CONTENTS
1 PNEUMATIC AND VACUUM 1-3
1.1 General 1-3
1.2 Safety Precautions 1-3
1.3 Full Pneumatic Systems 1-3
1.4 Vacuum Systems 1-5
1.5 Low Pressure Pneumatic Systems 1-5
1.5.1 Engine Driven Air Pump 1-5
1.6 Air Supply sources 1-6
1.6.1 Engine Bleed Air 1-7
1.6.2 Compressors or Blowers. 1-8
1.6.3 Auxillary Power Unit (APU) 1-8
1.6.4 Ground Supply 1-9
1.7 Pressure Control 1-10
1.7.1 Pressure Regulator 1-10
1.8 Distribution 1-11
1.8.1 Expansion Joints 1-12
1.9 Indications And Warnings 1-15
1.9.1 Overpressure 1-15
1.9.2 Overheat 1-16
1.9.3 Duct Hot Air Leakage 1-16
1.10 System Interfaces 1-16
1.10.1 Pneumatic Gyro Power systems 1-16
1.10.2 Emergency Landing Gear Systems 1-17
1.10.3 Pneumatic De-Icing systems 1-17
1.10.4 Air Conditioning And Pressurisation. 1-18
1.10.5 Air Driven Hydraulic Pumps. 1-18
1.10.6 Pressurising Of Hydraulic Reservoirs. 1-18
1.10.7 Waste And Water Systems 1-18
1.10.8 Pneumatic Stall Warning 1-19
1.10.9 Pneumatic Engine Starting 1-19
PAGE
INTENTIONALLY
BLANK
1 PNEUMATIC AND VACUUM
1.1 General
Pneumatic systems are fluid power systems that use a compressible fluid, air. These systems are dependable and lightweight and because the fluid is air there is no need for a return system
Some aircraft have only a low pressure pneumatic system to operate the gyro instruments, others use compressed air as an emergency backup for lowering the landing gear and operating the brakes in the case of hydraulic failure. Other aircraft have a complete pneumatic system that’s actuates the landing gear retraction, nose wheel steering, passenger doors and propeller brakes.
1.2 Safety Precautions
When working on bleed air systems, it is important to follow the precautions below:
· Bleed air is hot! Do not touch pipes and ducts.
· Always replace seals, (normally crush seals), when replacing joints.
· Tighten clamps to the torque figure quoted in the Maintenance Manual.
· Never lever against ducts, as dents cause hot spots.
· All duct supports and struts must not put any strain on to the duct.
1.3 Full Pneumatic Systems
The majority of aircraft use hydraulic or electrical power to operate landing gear systems, but some aircraft use air systems. Some advantages of using compressed air are:
· Air is universally available and in unlimited supplies.
· Pneumatic system components are reasonably simple and lightweight.
· No return lines are fitted resulting in a weight saving.
· There is no fire hazard and the danger of explosion is slight.
· Contamination is minimised by the use of filters.
Figure 1 shows a typical high pressure pneumatic system, that uses air compressors driven from the engines accessory drive. The compressed air is discharged through a bleed valve to a pressure relief (unloading) valve. The bleed valve is held closed by oil pressure. In the event of oil pressure failure the bleed valve opens to offload the compressor. The pressure relief valve maintains system pressure at around 3000 psi.
A shuttle valve in the line between the compressor and the main system makes it possible to charge the system from a ground source. When the engine is not running the shuttle valve slides over to isolate the compressor.
A Typical Pneumatic System
Figure 1
Moisture in a compressed air system will freeze as the air pressure drops when a component is actuated. To prevent this from happening, the water must be completely extracted from the air. A water separator is fitted which collects the moisture from the air onto a baffle and it is allowed to drain overboard. An electric heater prevents the water in the separator from freezing.
After the air leaves the water separator any remaining moisture is removed as the air flows through a desiccant or chemical dryer. The air is then filtered before it enters main system.
The air is then fed to each of the storage bottles, which provide the emergency air for several systems. A manually operated isolation valve allows the air supply to be shut off to so that maintenance can be carried out on the systems without having to discharge the storage bottles.
The air is stored at maximum system pressure around 3000 psi to supply the landing gear and brakes in an emergency. A pressure reducing valve is fitted to reduce the air pressure down to the operating pressure that the majority of the components work at 9around 1000psi) ie landing gear normal operation, the passenger door, the propeller brake and the nose wheel steering.
1.4 Vacuum Systems
A supply of air at a negative pressure can be required for a number of purposes. The supply of vacuum to instruments for example, usually comes from either a small vacuum pump attached to the (piston) engine of the aircraft or from a venturi jet pump, which obtains its power via a tapping from the (jet) engine. The low pressure caused by the venturi draws in air to supply the system.
Other requirements for a source of vacuum might be in a pneumatic de-icing system. This type of de-icing uses the inflation of flexible leading edge mats to break-off the ice, which has formed. To keep the de-icer boots, as they are called, in place, they are fed a negative pressure from a venturi, which ensures that the boots are sucked flat onto the wing leading edge, ensuring a smooth, aerodynamic surface.
1.5 Low Pressure Pneumatic Systems
These systems provide air for gyroscopic altitude and direction indicators and air to inflate the pneumatic de-icing boots. This compressed air is usually provided by a vane type engine driven air pump (Figure 2).
