Interiors, Legalities, and Part 91 Aircraft

AVweb reader Greg Amy refurbished the interior of his Grumman Tiger with automotive cloth and vinyl materials, but subsequently became concerned about whether those materials were legal for use in his aircraft. His research led him through a veritable morass of conflicting and contradictory guidance from FARs, Advisory Circulars, FAA-Designated Engineering Representatives, FAA-approved testing labs, and a National Resource Specialist from FAA Headquarters. Greg's ordeal in trying to get a straight answer to a simple certification question helps explain both why the FAA employs so many lawyers, and why so many pilots are drawn to homebuilt aircraft.


Here’s the issue: Suppose you want to refurbish the interior of your FAA-certificated light plane with new carpet and upholstery. Further suppose that you have found a reputable auto upholstery shop that does great work and will reupholster your seats and cut you new carpets if you bring the stuff to them. What does the FAA have to say about the materials you or the shop uses?

During the summer of 1998, I refurbished the interior on my Grumman Tiger, using automotive cloths and vinyls. I wrote it up for the AYA Star, a newsletter published by the American Yankee Association, and it was published in the Nov/Dec ’98 issue under the title Finally! Comfortable Seats!

Then the fun began. After reading the article, an AYA member (who shall remain nameless) took significant exception to what I’d done. Based on an article he read in Light Plane Maintenance (November 1998 issue), he claimed:

“Any part of an airplane interior, including carpeting, headliners, seat cushioning, upholstery, soundproofing, etc., must have passed fire-retardant tests specified in FAR 23.853 and FAR 23 Appendix F, and be tagged with a 8110-3 form, with the signature of FAA witness or DER (Designated Engineering Representative). … Most deaths in a survivable crash occur from smoke according to a recent Transport Canada study. And airliners use approved materials. Do you want to sit on something flammable in a Grumman?”

Interior of Greg Amy’s Grumman Tiger. Photo by Steve Williams.

I decided I better check into the question of legalities further. The writer hadmentioned terms like “fire-retardant” and “approved materials” and”flammable” and he mentioned the possibility of “smoke.” Trouble was,none of these terms are mentioned in the FARs he quoted. Nothing, nada. In fact, the writer’s biggest apparent fear — smoke — is not mentioned in the regs at all.

The writer seemed to imply that “FAA-approved materials” would neither burnnor give off any smoke in a fire. This is simply not true. I fired back that:

“Per FAR 23.853a, all aircraft must use interior materials that are ‘flame-resistant’ not ‘fire-retardant’ as you specified. The FAA does not define, nor does FAR 23 specify, the term ‘fire-retardant.’ ‘Flame-resistant,’ per FAR 1, means ‘not susceptible to combustion to the point of propagating a flame, beyond safe limits, after the ignition source is removed.’ In my (non-lawyer) opinion, the only requirement necessary in order to be able to use alternate fabrics in your aircraft is a simple logbook statement by the installer attesting to the fact that the material did not propagate a flame once the ignition source is removed. …

“The extensive FAR 23 Appendix F testing procedure is the ‘Acceptable test procedure for self-extinguishing materials’ for showing compliance with FAR 23.853. However, the need for this testing procedure is only called out in FAR 23.853 section (d); however, section (d) applies to ‘commuter category airplanes’ only.

“Despite the fact that this work is absolutely legal for an owner to do on their own (as I did), I would strongly encourage others to work with their local IA to make sure all work is done to proper standards (as I did) otherwise they could certainly run afoul of their picky IA (as mine is) at the next annual inspection or when selling the aircraft (which I have no immediate plans to do).”

Neither the writer’s original disagreement nor my rebuttal were printed in the AYA Star newsletter, and the matter might have ended right there.

Parsing The Regs

However, this got me thinking: Where did I get that rule interpretation? At first glance, FAR 23.853 seems pretty clear: All materials in our non-commuter-category FAR 23 aircraft must use “flame-resistant” materials. (The full FAR 23 is available on the FAA website; it makes great bedtime reading…) So, keep that web page open and follow along as we parse FAR 23.853:

(a) “The materials must be at least flame resistant”

OK, sounds easy enough. So, where’s the definition of “flame resistant,” and how do we prove it? Well a simple check of FAR 1, “Definitions and Abbreviations,” defines:

“Flame resistant” means not susceptible to combustion to the point of propagating a flame, beyond safe limits, after the ignition source is removed.

