For most of aviation history, aircraft fabric covering meant one thing: Grade A cotton. Cotton was the standard against which all fabric was measured, and the FAA's original certification standards were written with cotton in mind. Beginning in the mid-twentieth century, however, synthetic polyester fabrics entered the market and quickly demonstrated they could outperform cotton in nearly every measurable way. Today, two systems dominate the certified polyester fabric world — Ceconite and Poly-Fiber — and every AMT working on fabric-covered aircraft needs to understand not just how these systems work, but the legal and regulatory basis that allows them to be used on certificated aircraft in the first place.
This article walks through the material properties of polyester fabric, the approval mechanisms that make Ceconite and Poly-Fiber legal for certificated aircraft, the application processes unique to each system, and the inspection and airworthiness standards an AMT must apply when evaluating polyester-covered surfaces.
Why Polyester Replaced Cotton
Grade A cotton, as defined by FAA Technical Standard Order (TSO) standards and described in FAA Advisory Circular AC 43.13-1B, has a minimum tensile strength of 80 pounds per inch of width when new. The problem with cotton is biological and chemical: it is an organic fiber that absorbs moisture, is susceptible to mildew and rot, and degrades with exposure to ultraviolet light. A properly doped cotton covering on an aircraft operating in normal conditions might realistically last 15 to 20 years under ideal storage and maintenance, but real-world service life is often shorter.
Polyester, by contrast, is a synthetic polymer — essentially a plastic fiber woven into fabric. It does not absorb moisture the way cotton does, resists mildew and fungal attack, and retains its tensile strength far longer under UV exposure. Ceconite and Poly-Fiber polyester fabrics routinely achieve service lives of 25 to 30 years or more when properly installed and maintained. Their initial tensile strength also significantly exceeds Grade A cotton, with typical new polyester fabrics testing well above 100 pounds per inch of width — often in the range of 120 to 135 pounds per inch or higher depending on the specific product grade.
Beyond longevity, polyester fabric is dimensionally stable. Cotton fabric shrinks dramatically when doped, a property that is actually useful for achieving tautness, but which must be carefully controlled. Polyester does not shrink with nitrate or butyrate dope in the same way; instead, dedicated heat-shrink properties are engineered into the weave, and tautening is accomplished through the application of heat — typically with a household iron or heat gun — rather than relying solely on chemical doping. This makes the process more predictable and repeatable.
The Approval Basis: STCs and the Legal Framework
Here is where the regulatory knowledge becomes critical for the AMT: polyester fabric is not a direct replacement for Grade A cotton under the original type certificate (TC) of most certificated aircraft. The original TC specifies cotton. To legally use polyester fabric on a certificated aircraft, the work must be performed under an approved data source.
The primary approval mechanism is the Supplemental Type Certificate (STC). Both Ceconite and Poly-Fiber hold FAA STCs that authorize the use of their respective fabric systems as replacements for Grade A cotton on a broad category of certificated aircraft. These STCs are not aircraft-specific in the traditional sense — they are issued to the fabric manufacturer and apply to any aircraft that falls within the STC's eligibility criteria, which is typically stated as any aircraft originally certified with fabric covering and a never-exceed speed (VNE) below a specified threshold.
When an AMT performs a recover using Ceconite or Poly-Fiber under the applicable STC, the STC number must be entered in the aircraft maintenance records. The maintenance record entry must reference the STC, identify the fabric used, and describe the work performed — just as any major repair or alteration requires a completed FAA Form 337 for major alterations if the work falls outside approved data already in the aircraft records.
It is important to understand that recovering an aircraft with polyester fabric, even using an STC-approved product, is classified as a major repair or major alteration under 14 CFR Part 43, Appendix A. This means the work must be performed by or supervised by an appropriately rated AMT (Airframe rating), and the work must be documented properly. Under 14 CFR 43.7, a certificated mechanic holding an Airframe rating may approve for return to service a major repair or alteration only when the work was performed in accordance with technical data approved by the FAA and the mechanic is otherwise authorized to approve such work; in practice, return to service after a major repair or alteration typically requires an appropriately rated certificated repair station, an Inspection Authorization (IA) holder, or the manufacturer, rather than an Airframe mechanic's own sign-off.
In addition to STCs, AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair) remains an important reference for fabric work generally. Chapter 2 of AC 43.13-1B covers fabric covering in detail and provides the minimum tensile strength standards that both cotton and synthetic fabrics must meet to be considered airworthy.
