Recip Technology: Air Intake Filters

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:

  1. Remove the filter and inspect for damage or deterioration.
  2. Pre-clean using compressed air to blow off the dust and particulate.
  3. Wash and soak with water and detergent.
  4. Rinse the filter.
  5. Dry the filter.
  6. Re-inspect and re-install.

Wet media filters
Wet media filters generally fall into two different classifications: oiled foam and oiled cotton gauze. The oiled foam style filters contain a low-cost replaceable pad which is saturated with tacky oil which provides its filtration efficiency. The foam pads are contained inside of a filter frame for easy removal and replacement. These foam pad wet media filters have been primarily an aftermarket part, approved for installation by way of a supplemental type certificate (STC). The foam pads are required to be replaced on a regular basis, typically every 100 flight hours or when 50 percent of the surface is covered with contaminants or debris. The cost of the replacement foam filter pads are low and this type of air intake filter is popular on many GA aircraft models. There really is no maintenance servicing for this style of wet media foam pad filter — only remove and replace.

The other wet media technology is the gauze pleated filter. This media consists of layers of surgical cotton gauze that is pleated between wire screens and then coated with oil. This technology has migrated into the GA aircraft industry from the automotive industry. The highly permeable cotton gauze is used to support tacky oil to provide its filtration efficiency. The gauze pleated wet media filters have also been an aftermarket part, approved for installation by way of an STC. The following steps can be used as a guide when servicing the wet gauze pleated filter:

  1. Remove the filter and inspect it for damage or deterioration.
  2. Gently tap filter on a hard surface to remove loose dust that will easily fall off the filter.
  3. Apply the cleaner to clean side of filter.
  4. Apply the cleaner to dirty side of filter.
  5. Let the cleaner soak for 10 minutes.
  6. Rinse with water.
  7. Dry the filter without accelerated drying methods.
  8. Re-oil the filter substrate.
  9. Let sit for approximately 20 minutes and check for oil coverage.
  10. Re-oil any areas of the filter that were initially missed.
  11. Continue steps 5 thru 7 until a uniform color covers the entire filter media.

The cleaning procedures for this type of filter are recommended every calendar year or every 100 flight hours, and can be cleaned up to 25 times or a maximum of 2,500 flight hours.

No matter which type of air intake filter is used, when operating a reciprocating engine powered aircraft in sandy or dusty conditions, it may be necessary to service the air intake filter(s) much more frequently — even daily. Use only the cleaning procedures, cleaning fluid, and the correct type of re-oiling fluid that are recommended by the filter manufacturer or aircraft maintenance manual. Failure to follow the required cleaning instructions on any type of air intake filter can lead to poor filtering efficiency which can eventually lead to premature wear and damage of internal engine parts.

When choosing an air intake filter system for your customer's aircraft, consider all of the options. Calculate the initial costs, the cost of ongoing filter servicing tasks, and the cost of ongoing element replacement. Some air intake filter systems have service bulletins and airworthiness directives requiring certain maintenance actions. More information regarding care and servicing of air intake filters can be found by contacting the manufacturer of the aircraft and engine.

Information for this article was provided by Scott Petersen, account manager for the Donaldson Company's Aerospace and Defense Group.