Recip Technology: Air Intake Filters

Technologies used to keep contaminants out


  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.

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