AIRCRAFT UPHOLSTERY CONSTRUCTION GUIDE
Short answer: Aircraft seat upholstery is normally sewn with meta-aramid (Nomex-type) thread, not the bonded nylon or bonded polyester used in automotive and marine work. Aramid chars instead of melting. Nylon and polyester melt and drip, and flaming drippings are a specific fail criterion in the FAA vertical burn test that the finished assembly has to pass. Common aramid sizes run Tex 70 through Tex 135 (V-69 to V-138), matched to the seam and the machine. The thread your shop uses should match whatever was in the assembly your documentation is based on — confirm that with the A&P, IA, DER, or repair station signing off on the job.
Why bonded nylon and polyester are the wrong default
Both are thermoplastics. Nylon 6,6 melts at roughly 500 °F (260 °C) and polyester at roughly 482–500 °F (250–260 °C) — well below the temperature of a cabin fire. When a seam sewn with either one is exposed to flame, the thread does not simply burn. It liquefies, forms a bead, and falls. If that bead is still burning when it lands, it is a flaming dripping, and flaming drippings are scored against a hard limit.
Meta-aramid has no melting point at all. It decomposes in place at roughly 700–800 °F (370–430 °C), charring rather than flowing, and self-extinguishes when the flame is removed. That one property is the whole reason aviation upholstery uses aramid and automotive upholstery does not.
What the burn test actually measures
Under 14 CFR 25.853(a), upholstery, seat cushions, coated fabrics, and leather fall under the 12-second vertical Bunsen burner test in Appendix F, Part I. A specimen is clamped vertically, a 1.5-inch flame is applied at the lower edge for 12 seconds and removed. Four things are recorded:
| Measurement | Limit (average of specimens) |
|---|---|
| Burn length | 8 inches (203 mm) maximum |
| Flame time after the burner is removed | 15 seconds maximum |
| Flame time of drippings after falling | 5 seconds maximum |
| Self-extinguishing | Required |
The 60-second version of the test — 6-inch burn length, 3-second drip limit — applies to ceiling and wall panels, partitions, galley structure, and structural flooring, not to seat upholstery. The two get quoted interchangeably online. They are not the same test.
The point that matters for thread: the test is run on the specimen as it will be used. A seam is part of the specimen. A face material with excellent burn data can be assembled into something that fails, and thread is one of the ways that happens.
Thread is not covered by the face material's documentation
A burn certificate or test report from a vinyl or leather supplier describes that material, in the configuration that was tested. It does not extend to the thread, the foam, the fire blocker, the adhesive, the backing, or the finished seat. If a supplier's paperwork appears to cover your whole build, read it again — it almost certainly does not.
This is the most common documentation mistake in aircraft interior work: buying a burn-certified face material and assuming the seat is now compliant. The face material is one input. Compliance lives at the assembly.
What aramid thread documentation looks like
Aramid sewing thread is generally sold against U.S. Commercial Item Descriptions, and a supplier can usually provide a certificate of conformance to one of them:
- A-A-55217 — meta-aramid, spun staple. The Nomex-type thread most often used in cabin interior sewing.
- A-A-55195 — para-aramid, spun, intermediate modulus (superseded MIL-T-44100).
- A-A-55220 — para-aramid continuous filament, Kevlar-type.
Be careful about what a certificate proves. A conformance certificate to A-A-55217 says the thread meets that thread specification. It is not a 14 CFR 25.853 Appendix F test report on your seat, and neither is a European flame-spread result such as DIN EN ISO 15025, which is a protective-clothing test. Different test, different specimen, different pass criteria. Only the Appendix F test on the relevant specimen speaks to Appendix F.
Which rule applies to your aircraft
- Transport category (Part 25) — 25.853 applies. Aramid thread, fire-blocked cushions, documentation for everything. This is the strict end.
- Normal category (Part 23) — 23.853 and Part 23 Appendix F. Many small aircraft interior items are qualified against a horizontal burn test rather than the vertical one, and the requirement varies by where the item is installed and the aircraft's certification basis. Your aircraft's basis governs, not a general rule from the internet.
- Experimental / amateur-built — the flammability rules above generally do not apply as certification requirements. Plenty of builders still use aramid thread and fire-blocked cushions, because the fire does not check the airworthiness category.
If you cannot state which of these your aircraft falls under, that is the question to answer before ordering thread.
Sizing, once the fiber is settled
Thread size is a construction decision, and here aviation is no different from any other heavy upholstery work:
- Tex 70 (V-69) — lighter seams, tighter curves, finer topstitch, lighter face materials.
- Tex 90 (V-92) — general-purpose upholstery weight.
- Tex 135 (V-138) — heavy seams, thick stacks, bold decorative topstitch.
Heavier is not automatically better. Thread and needle that are oversized for the material perforate the edge, fight curves, and look coarse. Sew the actual stack on scrap from the real material before you cut the cover: build the thickest seam you expect, check tension balance, skipped stitches, puckering, curve control, and back-tacking, then write down the machine, needle, thread, and stitch length that worked.
What Carolyn Fabrics supplies — and what we don't
We want to be direct about this, because getting it wrong costs somebody a failed inspection.
We do not supply aviation-rated thread, foam, or adhesive. The thread we stock is AMANN Outdoor-Pro bonded polyester in Tex 90 and Tex 135. It is a marine and outdoor thread — UV-stable, strong, correct for boat seats and outdoor furniture — and it carries no FAA flammability data. Do not put it in a seat that has to meet a burn requirement. Source aramid thread from an aviation supply house that will send the conformance paperwork with it.
What we do supply is face material with manufacturer burn documentation: Spirit Millennium vinyl (documentation on request with the order) and Slipstream aviation leather (every hide ships with its burn certificate). If you tell us the aircraft, the seat, and the color direction, we can help with yield, yardage, and what documentation comes with the material. We do not determine airworthiness and we do not approve installations.
Frequently asked questions
What thread is used for aircraft seat upholstery?
Meta-aramid (Nomex-type) thread, typically Tex 70 to Tex 135, is the standard for aircraft seats that must meet a flammability requirement. Bonded nylon and bonded polyester — the normal choices in automotive and marine upholstery — melt and drip, and flaming drippings are a scored fail criterion in the FAA vertical burn test.
Does aircraft upholstery thread have to be fire rated?
The thread itself is not separately "rated." The requirement is on the assembly: the seat, as built, has to pass the applicable burn test. Because the thread is part of that assembly, a melting thread can cause a failure. That is why shops working to a burn requirement specify aramid.
Can I use bonded nylon or polyester thread in an aircraft seat?
On a certificated aircraft, not unless your approval basis and test documentation support it. Both melt and drip. On an experimental aircraft the certification rules generally do not apply, but the fire behavior is the same either way.
Does the vinyl or leather burn certificate cover my thread?
No. The certificate covers the material tested, in the configuration tested. Thread, foam, adhesive, and fire blockers are separate items with separate documentation.
Is Nomex thread the same as Kevlar thread?
No. Nomex is meta-aramid — flame resistant, used for sewing thread and flight suits. Kevlar is para-aramid — built for tensile strength, used where load matters. Both are aramids and neither melts, but they are specified under different Commercial Item Descriptions and are not interchangeable.
What thread size should I use?
Match it to the seam, the stack thickness, the machine's practical range, and the needle. Tex 90 is a reasonable general-purpose starting point for seat work; go to Tex 135 for heavy stacks or bold topstitch, and down to Tex 70 for finer seams and tight curves. Test on scrap from the actual material first.
After ordering, use the aircraft upholstery material receiving checklist before cutting, and see burn certificate vs. burn test for what the documents do and do not prove.