Modern aircraft are equipped with either a hydraulic or pneumatic system to control and power certain elements, such as the braking system, alternators, starters, and emergency devices. While hydraulics have predominated the market as of late, there are still many airplanes that implement pneumatic systems. In this blog, we will discuss the function and various components of pneumatic systems in aviation.
Pneumatic and hydraulic control systems operate on similar principles, particularly by forcing fluid through an enclosed space. However, while hydraulic systems employ incompressible oils to transmit power, pneumatic systems rely upon pressurized air. As the air is moved through the various lines and valves of the pneumatic system, it is used to power brakes, doors, landing gears, and more.
In order for air to reliably convey power across a system, it must be stored at high pressure, usually around 1,000-3,000 PSI in most cases. In the storage form, pressurized air is held in specialized metal bottles that contain two separate valves. The first is a charging valve, which allows air to be let into the bottle, and the second is a control valve, which, when engaged, prevents air from being let out. Both valves work together to ensure a steady pressure and flow rate during operation and idle time.
On aircraft, there are several important components that comprise a pneumatic system. The most common are explained below:
Relief Valves: Since the air in a pressurized system could have devastating moments of overpressure in a malfunction, relief valves are used to limit the pressure and protect associated lines.
Air Compressor: Since pneumatic systems have historically been limited by the quantity of air in the bottles loaded onto the aircraft before flight, many airplanes have been equipped with air compressors that are used to recharge the bottles.
Check Valves: Since damage or power loss could occur if pressurized air were to back up in the system, check valves are installed to only open in one direction.
Filters: Pneumatic systems are equipped with either micronic or screen-type filters to prevent dirt or other contaminants from entering the lines. The latter of which is permanent and does not require any replacement.
Desiccant: In addition to contaminants, excess moisture could also damage pneumatic components. To safeguard against this, a water-absorbing material is placed downstream of the compressor. Regardless of the material used, all desiccant filters must be replaced once fully saturated.
Chemical Drier: These components are similar to desiccants but are placed in other areas of the pneumatic system to protect against moisture damage.
Nitrogen Bottles: In the rare event of pressurized gas depletion, backup nitrogen bottles may be used to extend the landing gears.
Medium Pressure Systems: Drawing air directly from the compressor of a turbine engine, medium-pressure systems are used for short-term needs, such as engine start and hydraulic pump operation.
Maintaining pneumatic power systems requires preventative maintenance as well as regular inspection. In general, the compressor oil level should be checked daily by using a dipstick or other gauge. Additionally, the pressurized lines may occasionally be removed from the system to facilitate foreign matter disposal. It is also important to remove any components containing extreme amounts of contamination from oil or other particulates.
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