Aircraft Pneumatic Systems (Definition, Components, Operation)
Aircraft Pneumatic systems are one of those systems that must be functional for other systems to work. They provide much-needed power, pressurization, and so on for various aircraft systems, much like hydraulic systems.
In this post, I will talk about:
- The definition of pneumatic systems.
- The requirements of pneumatic system design.
- The components of aircraft pneumatic systems.
- The operation of aircraft pneumatic systems.
- Pneumatic Systems vs Hydraulic Systems
What Are Pneumatic Systems?
Just like hydraulic systems, pneumatic systems use pressurized fluid power to operate other systems. However, in this case, the fluid used is air which is very compressible. Additionally, the pressure in aircraft pneumatic systems, 30- 50 psi, is much smaller than that of hydraulic systems which have pressures in the range of thousands of psi.
Pneumatic systems use air to perform various functions such as pressurization, cooling, or actuation, as in hydraulic systems. However, the lower pressures in pneumatic systems mean they are not able to provide nearly as much actuation power as hydraulic systems.
In aircraft pneumatic systems, bleed air is taken from the engine compressors and used to power the ice protection systems that require hot air, the air conditioning systems, the water pressurization systems, and the pressurization of the hydraulic systems’ reservoirs.
Some pneumatic systems alternatively make use of compressed inert gases such as nitrogen.
The pressurized air in most transport-class aircraft has 3 sources:
- The bleed air from the engine compressor
- The Auxiliary Power Unit (APU) bleeds air
- The high pressure (HP) ground connection for when the aircraft is on the ground.
The aircraft pneumatic systems supply power to the wing and engine anti-icing systems to ensure that the wings and engines function properly.
This prevents engine failure and ensures the wing is aerodynamically efficient for flight. It is by the use of a functional APU bleed system that the engines are able to start and supply electrical power to the aircraft.
A lack of an electrical power supply can prove catastrophic during flight. The pneumatics system also contributes to a safe landing by the use of ‘pneudraulics’ operation of the landing gear in an emergency.
Within the system, the use of regulators and precoolers ensure that there is no overpressure or overheating as this can damage the aircraft components. Check valves within the system to ensure that bleed air does not enter the wrong place and cause damage to it.
Requirements for Pneumatic Systems Design
A pneumatic system can be said to have a good design if it has the following features:
- There should be redundancy in the system so that a single failure does not constitute total system failure.
- Failures should be able to be detected easily.
- Abnormalities such as leaks, overpressure, or overheating should be able to be detected before they lead to failure and corresponding safety measures should be put in place within the system.
- Airflow within the system should be controlled and regulated. Bleed air should be restricted to flow in the desired directions alone so that unwanted flow does not cause damage. The rate of bleed air flow should be able to be controlled.
- The sizes of the valve and ducts should be of the appropriate size to that the desired flow can be achieved.
- There should be a means of drying compressed air so that no icing, which can inhibit flow, occurs within the system ducts.
- The system should be safe enough for maintenance personnel to work on.
These requirements then guide the components and layout used in the pneumatic system.
Components of Aircraft Pneumatic Systems
A typical transport-class aircraft pneumatic systems have several components that work together to ensure good system operation.
Below are some of the components in the aircraft pneumatic systems, with the Airbus A320 aircraft as a reference.
- Computers and cockpit display: in the A320, there are two computers that control the systems, namely Bleed Monitoring Computers (BMC 1 and 2). The BMCs are connected to each other as well as to the information systems and pneumatically operated systems.
- Bleed valves: they open, when commanded, to let the engine or APU bleed air be supplied to the pneumatic systems. They are controlled by their associated computers and operated pneumatically. This means that it is only in the presence of pressure downstream of the valves that the valves open even if the computer commands it to. For the engines, there are two bleed valves, Intermediate Pressure (IP) and High Pressure (HP). When the IP bleed valve is open, it keeps the pressure at which the air is delivered between 40-50 psi, with the ideal pressure being 45 psi. When the HP valve is open, it keeps the pressure between 32 and 40 psi, with 36 psi being the ideal pressure.
- Overpressure valves: they are used to ensure the safety of the system. Whenever there is excess pressure in the system, the relief valves are used to vent out some of the air, thereby limiting the system pressure.
