Maintenance and Certification

Maintenance and Certification

Air Intake Filters: Technologies Used to Keep Contaminants Out

by Ronald Donner, Editor, Aircraft Maintenance Technology (AMT) on-line magazine ( This article originally appeared in the May 2010 issue of AMT magazine and is reprinted with permission.

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.

Photo of dry media in air intake filter.
The tight weave of the dry media traps particles by sieving the dust contaminates. 
Photo: Donaldson Aerospace and Defense Group


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 that 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 hr 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 min.
  6. Rinse with water.
  7. Dry the filter without accelerated drying methods.
  8. Re-oil the filter substrate.
  9. Let sit for approximately 20 min and check for oil coverage.
  10. Re-oil any areas of the filter that were initially missed.
  11. Continue steps 5 through 7 until a uniform colour covers the entire filter media.

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

3. Photo of dry media in air intake filter.
Wet media, 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.
Photo: Donaldson Aerospace and Defense Group

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. 

Fuel Tank Safety and Electrical Wiring Interconnection Systems—Considerations for Transport Airplane Modification and Repair Designs

The business of modifying and repairing transport category airplanes can be complex. A spectrum of engineering design challenges related to any specific modification or repair necessarily compete with the business realities of financial and time constraints. As always, it is necessary to be wary of aircraft level risks that may be inadvertently introduced with the installation and integration of new design changes to any aircraft.

Accident examples have raised awareness regarding the need for new best practices to protect against aircraft level safety risks associated with modification (and repair) of fuel tank systems, including adjacent areas, and the installation and maintenance of electrical wiring interconnection systems (EWIS). EWIS is defined in Airworthiness Manual (AWM) 525.1701 as “any wire, wiring device, or combination of these, including termination devices, installed in any area of the airplane for the purpose of transmitting electrical energy between two or more intended termination points.” EWIS does not include electrical equipment or avionics qualified to acceptable environmental conditions and testing procedures, portable electrical devices that are not part of the airplane’s type design, or fibre optics.

Photo of electrical wiring interconnection system (EWIS).
Electrical wiring interconnection systems (EWIS)

In the United States, the Federal Aviation Administration (FAA) has codified these best practices under Special Federal Aviation Regulation (SFAR) No.88 and 14 CFR 26.11 Enhanced Airworthiness Program for Airplane Systems (EAPAS). In particular, these requirements apply to transport category, turbine-powered airplanes with a type certificate issued after January 1, 1958, that, as a result of the original certification or a later increase in capacity, have:

(1) a maximum type-certificated passenger capacity of 30 or more; or

(2) a maximum payload capacity of 7 500 lbs or more.

In the case of fuel tank systems, a worldwide effort by transport airplane design approval holders (DAHs) to re-evaluate their designs has resulted in the development and promulgation of numerous design changes and Instructions for Continued Airworthiness (ICA) (including Limitations), most importantly by airworthiness directives (AD). Much knowledge of the specific vulnerabilities of fuel tank system designs with respect to the development of ignition sources was gained through this safety exercise. New best practices are now recognized as necessary to minimize the development of fuel tank system ignition sources stemming from possible heat sources, electrical arcing (including lightning-induced arcing), or mechanical sparking (each arising from normal operation, single failures or combinations of failures that are not extremely remote).

In the case of EWIS, DAHs had to make a similar large-scale effort to evaluate the need for, prepare, and make available any additional maintenance and inspection tasks, developed using the Enhanced Zonal Analysis Procedure (EZAP) methodology, that may be required for the EWIS.

The EZAP is an analytical procedure that identifies the physical and environmental conditions in each zone of an airplane, analyzes the effects of these conditions on EWIS, and assesses the possibilities for smoke and fire. From EZAP analysis, maintenance tasks can be developed to detect EWIS degradation issues, prevent ignition sources and minimize possibilities for combustion by minimizing accumulation of combustible materials. The resulting EWIS EZAP ICAs are to be presented in the form of an appropriate informational document and will be easily recognizable as EWIS ICA. The goal of the resultant enhanced cleaning and inspection tasks is to have fewer EWIS failures, which leads to safer operation.

During the zonal inspections, EWIS would be checked for unacceptable conditions, including:

  • wire bundle chafing, sagging or improper attachment and securing;
  • wire damage (obvious damage due to mechanical impact, overheat, localized chafing, etc.);
  • wiring protection sheath/conduit deformity or incorrect installation;
  • contamination, such as dust and lint accumulation, surface contamination by metal shavings/swarf, and liquids;
  • deterioration of splices, whether from production or previous repair;
  • inappropriate repairs (e.g. incorrect splice);
  • grommets missing or damaged;
  • lacing tape and/or ties missing/incorrectly installed; and
  • wires riding on, or inadequate separation from, fluid lines.