The Ceconite System
Ceconite fabric is a tightly woven polyester fabric available in several weights, designated by product number. Ceconite 101 is a heavier weight product suitable for aircraft with higher performance or VNE speeds, while Ceconite 102 is a lighter weight product generally used on slower, lighter aircraft such as ultralights and light sport aircraft. For certificated aircraft under the STC, the appropriate weight must be selected based on the aircraft's VNE, following the STC instructions for continued airworthiness (ICA).
The Ceconite system is compatible with both nitrate and butyrate dope — the same chemical coatings historically used on cotton — as well as with polyurethane-based finishing systems. This compatibility with traditional dope makes Ceconite appealing to shops already experienced with conventional doping practices. The process involves attaching the fabric to the structure using a combination of rib stitching (where required by the aircraft design), fabric cement, and surface tapes, followed by the application of multiple coats of dope or approved coating, with intermediate sanding between coats.
The Poly-Fiber System
The Poly-Fiber system and Stewart Systems are two separate, competing STC-holding fabric covering brands, each with its own proprietary process materials — they are not the same system, and one was not developed by the other. Poly-Fiber (Consolidated Aircraft Coatings) uses its own proprietary line of water-based coatings and adhesives rather than traditional nitrate or butyrate dope. The process begins with Poly-Tak adhesive for bonding fabric to structure, followed by Poly-Brush (a porous, brush-applied primer coat that penetrates the fabric weave), then Poly-Spray (an aluminum-pigmented UV-blocking coat), and finally Poly-Tone or compatible topcoats for the color finish. Stewart Systems, by contrast, markets its own distinct water-based process built around products such as EkoBond, EkoFill, EkoPoxy, and EkoFinish.
Because the Poly-Fiber system does not use traditional nitrate or butyrate dope, there is no significant fabric shrinkage from the coatings. Tautening is accomplished using a covering iron at specific temperatures — typically around 250°F for initial shrinking and up to 350°F for final tautening, following the manufacturer's detailed instructions. The water-based coatings are also considerably less flammable during application than nitrate dope, which is a meaningful safety advantage in the shop environment.
Airworthiness Standards and the 70% Rule
Regardless of which system was used for the original cover, an AMT performing fabric inspections must evaluate the fabric against the minimum airworthiness standard: the fabric must retain at least 70% of the minimum tensile strength required for the original covering material. For Grade A cotton, that original minimum is 80 pounds per inch, so the fabric must test at or above 56 pounds per inch to remain airworthy. For polyester fabric, the original specified minimum (per the applicable STC) is typically higher, so the 70% threshold applies to that higher starting value.
Field testing is accomplished using a calibrated fabric punch tester, also called a Seyboth tester or similar device. The instrument is pressed through the fabric, and the force required to puncture it is compared against a reference chart. Testers must be calibrated and used strictly per the manufacturer's instructions to produce reliable results. A failing test result grounds the aircraft until the fabric is replaced.
- Grade A cotton minimum new strength: 80 lb/in — airworthy threshold is 56 lb/in (70% of 80)
- Polyester (Ceconite/Poly-Fiber) typical minimum new strength: varies by product but often exceeds 100 lb/in — airworthy threshold is 70% of that specified minimum
- Approval basis for polyester on certificated aircraft: STC required; document with Form 337 if a major alteration
- Regulatory classification of recovering: Major repair or major alteration per 14 CFR Part 43, Appendix A
- Heat shrink temperature for Poly-Fiber: approximately 250°F initial, up to 350°F final (per system ICA)
Common Test Traps
- Assuming polyester is a drop-in replacement: Polyester fabric cannot simply be substituted for cotton without STC authorization. The STC is what makes the work legal on a certificated aircraft — do not confuse experimental or ultralight practices with certificated aircraft requirements.
- Confusing the 70% rule threshold: The 70% minimum applies to the original specified strength for the covering material, not to the new strength of the fabric actually installed. Know the baseline for each material.
- Mixing systems: Ceconite fabric is compatible with nitrate or butyrate dope, but Poly-Fiber coatings are a proprietary system that must be used as a complete system. Mixing Poly-Fiber fabric with traditional dope, or vice versa in ways not approved by the ICA, can result in unapproved data and an airworthiness issue.
- Forgetting Form 337: Recovering is a major alteration. Failing to complete and submit FAA Form 337 when required is a regulatory violation and leaves the aircraft in an unapproved configuration.
- Ignoring VNE limits on STC eligibility: Each STC specifies the aircraft performance envelope it covers. Using a lightweight polyester product on a higher-performance aircraft outside the STC's eligibility criteria makes the installation unapproved, even if the fabric brand is otherwise FAA-recognized.