressure actuated and electrically controlled by a torque
motor.
The APU ECB ( Electronic Control Box ) controls the torque motor according to the
aircraft pneumatic demand signals for air conditioning operation or engine start. The
bleed air quantity is limited by the APU Exhaust Gas Temperature (EGT). The EGT-
limit varies with the APU air inlet temperature.
o The actual EGT increases with the APU load. That means with a high APU generator
load the APU bleed air supply will be automatically reduced. (APU electrical power
supply has priority). If no APU bleed air is selected or during APU start sequence, the
IGVs will be controlled to close to unload the APU shaft.
o Above 20,000 ft flight altitude, the IGVs will be controlled to close. For this function
the ECB uses the pressure signal from the air inlet pressure sensor. (APU bleed air can
normally be used up to a specified flight altitude only).
Bleed Control System
o The APU bleed control valve controls the air from the load compressor to the
pneumatic system or to the APU exhaust duct.
o The APU ECB controls the torque motor in order to avoid load compressor
surge condition and to supply the aircraft pneumatic system.
o To provide this function, the ECB needs input signals from several sensors.
Ground Supply
o For use on the ground when the engines are not running.
This unit will run until the aircraft is independent of the trolley.
o A quick release hose is connected from the cart to the aircraft service panel.
o Instructions for operating the ground cart will be found on a panel on the carts
control panel.
BLEED AIR SYSTEM
DISTRIBUTION
DISTRIBUTION
o Distribution is achieved by ducting and pipelines that carry the charge air from
the engine compressors to the various services that require air for their
operation.
o Due to the heat of the bleed air , any leakage of the ducts will cause an extreme
temperature rise in the area of the leak with the possibility of fire or damage to
the surrounding structure and equipment. Leak detection systems are therefore
incorporated.
o They are constructed of thin wall material and clamped together with joints that
allow for thermal expansion.
o Engine bleed air system ducts are manufactured from stainless steel and the
ducts and pipelines are usually manufactured from titanium as they are able to
withstand higher temperatures and are lighter in weight.
o The duct sections are supported throughout their length by clamps and tie rod
attachments to the aircraft structure
Ducts Supports
Expansion Joints - Ducting :
o Joints are assembled cold and when in use the temperatures in the ducting can
reach up to 350 degrees F.
1. Pre-Stressed Joint
o One method is to have the duct sections installed slightly shorter in length and
allow them to expand with the heat to fit correctly.
o The joint is designed to allow for slight flexing and misalignment as well as
expansion.
o A flange on one end of the duct is connected to a bearing nut on the other and
screwed together to form the joint.
o Shims are used to ensure adequate clearance is maintained for the expansion and
flexing and a crush type metal seal is used to prevent air leakage at the joints.
Flexible Ball Joint
3. Cable Attachment Joint
o The cable attachment type joint is used where large temperature changes
exist,ie : from cold soak at high altitudes to maximum working
temperatures when the pneumatic system is selected on. This joint has
bosses attached at each end of the duct.
o There are usually 3 short cables equally spaced around the duct.
o The cables have a swaged ball end fittings at one end and a swaged
threaded fitting at the other.
o Each end is located in a bracket on the ducting. A seal is fitted around the
duct before the ducts are connected. A nut is fitted on the threaded end
and tightened.
o This pulls tightens the cables and seals the duct. A small gap is left at the
seal ends to allow for expansion.
Cable Attachment Joint
Pneumatic Duct Leak
Warning Systems
Pneumatic Duct Leak Warning Systems
o In modern aircraft the system is also used to provide an automatic shut off of
the affected pneumatic system.
This method use thermal switches connected in parallel to the warning light and if
applicable to the automatic shut off circuit.
The thermal switches close if the overheat setting is reached and open after cool
down
o This method is used in modern aircraft. The manifold failure loop is a grounded
flexible metallic tube filled with a salt mixture.
o Included in the tube is a conductor insulated by the salt crystal. The conductor is
connected via plugs and wires to the sensing device.
o If the temperature of the salt mixture reaches the overheat setting, the salt melts and
provides a current flow to energize the sensing relay or amplifier.
o After cool down the salt will crystallize again and interrupt the current for the sensing
relay. The sensing device with the loop test circuit is normally incorporated in a
pneumatic controller. But it can also be a separate unit called the Manifold Failure
Controller (MFC).
o The advantage of this detection system is that in case of a single open loop, the leak
warning is not lost.
o The overheat setting of the loops may by different depending on the type of salt
mixture.
o The test relay opens the loop circuit and sends a ground signal through the loops to
energize the sensing relay.
o The test makes sure that no loop has an open circuit and the sensing relay and the
warning light is functional.
Leak detection by manifold failure loops
Leak detection by pressure switches
o There are aircraft which have, for safety reasons, double walled pneumatic
ducts in the pressurized zones.
o After repair of the leaky duct, the DUCT LEAK light must be reset by pressing
the RESET BUTTON on the maintenance panel.