Most manufacturers will conduct the EZAP through the Maintenance Review Board (MRB) process (using MSG-3 v2005.1 or a later version). However, Supplemental Type Certificate (STC) applicants will likely conduct the EZAP by other means outside the MRB process, such as via a Maintenance Type Board (see TP 13850). Typically, Transport Canada Civil Aviation (TCCA) aircraft evaluation or regional maintenance inspectors would participate in and/or review the results of the EZAP, with input from headquarters or regional aircraft certification engineers. The EWIS ICA would be included in an approved section of the ICA document(s) pertinent to each design approval.

Once generated, these EWIS ICAs are required to be placed in Canadian commercial air operator-approved maintenance schedules, pursuant to CAR 605.86, to meet the requirements of Standard 625, appendices C and D.

These wiring lessons have been learned and documented as recommendations from the Transportation Safety Board of Canada’s (TSB) investigation into the Swissair 111 accident. Among other findings, the TSB asserted that wiring discrepancies found on many aircraft reflected a shortfall within the aviation industry in wire installation, maintenance, and inspection procedures. In particular, the TSB identified that:

  • current maintenance practices did not adequately address wiring components;
  • wiring inspection criteria were too general;
  • maintenance instructions did not describe unacceptable conditions in enough detail; and
  • airplane wiring needed to be considered as a discrete system and given the same level of scrutiny as other airplane systems.

Photo depicting wire chaffing.
Wire chaffing issue

To ensure that the achieved safety objectives of the fuel tank system and EAPAS industry-wide safety reviews and retrofits are maintained for the operational life of the reviewed airplane models, we need to ensure that future design changes do not degrade the achieved level of safety in the fleet.

On a go-foward basis, the FAA is applying the fuel tank system and EWIS EZAP ICA requirements to all new design changes to transport category airplanes, pursuant to specific regulations. These requirements may be over and above the requirements of 14 CFR Part 25/AWM 525, or those otherwise established in the airplane’s basis of certification. Transport Canada (TC) and the European Aviation Safety Agency (EASA) are also applying these same design requirements for new design approval applications, citing that the design may have (unsafe) features that were not foreseen in the existing certification basis; for that reason, it establishes these new design requirements as applicable standards for a design approval application. Moreover, there are existing requirements that provide that there may not be design features or details that experience has shown to be hazardous or unreliable. Further, in view of the hundreds of ADs issued to correct in-service deficiencies relating to fuel tank safety, failure to follow the revised “best practices” would be considered an unsafe feature or characteristic, and on that basis TCCA or EASA may refuse to issue the design approval.

EASA has further clarified in NPA 2007/01 that it supported the retrospective design reviews and would send letters to request review of ICA to incorporate results of EZAP by DAH holders. ADs would be issued to non-cooperative DAHs, pursuant to the EASA Implementing Rule (IR) 21A.3B(c)(1). In addition, pending modification of type certificate data sheets, generic special conditions quoting the relevant paragraphs of CS-25 as modified by the EWIS NPA will be systematically issued for approvals of modifications affecting EWIS when application is after the amendment to CS-25 (September 5, 2008). As in Canada, any EWIS ICAs developed under the EZAP must be placed in European operators’ maintenance programs, pursuant to IR Part-M.302.

Each new design change that may affect the airplane fuel tank system should not introduce additional fuel tank ignition hazards to those that may already be present in the unmodified design. The design change applicant must demonstrate compliance with the design standards of AWM/FAR/CS 525/25.981(a), (b) and Appendix H525/25.4 (change 525-11, equivalent to FAR Amdt. 25-102).

Similarly, for each aircraft zone containing EWIS that is affected by the design change, especially where the characteristics of the zone (e.g. susceptibility to systemic accumulation of dust and lint, proximity to hydraulic and mechanical flight controls, zone density) may be affected, it must be determined whether any specific EWIS ICA may be required, using the EZAP methodology in accordance with AWM Appendix H525.5(a)(1) and (b) (change 525-16, equivalent to FAR Amdt. 25-123).

These actions are the cumulative result of past experience and in-depth reviews. They are intended to promote safety of the transport airplane fleet through certification and continued airworthiness processes. 

Floats—a Seasonal Problem

The following article was originally published in Aviation Safety Maintainer Issue 1/1988, and is republished in this issue for its pertinence to this day.

Spring is fast approaching and, with the melting of ice on lakes and rivers, aircraft owners and operators scramble to change over from winter ski and wheel kits to floats or amphibious landing gear. A search through some accident files suggests this can spell big trouble for the unwary AME after installation of an unserviceable or incorrect kit.

Accidents caused by faulty float or amphibious gear maintenance include those of an amphibious Cessna 185 that flipped onto its back during a water landing. This accident was due to a hung landing gear wheel that did not retract because of a defective pin in the mechanism. Also, a Cessna 185 Skywagon lost a panel of a float during flight because it was improperly fastened.

