Engine Air Filters: Dry Pleats or Oiled

Since performance gains are minimal, think in terms of serviceability, cost. We favor Donaldson’s dry media design for simple upkeep and good airflow.

Ask a typical owner what type of inlet air filter he has and you’ll likely get a shoulder shrug. It’s one of those service items that many rely on the maintenance shop to look after.

But we think it’s critical enough to at least know what type of filter is on your engine and that it’s being properly maintained. It’s a required component to check during annual inspection, but we once spotted a paper air filter on a Cessna 210 that was so dirty and deteriorated we wondered if it was ever replaced. Incidentally, that engine needed overhaul long before its new owner planned. Clearly, air filter maintenance is an easy way to help protect the inside of an engine. According to Lycoming, dust or some form of dirt is frequently the principal factor in premature piston ring and cylinder wear.

There are only three brands of FAA-approved air filters—Brackett Aero Filters, Challenger Aviation and Donaldson Filtration. We sampled one of each (approved replacements for Cessna part number 120009, used on the 172-series) to look at design, build quality and each manufacturers performance claims. We didn’t perform laboratory testing, given the costs, but instead collected manufacturers’ performance data, based on its claimed lab testing. We also did some basic experimenting of our own and talked with maintenance shops, aircraft owners and engine shops.

Filtering media
There are two major differences in engine air filter design—dry media and wet media. The dry media design used by Donaldson uses a more traditional pleated synthetic fiber element, while the wet media filters from both Challenger and Brackett are drenched in a tacky oil solution.

Wet filters rely primarily on the oil to trap the dust particles and other contaminates, while a dry filter uses the filter fibers to trap contaminates, as most traditional air filters are designed to do.

There’s an ongoing FAA Airworthiness Directive (AD 84-26-02)—dating back to the 1980s—that applies to all paper induction air filters. It says, in summary, that filters must be replaced at 500-hour service intervals. The replacement filter must be FAA-approved, of course, and the AD doesn’t override instructions for continued airworthiness, which requires 100-hour and annual inspection of the filter. It’s the deterioration of the paper element and ultimate digestion and engine stoppage that’s of concern. There’s also the potential for a paper filter to catch fire during engine backfire.

None of the filters evaluated here fall under the paper filter AD (technically, none of them use paper media), although Donaldson’s instructions for continued airworthiness is in line with the AD, recommending that it’s replaced at 500 flight hours.

The key is that no matter which filter you use, a restriction in airflow will be detrimental to engine power. Some argue that surface-loading paper elements are more restrictive to airflow by design because the filtering fiber must be tightly compressed and dense enough to effectively trap the harmful contaminates. It’s also said that ingested contaminates of 10-20 microns are responsible for the most damage (the industry standard SAE J726 air-filter test uses 0-to 5-micron test dust as a benchmark). Keep in mind that once a surface-loaded filter becomes clogged, it’s pretty much spent and the engine—starved for air—can ingest pieces of the filter.

Every technician we spoke with noted the first item to check when an engine doesn’t make full static power is the air filter. If it’s wet, plugged with dirt and debris or has any restriction, it will starve the engine of air, likely creating a rich mixture. But what about gains in performance? Challenger was the only filter manufacturer that definitively told us that years of flight testing (during the STC approval process) revealed slight increases in horsepower.

“In our testing, we found increases in performance, but there are many variables that come into play when you get out in the field. While we get claims from some owners boasting of higher performance numbers, we can only speak to what we have seen in our tests,” said Linda Rocco, Challenger’s president.

She told us that some engines could gain two to four horsepower, but concedes that depends on restrictions in the air intake system. The company provided flight test results that show slight increases in manifold pressure during its initial STC testing in a Mooney M20J and also in a Cessna.

Additionally, Challenger provided airflow versus restriction data that was in line with test data provided by Donaldson. As the chart in the sidebar on page 21 shows, the Challenger and Donaldson filters share similar pressure drop characteristics.

Such slight changes in manifold pressure may be difficult to see on some stock gauges. Additionally, at wide-open throttle there is less than one inch of mercury drop between the ambient and the pressure inside the manifold. Regardless, restrictions in airflow can adversely affect performance characteristics that you might notice on takeoff, in climb and during cruise. See the sidebar on page 21 for more on intake restrictions.

Filters from Brackett Aero Filters have been in service since the late 1960s and are installed as original factory equipment on a wide variety of aircraft. There is PMA approval in place on 88 different models.
The Brackett filter consists of a dual-stage throwaway foam element that fits into an aluminum frame with a stainless steel screen. The filter assembly is sealed to the airbox with a gasket, while a removable grill fits over the foam element.

The current Brackett design has been improved in response to earlier ADs that were issued on Brackett filters due to failing gaskets and filter screens that were coming loose and being ingested into engines.

For example, there’s now a protective lip that surrounds the edges of the airbox gasket should the glue fail and the gasket becomes loose. Since the Brackett filter element is foam, the paper filter AD doesn’t apply.

The oiled (saturated, really) element is required to be replaced yearly, for a minimal cost. We purchased a replacement foam element that fits a Bonanza for under $10—that’s a huge savings compared to the others that cost hundreds of dollars. Aside from getting your hands gooped up with sticky oil, replacement is painless and straightforward. The same can’t be said for the K&N element in the Challenger assembly. It requires cleaning, drying and re-oiling. More on that in a minute.

The Brackett foam element has a shelf life of four years, even if it’s been stored in the original bag.

