Keeping dust from reaching the internal workings of any reciprocating engine is critical. According to publications from both Lycoming and Continental unfiltered air contains contaminates which are very abrasive to engines, especially reciprocating engine cylinder walls and piston ring faces. If a worn, poorly fit, or poorly functioning inlet air filter allows as much as a tablespoon of abrasive dirt in the cylinders, it will cause wear to the extent that wear to internal parts of the engine will prematurely occur and an overhaul will be prematurely required.
For most general aviation (GA) aircraft powered by reciprocating engines there are four different technologies currently in use to protect today's reciprocating engines. These four technologies can be further broken down into two different categories: "dry media" and "wet media." Let's take a closer look at these two basic types of inlet air filters.
We'll begin with the dry media filter. As its name implies "dry media" filters feature a filtering medium that — well is dry. A dry media filter does not require the use of oil as part of the filtering process. Historically, the filtering media has been made using cellulose or paper fibers. Today a large portion of these filters have a man-made synthetic fiber, or fiberglass as the filtering media. This media, regardless of the material type, is then pleated into the "accordion" shape to make the filter. The filter media is then encased in a frame designed to fit the specific engine and aircraft application. This style of filter is currently found on multiple GA reciprocating engine aircraft applications.
Next is the wet media filter, which is the other popular filter technology which is found in use on GA aircraft today. Wet, as its name implies, is a type of media that requires a tacky oil to be applied to a substrate to act as the dust trapping agents. The substrate is most commonly either a foam pad or pleated cotton gauze. Typically this filter substrate alone offers only a limited portion of filtration protection. However, once tacky oil is applied to the substrate the effectiveness increases dramatically. Wet media filters require that oil is always present on the substrate in order to ensure the best filtering action. Consequently as the filter media dries out, the efficiency of these filters becomes modified. In some cases wet media filters will require that oil is re-applied as part of the normal servicing for the aircraft. Additionally, care must be taken to not wash away the oil from the foam pads.
The following is a general description and guidelines to follow when inspecting and servicing induction air filters:
Dry media filters
Dry media filters can be either a cellulose or synthetic media. The tight weave of the media traps particles by sieving the dust contaminates. The pleated style of the media maximizes the surface area of the filter providing the engine maximum area to breathe. Most GA original equipment manufacturers (OEM) use a dry media pleated filter on their equipment. The dry media pleated filters are designed to offer long life, approximately 500 flight hours or three years of service, and they can be cleaned up to five times before replacing them. Cleaning can be initially performed by using compressed air to expel any dust and particulate that has been trapped in the filter pleats. Once all of the dust and particulate has been blown away, you should hold the filter up to a light source and inspect the condition of the media for deterioration. If the media is in satisfactory condition further cleaning can be accomplished by washing the filter in a solution of water and general purpose low-suds detergent. After washing, the filter should be dried and once again inspected for contamination and general condition. The following steps can be used as a guide when servicing the dry media filter:
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