Leak detection by pressure switches
Pneumatic System
Indications and Warnings
Indications and Warnings
Indications
o To monitor the pneumatic system, normally pneumatic pressure and temperature
indicators are installed in the cockpit. On newer aircraft this indications may be
shown on a system display screen. (See Figure 16.27)
Warnings
o To alert the flight crew if dangerous or abnormal conditions in the pneumatic
system exists warning and caution lights are provided in the cockpit.
"PNEUM TEMP HI" This warning light comes on, if the pneumatic
temperature exceeds a set threshold (2551)
"PNEUM MANFLD FAIL' This warning light comes on, if a duct rupture
or a leak in a pneumatic manifold is detected.
Typical Caution lights are:
"USE ENG PNEUM SUPPLY" This caution light comes on, if the
pneumatic system is still supplied by the APU to remember the pilots that
they must use the engine pneumatic supply system
Typical Pneumatic Indication and warning
High Pressure Pneumatic Systems
( FULL PNEUMATIC SYSTEM )
Landing gear
Nose Gear Steering
Wing flaps
Wheel brakes
De-icing boots ( at reduced pressure ).
High-Pressure Systems
o For high-pressure systems, air is usually stored in High Pressure metal bottles at
pressures ranging from 1,000 to 3,500 psi, depending on the particular system.[Figure 12-
70]
Relief Valves
o They act as pressure limiting units and prevent excessive pressures from bursting lines
and blowing out seals
Fig. Relief valve
Control ( Selector ) Valves
o Figure 12-72 ,
Illustrates how a valve is used to control emergency air brakes.
Figure 12-72A
Control ( Selector ) Valves
Emergency
brake control -
Figure 12-72B
Control ( Selector ) Valves
o In Figure 12-72B,
The control valve has been placed in the “ on position “
One lobe of the lever holds the left poppet open, and a spring closes the
right poppet.
o Compressed air now flows around the opened left poppet, through a drilled passage,
and into a chamber below the right poppet.
o Since the right poppet is closed, the high-pressure air flows out of the brake port and
into the brake line to apply the brakes.
o To release the brakes, the control valve is returned to the off position.
[Figure 12-72A] - The left poppet now closes,stopping the flow of high-pressure
air to the brakes.
o At the same time, the right poppet is opened, allowing compressed air in the brake
line to exhaust through the vent port and into the atmosphere.
Check Valves
o Air enters the left port of the check valve, compresses a light spring, forcing the
check valve open and allowing air to flow out the right port.
o But if air enters from the right, air pressure closes the valve, preventing a flow of
air out the left port.
o Figure 12-74 illustrates an orifice-type restrictor with a large inlet port and
a small outlet port.
o The small outlet port reduces the rate of airflow and the speed of
operation of an actuating unit.
o It contains an adjustable needle valve, which has threads around the top and
a point on the lower end.
o Depending on the direction turned, the needle valve moves the sharp point
either into or out of a small opening to decrease or increase the size of the
opening.
o Since air entering the inlet port must pass through this opening before
reaching the outlet port, this adjustment also determines the rate of airflow
through the restrictor.
Fig. Variable Restrictor
Filters
o Normally, air enters the inlet, circulates around the cellulose cartridge, and flows
to the center of the cartridge and out the outlet port. If the cartridge becomes
clogged with dirt, pressure forces the relief valve open and allows unfiltered air
to flow out the outlet port.
o A screen-type filter is similar to the micron filter but contains a permanent wire
screen instead of a replaceable cartridge.
o In the screen filter, a handle extends through the top of the housing and can be
used to clean the screen by rotating it against metal scrapers.
Desiccant/Moisture Separator
o The check valve protects the system against pressure loss during the dumping cycle
and prevents reverse flow through the separator.
Chemical Drier
o Each drier contains a cartridge that should be blue in color. If otherwise noted,
the cartridge is to be considered contaminated with moisture and should be
replaced.
o 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.
o Nitrogen used for emergency landing gear extension is stored in two bottles, one
bottle located on each side of the nose wheel well.
o The handle is located on the side of the copilot’s console and is labeled -
EMER LDG GEAR.
o Pulling the handle fully upward opens the outlet valve, releasing compressed
nitrogen into the landing gear extension system.
o Pushing the handle fully downward closes the outlet valve and allows any
nitrogen present in the emergency landing gear extension system to be vented
overboard.
o When activated, a blue DUMP legend is illuminated on the LDG GEAR DUMP VLV
switch, located on the cockpit overhead panel.
o A dump valve reset switch is used to reset the dump valve after the system has
been used and serviced.
o Figure 12-76
Emergency Extension Sequence:
5. Pneudraulic pressure actuates the dump valve portion of the landing gear selector/dump valve.
8. Pneudraulic pressure is routed to the OPEN side of the landing gear door actuators, the UNLOCK side of the
landing gear uplock actuators, and the EXTEND
side of the main landing gear sidebrace actuators and nose landing gear extend/retract actuator.
12. Three green DOWN AND LOCKED lights on the landing gear control panel are illuminated.