- Cross-bleed valves: they are located between the two engines and allow the air supply of the two engines to be connected or isolated. This is especially useful in the event of leaks. A rotary selector in the cockpit’s air conditioning panel electrically controls the bleed valve. The cross-bleed ducts allow for the air to be supplied from the APU and HP ground connection when needed.
- Control valves: they are used to control the movement of the air within the system. A control valve assembly comprises a three-port housing and a control lever with two lobes. Depending on the command given, a spring within the control valve will either hold a poppet valve closed or move to leave it open so that the compressed air may or may not be supplied to the pressure port which is connected to the components. When the component has finished operating, the poppet valve which supplies the air closes and the other poppet valve closes so that the air used by the component can be vented off into the atmosphere through the vent port.
- Check valves: they allow air to flow in only one direction. An application of this is when the HP bleed air is selected. The check valves prevent the air from the HP compressor from entering the IP compressor. It is also used to prevent air from other sources from entering the APU.
- Precooler: it regulates the bleed air temperature. It functions as a heat exchanger and uses cooler air from the engine fan to restrict the temperature to 200 ̊C. The fan air valve which is controlled by the movement of the spring (as in a control valve) controls the airflow. The valve only opens when there is adequate pressure.
- Restrictors: they are a type of control valve used in pneumatic systems. It is used to control the rate of airflow and therefore, the operating speed of the actuating unit it is connected to.
- Filters: they are used to remove impurities in the micron-scale from the system. This is important as impurities can cause damage to the system as they pass along
- Moisture separator: it is located downstream of the compressor as it is used to remove any resulting moisture from the compressor’s operation. It comprises a reservoir, pressure switch, dump valve, check valve, relief valve, and regulator. The pressure switch is used to energize or de-energize the dump valves. When de-energized, it purges the separator reservoir of all moisture. The check valve ensures there is no backward flow to the separator and that no pressure is lost during the dumping cycle. The moisture from the moisture separator is then dried by means of a chemical drier. This is important as the water can otherwise turn to ice and cause obstruction of flow in the system.
- The emergency ‘pneudraulics’ landing gear system: in the event of engine 1 failure, the green hydraulic system which operates the landing gear will be supplied with no power. As a backup, pressurized nitrogen gas is used to move the hydraulic fluid to the landing gear actuator. The nitrogen is kept in two bottles, one on each side of the nose wheel well. The nitrogen is released by the movement of an outlet valve. The pressure of the gas is about 3, 100 psi and is only enough for one cycle. The outlet valve is connected to a cable and a handle which is operated by the pilot. When totally raised upwards, the outlet valve opens and releases the compressed nitrogen. When the opposite is done, the outlet valve closes and the nitrogen can be vented off into the atmosphere for a total duration of about 30 seconds. The dump valve ensures that the landing gear system is separated from the whole hydraulic system.
Operation of an Aircraft Pneumatic System
The bleed air from the pneumatic systems of aircraft is used to power the following systems:
- Ice protection systems: hot bleed air is needed for anti-icing functions in the wings and engines.
- Air conditioning systems: bleed air is cooled through a series of actions before it is distributed to the aircraft.
- The water pressurization systems: bleed air is used to provide pneumatic power for pressurization. This eliminates the need for a pump in the water holding tanks where the air is distributed to the galleys and lavatories.
- The hydraulic systems’ reservoirs: bleed air is used to provide pneumatic power for pressurization so that the reservoirs can effectively deliver hydraulic fluid to their systems.
- Cargo compartment heating (via the ventilation system).
This means there are bleed valves for each of these systems. The air moves through the bleed valves. When the demand is made, the valves open to let air flow to any of the above-mentioned systems where they are needed. If there is no adequate pressure, the valves do not open. Several features are put in place to ensure that the air flows to the desired paths alone.
The Airbus A320, a narrow-body passenger aircraft will be used for explaining this section. Other aircraft pneumatic systems follow similar operating principles.
The A320 aircraft has 2 identical engine bleed air systems. Each engine is connected to 1 BMC which receives information about the bleed air temperature and pressure and the position of the valve. In the event of failure of one of the BMCs, the other one takes over most, but not all, of the monitoring functions. Air is normally taken from the intermediate pressure compressor (IPC). At low engine speeds, the pressure and temperature of the IP stage are too low, therefore the bleed air must be taken from the High-Pressure Compressor (HPC). The pressure is kept lower. The temperature and pressure of the air within each bleed system are regulated.
