Pneumatic systems use compressed air as a working fluid and it acts much like the hydraulic systems we already described previously in our articles. In comparison with hydraulics there are advantages to be found by using air instead of hydraulic fluid to transmit power.
The principles (Pascal's law) in using air are the same as for hydraulic fluids but with a remarkable difference. Below we will discuss them and see where these systems are used in modern day aircraft.
In small experimental homebuilt aircraft you will find pneumatic systems in the form of the gyroscopic vacuum system and the tires the aircraft stands and rolls on. As the pilot in command of your aircraft you will need some knowledge of where and how pressurized air is used and how to inspect them during the preflight. This also includes the vacuum systems to power the gyro's in the instrument panel.
Pro's and cons
Outside air is pressurized by an engine driven pump (compressor) and stored in bottles. This shows a striking difference with a hydraulic fluid: air is compressible and this property gives it some unique (dis)advantages.
One disadvantage is that air does not transmit pressure as easy as a fluid would do. It is springy in nature and therefore not really suitable for precise control operations. This, however, is an advantage when air is used in tires and damping in undercarriage to smoothen the ride when taxiing on rough surfaces.
It is not suitable for large and heavy mechanical devices. Air must be compressed to a large degree to have enough energy and this would require large air tanks and actuators with very high working pressures. Not very practical on small aircraft.
Advantages of air
In contrary to the disadvantages there are some properties which make the use of compressed air very desirable, they are:
- No fire problem - Air will not burn by itself, however, bottles can explode and cause damage
- Light weight - Air systems do not need a return line as is the case with hydraulic fluids
- Simple - Pneumatic systems are by design very simple
- Supply - Air can be pumped in from around us and is virtually unlimited in supply
Some aircraft engines have an airstart system in where pressurized air in a bottle starts the engine, and as long as the engine runs it keeps the bottle topped up for the next start.
Larger aircraft use bleed air from the engines to actuate a number of systems from deicing boots, cabin pressurization, backup and main pneumatic systems. See the image for mor detail.
Used when, for example, the landing gear hydraulic systems fails for a reason. A backup pneumatic system uses a storage bottle with air and an actuator as an emergency means to extend the gear prior to landing.
Low pressure systems
These are pressurized up to about 1000 psi and use an engine driven vane type pump. They are mainly used to drive the air-conditioning, door seals, de-icer boots, mainly small low power applications.
High pressure systems
An engine driven compressor feeds air via an unloading valve to the system keeping the pressure around 3000 psi, as expected this may vary from one aircraft manufacturer to another. There will usually also be a ground valve on the aircraft to enable the system to be pressurized when the main engines are not running.
You will also find a moisture separator, dryer (desiccant) and filter to keep the air clean and free from water before it is stored in the high pressure bottles.
Pressurized air at 3000 psi is reduced before it is routed to valves and actuators, this reduced pressure is monitored by gauges. Actuators can be a single acting device where air moves them one way and a strong spring inside pushes the piston actuator back, or can they be double acting. These are sometimes used with flap extension systems.