The company provided us with laboratory test data that said one of its synthetic fiber filters (for a Cessna 172) had the lowest restriction—or best airflow—compared to the Brackett foam and Challenger gauze-type filters. Donaldson says it’s more filtering media and less frame area, plus deeper filter pleats and the evenly distributed size and shape of the fiber that contributes to better air flow. Plus, there’s no layer of oil to restrict the flow.

On the other hand, Donaldson doesn’t specifically state a value for gains in horsepower. Instead, it notes the obvious: A lower air restriction would theoretically provide a fractional performance advantage. Donaldson told us that the difference in airflow restriction between filters must be significant to realize even a one percent gain in horsepower and that sizable difference didn’t exist when it tested its synthetic filter media filter against the Challenger’s K&N gauze filter. Donaldson said there was, however, a significant difference in pressure loss when it tested its filter against a foam filter. This loss in pressure, or delta-P, can be detrimental to performance. This jives with field reports we got from a variety of owners that used both style filters.

The Donaldson filter is rated to last 500 hours and while the synthetic filtering media isn’t governed by the air filter AD, it still must be replaced at 500-hour intervals, after three years or after five cleanings—whichever comes first.

The filter can be cleaned either by compressed air when there’s dust on it or washing it in a solution of water and general purpose detergent when there’s a combination of dust and oil or carbon.

When it’s clean, Donaldson claims that the filter delivers 98.5 percent or greater filtration efficiency (according to the ISO 5011 requirements) and the filter meets or exceeds FAA induction air filter fire safety regulations (FAR 25.853 and FAR 23.1107).

The Donaldson filter for the Cessna 172 application typically sells for around $140 and the company offers over 75 approved filter models for a wide variety of aircraft. It’s worth noting that Donaldson filters have been stock equipment on many Cessna, Beechcraft, Mooney and other aircraft for decades.

These filters use the K&N oiled filter media. Most anyone that’s familiar with high-performance engines will recognize the K&N name and its High-Flow air filters and air intake systems. For automotive and motorcycle applications, K&N claims that its filtering media will generally increase horsepower by one to four percent, compared to OEM filters.

The filter is made of four to six layers of cotton gauze sandwiched between two epoxy-coated aluminum wire screens. The cotton is treated with oil, causing tackiness throughout the cotton’s microscopic strands.

K&N claims that the nature of the cotton allows high volumes of airflow, and when combined with the tackiness of the oil barrier, creates a more effective filtering media—achieving an overall filtration efficiency in the range of 96 to 99 percent, based on its ISO 5011 testing procedure. Of course, there are factors that can deteriorate the filtering efficiency, particularly the oil saturation (wash-off) that can be affected by flight in rain, for example.

Even the location of the airbox in relation to the aircraft cowling can have an effect on filter performance and longevity. After a while, the wet, sticky oil that coats the filter begins to dry and excess might even get drawn into the intake.

Depending on what you fly and the conditions you fly in, Donaldson’s Scott Petersen makes a good case for selecting a dry filter. “If the filter is in close proximity to the propeller, as it often is on a Cessna single, for example, the filter is being impacted by rain, snow and moisture. That can cause an oiled filter to dry out, drastically reducing its effectiveness,” he told us. We concur, based on our experience using K&N filters on two motorcycles. In one application, the filter is bolted to a naked high-flow airbox that’s unprotected from rain. After distance-riding in a few rainstorms, it’s clear that some of the oil washes off. In another application where the filter is in an enclosed airbox, it’s notably wetter for longer periods of time.

Petersen also noted that the K&N media used in the Challenger has lower resistance for better airflow, but because the Donaldson filter has more surface area for the air to flow through, it has a slight advantage.

Our thanks and appreciation to reader Bill Foley who bravely offered up his V35B Bonanza for a series of eye-widening aborted-takeoff time trials. Incidentally, Foley is an A&P that made the switch from the Brackett to the Donaldson filter a few years ago for the perceived improvement in performance and simplicity and has never looked back.

“I have been making frequent trips out West from Connecticut and, hence, need all the inlet ram air I can get. I’m satisfied that a slight overall performance improvement is there. The filters are easy to clean, the messiness is gone and there has been no increase in silicone in the oil when I get oil analysis at each oil change,” he told us.

Based on our evaluations of the three filter designs, we think choosing one might also depend on the environment you operate in.

If you fly in an area that has a lot of dust, including the desert, for example, or operate in and out of dirt and unpaved runways, our sense is that a well-maintained Brackett foam element is going to catch nearly any piece of impurity your inlet is likely to encounter, but with a performance penalty. We like its inexpensive price and that it’s a throw-away, easy drop-in replacement. That saves money during annual inspections.

Based on K&N’s long-standing reputation for increasing horsepower in non-aviation applications, we don’t doubt Challenger’s claims of slight gains in horsepower for some applications. If performance was everything—and for some folks it is—we’d likely try one. Plus, we like its high-quality build, including the frame that’s built and assembled by Challenger. But for basic applications, we’re not convinced the claimed performance gain is sizable enough to justify the initial acquisition cost (the CP-1172 filter for a Cessna 172 is a whopping $219), in addition to the time-consuming effort and cost to service it.

While the Donaldson, at around $140, isn’t exactly cheap, we think it offers decent airflow, good filtering properties and is convenient to service.

Larry Anglisano
Editor in Chief Larry Anglisano has been a staple at Aviation Consumer since 1995. An active land, sea and glider pilot, Larry has over 30 years’ experience as an avionics repairman and flight test pilot. He’s the editorial director overseeing sister publications Aviation Safety magazine, IFR magazine and is a regular contributor to KITPLANES magazine with his Avionics Bootcamp column.