Problem is, while the FAA defines what it is, they don’t tell us how to demonstrate it. Traditionally, the simple “Bic test” was sufficient … you ignite the edge of the material on fire with a lighter, and see if it goes out by itself.

(b) “(Reserved)”

There probably was a rule here in the long past, but they deleted it and left the other sections in place.

(c) “If smoking is to be prohibited, there must be a placard so stating, and if smoking is to be allowed…”

This one’s an easy fix. Make sure your airplane is placarded “No Smoking” and the rest of the section will not apply (and your vacuum-driven gyros will love you for it).

(d) “In addition, for commuter category airplanes, the following requirements apply:”

Since our airplanes are not commuter category, this section does not apply.

(e) “Lines, tanks, or equipment containing fuel, oil, or other flammable fluids”

Not applicable to our discussion about interior fabrics et al. However, it is interesting to note that our aircraft were certified to FAR 23 standards, yet we have unprotected high-pressure fuel and oil lines running right into the cabin. (And we’re worried about the flammability of our seat covers…)

(f) “Airplane materials located on the cabin side of the firewall must be self-extinguishing, or be located at such a distance from the firewall, or otherwise protected, so that ignition will not occur if the firewall is subjected to a flame temperature of not less than 2,000 degrees F for 15 minutes. … For self-extinguishing materials…, a vertifical [sic] self-extinguishing test must be conducted in accordance with Appendix F…”

Must be “self-extinguishing” or far enough from the firewall. Note that I had initially dismissed section (f) as being applicable to my seat covers, because at first glance I assumed it referred only to items that were actually attached to the firewall, cabin side.

We have a definition of “self-extinguishing.” It’s defined in FAR 23, Appendix F, labeled “Acceptable test procedure for self-extinguishing materials for showing compliance with Secs. 23.853, 23.855 and 23.1359.” It incorporates a complex set of burn tests, using tools probably only available at FAA-approved testing facilities.

So, now we know what we have to test to for “self-extinguishing.” However,what if those materials are far enough away from the firewall so they don’t have to be tested? How can we prove they’re far enough away? In addition, what about “flame resistant”? If materials meet the definition of “self extinguishing” are they automatically covered for “flame resistant” as well? Or vice-versa?

So the questions lie:

  • Are our materials of sufficient distance from the firewall (and how do we prove it)? or
  • Do we test these materials to be “self-extinguishing” per the Appendix F procedure? and
  • How do we prove our materials are “flame resistant”?

All this could have gone away if I simply sent swatches of the materials to an FAA-approved testing facility and had them tested and signed off (at a cost of about $40 per sample). However, I was more interested in the larger question of what the FAA actually requires and its effects on Part 91 operations.

Who Ya Gonna Call?

Interior of Greg Amy’s Grumman Tiger. Photo by Steve Williams.

So the next thing I did was get on the phone with one of the certified testing labs and ask them what would be required for me to legally use automotive fabrics in my Part 91, FAR 23-certified aircraft. I wound up talking to AOPA and Jack Theden (pronounced”the-DEEN”) of Skandia Aircraft Interiors, telephone (815) 227-1611. (Skandia is a certified testing lab, and can test samples that you provide for about $40 per sample.) I was surprised at the answers I received.

I was directed to FAA Advisory Circular 23-2, “Flammability Tests.” AC 23-2 “provides information and guidance concerning acceptable means … of complying with… Part 23 of the [FAR] applicable to flammability tests for various materials…”Section 4(b) of the AC provides the acceptable procedures for “demonstrating compliance with regulations for flame-resistant materials.”

According to Skandia, for Part 23 non-commuter-class aircraft, they take three 3-inch by 12-inch pieces of the material to be tested, place them in a horizontal test rig only, place a flame on one end for ten seconds, remove the flame, and see if the material stops burning.

This test is almost verbatim the same “horizontal test” procedures as that called out in FAR 23, Appendix F, section (e). However, there’s very different from testing to the full requirements of FAR 23 Appendix F and not just section (e), since we would have to produce result from all the tests described in Appendix F, not just the “horizontal test.”

Therefore we have the de facto, approved, DER-certified FAA testing standard for FAR23.853(a) definition of “flame resistant.” Further, this implies that the FAR 23.853(f) test for “self-extinguishing” is not required for Part 91 aircraft.

How Can That Be?