1.5.1 Engine Driven Air Pump
On early aircraft engine driven air pumps were used primarily to evacuate the casings of air-driven gyroscopic instruments so they were more commonly known as vacuum pumps. On later aircraft the discharge air was used to inflate de-icing boots on control surfaces and are now more correctly called air pumps.
There are two types of air pumps that are used, these are wet air pumps and dry air pumps.
Vane Type Air Pump
Figure 2
· Wet Air Pumps
Wet pumps have steel vanes that are lubricated and sealed with engine oil which is drawn in through the pump mounting pad and exhausted with the discharge air. This oil is removed from the discharge air with an oil separator before it is used for de-icing or driving the instruments.
· Dry Air Pumps
Dry air pumps were developed so that there was no oil in the discharge air and therefore there were no requirements for an oil separator. The pump vanes are made from carbon and are self lubricating. The main problem with this kind of pump is that the vanes are easily breakable by any contaminants that enters the pump. To prevent this form occurring the inlet air is filtered.
1.6 Air Supply sources
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:
1.6.1 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 pressure and temperature control system which reduces its pressure and temperature and is then mixed with ram air.
Because of the great variation of air output available from ground to maximum flight rpm there is a need to maintain a reasonable supply of air during low rpm operation as well as restricting excessive pressures when operating at full speed. Two tappings are taken from the engine, one form the LP stages and one form the HP stages to maintain a reasonable pressure band at all engine speeds. Figure 3 shows a typical 2 stage bleed air system.
At low engine rpm the LP air is of insufficient pressure for use in the pneumatic systems, so air will be tapped from the HP stages. When engine speed increases the LP air pressure will also increase and at a pre-determined pressure the HP air will be shut off and when operating at maximum engine speeds the air will be taken purely from the LP stages. In all normal stages of flight therefore the bleed air will come form the LP stages.
Typical Two Stage Bleed Air System
Figure 3
1.6.2 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. The compressor inlet duct is connected to an air scoop and its outlet is connected to the pneumatic manifold. The unit is controlled by a shut off valve which is operated from the cockpit.
When insufficient LP air pressure is available for the pneumatic systems at low engine speeds the aircrew will select the shut off valve to open. This will direct the LP air to drive the turbo compressor. A pressure regulator is incorporated to ensure a constant output at the required pressure.
On large multi-engine aircraft only some of the engines will have a turbo compressor (Figure 4) which is normally mounted with its associated controls in an engine bay.
Turbo Compressor
Figure 4
1.6.3 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 (Figure 5). On some aircraft the APU can be started in flight and act as a back up source of air, hydraulics services in the event of a loss of an engine.
Typical APU Setup
Figure 5
1.6.4 Ground Supply
For use on the ground when the engines are not running. This unit will run until the aircraft is independent of the trolley. The ground cart is basically a compressor driven by an engine, usually a diesel. The compressor output pressure is regulated to match the aircrafts system pressure. A quick release hose is connected from the cart to the aircraft service panel. The maximum aircraft systems pressure and operating instructions including safety precautions are detailed on the inside of the service access panel.
Ground Cart Panel
Figure 6
Instructions for operating the ground cart will be found on a panel on the carts control panel. Figure 6 shows a typical ground cart control panel.
1.7 Pressure Control
In many bleed air systems the pressure is regulated only by the operation of the high pressure shut off valve. The range of pressure may be from 10psi at ground idle to 65 psi at take off power. Many modern aircraft use bleed air for many systems that are sensitive to pressure variations and therefore some form of regulation is required.
The pressure regulator is a pneumatically operated valve which will give a pre-determined output pressure form the engine bleed air system. The regulator may also perform as the shut off valve. This is then called a pressure regulating and shut off valve.
1.7.1 Pressure Regulator
This valve operates on the principle of a balance between air pressure and spring pressures. Referring to Figure 7. Assuming the piston has an area of 1 square inch and is held in its seat by a spring that pushes with a 100 pounds force. The piston has a shoulder of 0.5 square inches and this area is acted on by a system air pressure of 1500psi. The cone shaped seat of the valve has an area of 0.5 square inches and is acted on by a reduced pressure of 200psi.
A bleed orifice in the piston allows air pressure into the piston chamber. A relief valve being acted on by the reduced 200psi pressure and relief valve spring pressure, maintains the air pressure in the piston chamber at 750psi.
Pressure Regulator
Figure 7
When the air supply is used by a pneumatic service, the reduced downline pressure of 200psi reduces further. This reduced pressure is now insufficient to keep the relief valve closed. The 750psi piston chamber pressure unseats the relief valve and reduces the piston chamber pressure.
The reduced piston chamber pressure unseats the piston cone piston which allows the system pressure to bleed into the down lines. Once the downline pressure rises to 200psi, the piston cone and the relief valve re-seat and the system is once again in balance.
1.8 Distribution
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. 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. Figure 9 shows a typical distribution layout.
Ducts Supports
Figure 8
The ducting is made up of many sections for ease of maintenance and cheapness of replacement. They are constructed of thin wall material and clamped together with joints that allow for thermal expansion.
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. The duct sections are supported throughout their length by clamps and tie rod attachments to the aircraft structure as shown in Figure 8.
Bleed Air Distribution Manifold
Figure 9
1.8.1 Expansion Joints
Joints are assembled cold and when in use the temperatures int eh ducting can reach up tom 350 degrees F. Expansion devices must be incorporated into the systems to prevent any distortion or buckling of the ducts. This expansion can be allowed for in several ways.