There is a leak detection system that checks for any overheating around the hot air ducts in the fuselage, pylons, and wings and sends information to the BMCs. There is no leak detection when the engine is being started. BMC 1 protects the left engine, wing, and APU. BMC 2 protects the pylon, the right wing, and the engine.
APU bleed air is usually used on the ground to power the air conditioning systems and to start the engines. If the APU is not available, the ground power connection is used. APU bleed air is also used in flight. The APU bleed valve will only be able to open when the APU bleed switch is on, APU speed is greater than 95 % of its maximum speed and there is no left-wing or APU leak.
After the engines are started, the APU BLEED is switched off. The cross bleed valve closes automatically and the engine bleed valves are automatically opened. The APU can only be switched off at least twice, minutes after its bleed is used.
The engine bleed valves will open when:
- Upstream pressure is more than 8 psi.
- APU BLEED valve is closed and the APU pushbutton off.
- There is no associated leak, overpressure, or over-temperature.
- The engine fire pushbutton is not on.
- The engine start valve is closed.
If the opposite of any of this is the case, the associated FAULT light will come on in the cockpit.
In the event of overheating of one engine bleed valve, the corresponding BMC causes the valve to close in response, thereby isolating the engine’s bleed air system. On the cockpit display, the temperature shows as normal and the pressure as zero since the valve is closed. The cross bleed valve is then opened by the pilot switch, allowing both sides to be supplied bleed air by the normal engine.
If BMC 1 fails, BMC2 will take over its leak detection loops, except for the APU. If BMC 2 fails, BMC 1 will take over, except for the pylons.
Pneumatic Systems Vs. Hydraulic Systems
The table below compares some of the qualities of pneumatic systems to hydraulic systems.
| Parameter | Pneumatic Systems | Hydraulic Systems |
| Design simplicity and control | Pneumatic machines are more easily designed and operated. | Hydraulic systems typically have much more components and have a complex design. |
| Reliability | They have longer operating lives than hydraulic systems and require less maintenance. This is because of the compressibility of gas which makes the components less susceptible to shock damage; the gases absorb excessive forces | Hydraulic fluid transfers excessive force rather than absorbing it. This results in more wear and tear of the components. |
| Safety | There are less susceptible to fire outbreaks | Hydraulic fluid is more flammable so the system is more susceptible to fire outbreaks. |
| Energy absorption | The fact that the gases absorb forces serves as a disadvantage in this regard. Not all the supplied energy is transferred to the systems it powers. | The fluid transfers all of the supplied energy to the systems that need it. |
| The magnitude of power /pressure | Pneumatic systems provide much less power supply than hydraulic systems. | Hydraulic systems can move much higher loads and supply much more power since the fluid is incompressible. Hydraulic system pressures can be up to 100 times higher than that of pneumatic systems. |
| Responsiveness | They respond more slowly | Hydraulic systems respond more quickly to pilot commands and changes in the system. |
| Multi-functionality | Pneumatic systems not only provide power but also cooling, heating and pressurization for other aircraft systems. | Hydraulic systems are mostly used for powering other aircraft systems, however, hydraulic fluid can also provide lubrication and cooling for fuel via heat exchange. |
Despite the advantages hydraulic systems have over pneumatic systems, pneumatic systems still prove extremely useful in aircraft with their unique qualities.
References:
- Pneumatic. (n.d.). [ebook] Airbus training. Available at: http://www.smartcockpit.com/docs/A320-Pneumatic.pdf
- Youtube (2016). A320 CBT #12 Pneumatic System Normal Operation. Available at: https://www.youtube.com/watch?v=OAp6R8VutDs&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=37
- Youtube (2016). A320 CBT #13 Pneumatic System Abnormal Operation. Available at: https://www.youtube.com/watch?v=9PfTsRFpxV0&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=38
- Youtube (2016). A320 CBT #12 Pneumatic System Normal Operation. Available at: https://www.youtube.com/watch?v=OAp6R8VutDs&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=37
- Youtube (2016). A320 CBT #11 Pneumatic System Description. Available at: https://www.youtube.com/watch?v=3my5xqHvtfA&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=36
- Wikipedia. 2022. Pneumatics – Wikipedia. [online] Available at: <https://en.wikipedia.org/wiki/Pneumatics> [Accessed 21 June 2022].