Again, a Cessna 185 floatplane veered right and rolled over during takeoff on a test flight because previous float repairs failed.

An amphibious Beaver was being flight tested following maintenance on the landing gear system. A circuit was flown, the gear cycled, and landing on the runway was completed. Following another takeoff, the pilot attempted a water landing; after touchdown, the aircraft cartwheeled and overturned. The investigation revealed that the left rear gear had failed to retract for some undetermined reason. In this case the pilot did not check the position and locking of the gear to determine if it was in the UP position. Inadequate inspection or other maintenance factors cannot be ruled out as contributory factors in the accident.

Photo of floatplane.

A review of one aircraft log book in the Western region did not uncover any entries related to float inspection, nor were the inspection items for floats added to the aircraft inspection schedules. Additional inquiries to other operators revealed that many were aware of neither maintenance requirements nor formal inspections on floats.

There seems to be a general lack of seasonal maintenance coupled with poor record keeping on float kits. This leaves floats transferred from one aircraft to another particularly vulnerable, since little service history accompanies the floats when this occurs.

What can the AME do to improve the safety of float installations? The place to start is with a close look at the nameplate on the float and then at the type certificate, to make certain the aircraft/float installation you are certifying is approved. Following this strategy, check that the installation conforms to available manufacturer’s information or drawings, and that all the hardware used is new or in good condition. Also include a check for proper installation of any supplementary type approvals (STA) and/or supplementary type certificates (STC) and that all applicable Airworthiness Directives (AD) have been carried out. Next, inspect the floats for evidence of repairs or corrosion, and, if repairs are found, make sure they comply with an approved repair scheme and that any skin replacement conforms to the manufacturer’s specifications. If corrosion is found, remove it and repair the floats as necessary. If in doubt, contact the float manufacturer or an approved overhaul facility for advice.

The following lists of items are compiled under the headings: Floats, Amphibious gear and Aircraft. These lists are not intended to be used as a formal checklist, but to serve as a reminder to the AME that considerable study and investigative work is required to install floats correctly and enhance safety.


  • Check for attachment of the manufacturer’s nameplate.
  • Check for required spacers, condition of attachment bolts, attachment fittings and associated structure for evidence of corrosion.
  • Check springs, cables, rigging and attachment points of water rudders.
  • Look for assemblies that have an excess number of washers and corroded bolts or fittings, and reassemble correctly using new parts.
  • Assemble all mechanisms using plenty of grease.
  • Check convenience items, such as steps or handles for proper approval.
  • Check for bogus parts and verify manufacturer’s part numbers and parts manufacturer approval (PMA) stamp where bogus parts are suspected. This is very important.
  • Watch for parts incorrectly heat treated, particularly important when repairs are being made; major repairs must be certified by an authorized inspector.
  • Examine spray rails for condition and evidence of repairs.
  • Look for patched or repaired spreader bars; in most cases this type of repair is illegal and susceptible to corrosion.
  • Check condition of streamlined brace wires and cables. Remove from service any streamlined brace wire showing evidence of repair by welding.
  • Check composite floats for evidence of delamination and loosening of fittings (special expertise is needed when repairing composite floats).

Amphibious gear:

  • Check for attachment of the manufacturer’s nameplate.
  • Check for proper operation and condition of gear and water door mechanisms, particularly for correct gear position and associated cockpit position indications.
  • Lubricate and retract mechanism paying particular attention to springs, bolts, pivot arms and worn or corroded detents. Test the hydraulic system for correct operation.
  • Check brakes and replace worn or heavily corroded disks or other parts.
  • Check microswitches for cleanliness and correct electrical operation, including operation and indication of the gear position indicator.
  • Inspect and lubricate all pulleys, slide tubes and cables.
  • Check baggage compartments located in floats. These must maintain structural integrity and be approved.


Don’t forget to check for the extra items that must be considered for the aircraft when installing float kits. Again, start with the aircraft approval, supplementary type approvals, and float approval documents or certificates and verify, where applicable, the conformity of items such as:

  • vertical fin modification (if required);
  • correct propeller installation;
  • exhaust extension (if required);
  • correct type of springs on water rudders;
  • changes to rigging such as flap limits in float configuration;
  • whether flight controls need re-rigging;
  • instrument markings for float operation—particularly the airspeed indicator;
  • any extra bracing—V braces in the cockpit, etc.;
  • corrosion proofing (if required);
  • float fittings or other attachment parts (look for bogus parts); and
  • any items that call for dye penetrant inspection prior to installation.

Finally, after the installation is complete, go back and recheck all the items called for on the inspection sheet and applicable airworthiness directives. Then complete the necessary log entries indicating that a new float or amphibious installation or re-installation is released for flight in a serviceable condition, and that all airworthiness directives have been complied with. 

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