Steve Willams, editor of the AYA Star, made further contact with Skandia, and came up with the following:

I described the statement in Light Plane Maintenance that only parts (a) and (b) of 23.853 apply to GA aircraft. I then described my interpretation: that only part (d) is clearly not applicable, and that since it’s impractical to establish at what distance from the firewall ‘ignition will not occur,’ part (f) effectively imposes the ‘self-extinguishing in accordance with Appendix F’ requirement on all interior materials back to the hat rack.

“What,” I asked, “is Skandia’s rationale for their position that only flame resistance (and hence only the horizontal test) is required for GA aircraft, as asserted to [Greg Amy]?”

He didn’t know, and ran his question by their DER. The DER said it’s an excellent question, but couldn’t answer it. … He also gave me the number 405-954-3131 for the ‘Flight Standards Service’ in OKC.

“But which part of this is actually important? What should you do?”

“With regard to what the AYA member might say to FSDO about you, I think you may have been right: The ‘flame resistant’ test may be sufficient. Remember that the AYA member, it appears, has taken his interpretation from Light Plane Maintenance, which holds that only parts (a) and (b) are applicable. My rationale that part (f), too, is required, probably hasn’t occurred to him. Assuming FSDO has the same interpretation as the AYA member and Light Plane Maintenance, the ‘flame-resistant’ test is all they’re likely to ask for.”

“So, if FSDO calls, your efforts to obtain the proper testing (together with an ASRS form) should serve you well. But get both tests done anyway.”

I’d still like to understand the rationale.

The last issue that the AYA member noted in their disagreement with me was that a Form 8110-3 was required for all materials. I could find no requirement for this anywhere. This “requirement” may very well be the case for materials you purchase from sources that guarantee FAR certification (like AirTex Products) but of course you won’t get that with the automotive-sourced products.

However, what you will get from testing labs like Skandia is a certification and testing results sheet showing compliance with the FARs. Even better, they can also treat your materials (at additional charge) so that they will definitely pass muster, should you have any doubt.

The “Aft Side Of The Firewall…”

So there we were, trying to figure out why FAR 23.853(f) apparently does not apply to our aircraft. I wasn’t complaining, mind you, just curious. Remember, at this point we pretty much have our standard by which to test and make the FAA happy…

Then I got to thinking about that Section (f) verbiage again: “Airplane materials located on the cabin side of the firewall…”

“Located on the cabin side of the firewall”? Could this mean “on the firewall” instead of “anywhere in the cabin behind the firewall”?

I wrote to Steve with this thought:

“… Section (f) deals with materials attached to the … firewall, cabin side.

“Think of it this way: 23.853 deals solely with ‘Passenger and crew compartment interiors.’ Agreed? In your opinion, 23.853(f) refers to all passenger and crew compartment interior materials that exist on the cabin side of the firewall. Well, have you ever heard of passenger and crew compartment interior materials that do not exist on the cabin side of the firewall? If not, then why the explicit verbiage?

“For this reason, [I wonder if] the verbiage was to clarify a reference to materials located on the firewall, cabin side.”

Steve disagreed; his response:

“The regulation doesn’t use the word ‘attached.’ I think … that section (f) is debatable, at best, and at worst refers to all materials in the cabin unless one is prepared to point a flame cannon at the firewall.

“…(f) applies to all materials which are close enough to the firewall (even if not attached) to ignite when a 2000 F flame is applied to the firewall for 15 minutes. Unfortunately, we’re missing two sets of data:

  • a plot of distance vs. temperature, and
  • the ignition temperature of the fabrics.

“Since we can’t prove that the fabrics are far enough from the firewall, we must apply (f) to all fabrics in the cabin. (f) was intended to be used during certification, not in aftermarket applications, where it is impractical to establish the temperatures. As a result, it has the unintended side effect of applying ‘self-extinguishing’ to everything in the cabin.

“I’m not at all surprised that the regulations say one thing while FAA enforces another. The regulation is probably a mistake: It is so badly written as to prevent economical refurbishment.”

Oh, well, it sounds like a religious debate anyway. We have our semi-official “answer.” Maybe we’ll let sleeping dogs lie…

Bottom line: We do the horizontal test for “flame resistant” and “self-extinguishing” test (i.e. FAR 23 Appendix F) does not apply.

End of story, right?

Bureaucracy Strikes Again!

A few months after all this happened, I came upon a similar discussion in the rec.aviation.owning newsgroup, dealing with the same issues. The author of posting, Rod Farlee, pointed out revisions to AC 43-13-1B, Paragraphs 9-60 and 9-62, which state,

Paragraph 9-60: “Only materials that are flash resistant should be used in cabin interiors.”

Which tosses yet another definition at us. “Flash-resistant,” per FAR 1, is defined as “not susceptible to burning violently when ignited.” Note however, that paragraph 9-60 continues on to say:

“The requirements related to fire protection qualities of cabin interior materials are specified in CAR 3.388, fire precautions or 14 CFR part 23, section 23.853 compartment interiors.”

This effectively defers legal requirements back to FAR 23.853.

The kicker is in AC 43-13-1B paragraph 9-62:

“Materials used in Part 23 aircraft interiors must meet the requirements of Section 23.853, and the burn test requirements called out in Part 23, Appendix F.”

YIKES! Although FAR 23.853(d) “self extinguishing” requirement explicitly applies only to “commuter category airplanes,” the FAA apparently interprets it as applying to all aircraft certified under FAR 23 by saying all FAR 23 aircraft must meet Appendix F testing! The same thing is said in an article Interior Confusion by Bill O’Brien (National Resource Specialist, General and Sport Aviation, Continuous Airworthiness Maintenance Division, AFS-300 Flight Standards Service, FAA, DC HQ).

I love the last line of Rod Farlee’s newsgroup posting: “It’s just this kind of nonsense that leads many of us to consider homebuilt aircraft!”

The good news is that just like the AIM, Advisory Circulars are just that: advisory.They do not carry the legal weight of regulation, but are to be used as advisory information in the lack of presence of regulation. Therefore, all FARs take precedence over Advisory Circulars.

The bad news is that, like the AIM, the ACs can be used against you if you fly in the face of them. Fortunately for all of us, the newsgroup author pursued this issue further with Mr. O’Brien. The response from Mr. O’Brien (PDF of complete text below) to Rod Farlee clarified the FAA’s position:

“…you are right. Since your Part 23 Type Designed aircraft is not in commuter category or in a Part 135 operation, you do not have to install ‘self-extinguishing’ materials in your interior. The material for your aircraft must be ‘flame resistant.’

“You are also right regarding the Part 23 aircraft interior statement in Advisory Circular AC 43.13-1b that require all materials to meet the Appendix F burn test. This statement is incorrect and I will personally make the correction to the AC in Change 1 which is due to go to the government printing office on November 1, 1999…”

The Bottom Line

Here’s my train of thought:

  • Effectively for this discussion, as Category 23, Part 91 operations, we are bound by 23.853(a), (b), (c), and (e); not (d) and (f).
  • We can use any interior materials we wish, as long as they are “flame-resistant.”
  • “Flame-resistant” is defined in FAR 1.
  • AC 23-2, Section 4(b) provides guidance concerning acceptable means, but not the only means, of complying with the FAR 1 definition of “flame-resistant.” These procedures mimic FAR 23 Appendix F Section (e), the “horizontal test.”

Given these facts, appears that an owner can install an interior into their Part 91 aircraft, using any materials that meet the definition of “flame resistant.” Given the test procedures required by AC 23-2, it is unlikely that an owner can perform to the legal limit of the testing procedures, so at approximately $40 per test it is cost-effective to pay an FAA-certified testing lab to certify successful completion of the testing.

Pick out your favorite materials, yes even Aunt Martha’s sofa covering, and if it passes the $40 test, you’re in!

Why do all this research? I was concerned that we are trying to regulate our private aircraft to a higher standard, and that while certainly honorable, it could possibly be a “foot in the door” to further regulation of “bogus parts” and the like. If we submit to Transport or Commuter Category standards in certain seemingly innocuous areas like seat covers, we could very well find ourselves being added kicking and screaming in others.

A Happy Ending

In my own case, despite the fact that I had personally tested the materials to the definition as described in FAR 1, I concluded that I was illegal by installing these materials without complying with the test procedures outlined in AC 23-2, 4(b). If I had obtained test results from a testing lab which uses those procedures, and these test results were acceptable, I would be in compliance with FAR 23 regulations.

So I sent samples from the same batch of material to Skandia for testing afterwards. They passed, and I got my pretty certificates that made me legal. I filed an ASRS report to describe my misinterpretation, and all is well in Mudville tonight.