Click here to upgrade to a newer version of Internet Explorer or Microsoft Edge.
By Tony Bingelis (originally published in Experimenter, March 1995)
The firewall is an important component in your aircraft. Essentially, it is a fire-resistant bulkhead that separates the engine compartment from the cockpit area. This special bulkhead must be constructed so that no hazardous quantity of liquid, gas or flame can pass through it. In theory, at least, should a fire occur in the engine compartment, the firewall would protect the aircraft’s occupants from the flames long enough for an emergency landing to be effected.
In addition to providing protection, the firewall has other useful functions. It provides a convenient surface on which to mount accessories and other essential units that are normally located in the engine compartment. And, since sooner or later it may be necessary to remove the engine or to replace other parts, it makes a handy junction for the disconnection and removal of engine control linkages, fuel lines, and various electrical and ignition wires.
Must I have a Firewall?
Generally speaking, yes. From the foregoing introduction the reasons are obvious why certificated aircraft are required to have firewalls, so why should homebuilt aircraft be excepted?
Although experimental aircraft need not comply with the same stringent federal regulations expressly established for production-line aircraft, most FAA inspectors will insist that a suitable firewall be installed in the aircraft they inspect. And, why not? Compliance with the portion of the Federal Aviation Regulations pertaining to firewalls is relatively simple and highly recommended.
Should you want to read the exact language, refer to paragraph 23.1191, Firewalls (FAR Part 23, Airworthiness Standards: Normal, Utility and Acrobatic). For your convenience, however, I have paraphrased the regulation.
1. Any engine operated in flight must be isolated from the rest of the airplane by a firewall or equivalent means.
2. The firewall must be constructed so that no hazardous quantity of liquid, gas or flame can pass from the engine compartment to other parts of the airplane.
3. Each opening in the firewall must be sealed with close-fitting, fireproof grommets, bushings, or firewall fittings.
4. Fire resistant seals may be used on firewall for small engines (1,000 cubic inch displacement or less.)
5. A firewall must be fireproof and it must be protected against corrosion.
Acceptable Firewall Materials
The following materials may be used in firewalls WITHOUT BEING TESTED;
• Stainless steel sheet - .015 inch thick (.284 lb./cu. in.)
• Mild steel sheet (coated with aluminum or otherwise corrosion protected) - .018 inch thick
• Terne plate - .018 inch thick
• Steel or copper base alloy fittings
• Other materials may be used if they can pass the fireproof test.
The Fireproof Test
If you want to use some other material for your firewall, you may have to prove its ability to meet certain criteria for compliance as a fireproof material. Simply stated, the material must be given a flame test, in which a piece of it, approximately 10 inches in diameter, is subjected to a five-inch diameter flame. Firewall materials and fittings must resist flame penetration for at least 15 minutes.
That’s all there is to that. Compliance is relatively easy and no homebuilder wanting to try a new firewall treatment should slight the requirement.
Choosing Your Firewall Material
Galvanized sheet is the most common, most economical of the acceptable materials used in small general aviation aircraft. It gets its corrosion resistance from a zinc or hot-dip galvanizing coating obtained by immersing the sheet steel in a mixture of molten zinc at a temperature of approximately 865 degrees F. until the base metal temperature of the immersed steel sheet reaches that of the hot-dip bath.
Local sources for galvanized sheet include metal shops and air conditioning duct fabricators.
Don’t be surprised if the folks operating these metal shops refer to the thickness of their galvanized sheets in term of gauges rather than inch sizes such as .018 inch or .015 inch. If that be the case, 26 gauge is pretty close to what you want (about .018 inch thick). You could assure yourself that you are obtaining the correct thickness if you bring your own micrometer to check the various sheets. There are a number of so-called "standard gauges" and some variations in the actual thickness for the same gauge number may crop up.
No useful purpose would be served by using a heavier firewall sheet than that required to meet acceptable minimums. Never lose sight of the fact that the addition of unnecessary weight, no matter how slight, degrades the aircraft’s performance. The firewall metal for a two-seat aircraft will probably weigh as much as 4-1/2 pounds anyway, so why make it seven?
In order of preference for firewall material, stainless steel gets top billing in both quality and appearance. It also costs two to four times more than galvanized sheet. However, as the "cadillac" of firewall materials, it is the first choice among builders who want a "show airplane." Builders with economy in mind will ordinarily use galvanized sheet for their firewall and may console themselves by claiming that galvanized sheet is lighter than stainless. I doubt that the weight difference could be measured on the crude scales most of us have around our shops, but there is that difference.
Another fireproof material, often used in the past and sometimes still found on older aircraft, is terne plate. It is not used much today because it isn’t as commonly available as galvanized.
Monel and Inconel, although acceptable as firewall materials, are difficult to obtain. As far as homebuilders are concerned, these may be regarded as rare materials.
Concern over weight leads some builders to consider the use of aluminum. However, using aluminum sheet by itself for firewall material is not acceptable to most FAA inspectors. The official reason is, aluminum will not pass the flame test because its melting point is just slightly above 1,200 degrees F. In some applications . . . for very slow, light aircraft (ultralights) with small engines, it might be considered sufficient. However, before you decide to use it in your own aircraft, check it out with your local FAA inspector. Although the standards established by regulation for production-line aircraft do not necessarily apply to experimental aircraft, deviation from accepted practices may result in your having to get into a proof testing demonstration. Is it worth the trouble?
Recently, a new space-age ceramic paper called FIBERFRAX has been developed and is finding its way into homebuilding circles. It can withstand temperatures of approximately 2,300 degrees F. and may be considered as effective a fire barrier as stainless steel.
It has been used primarily in VariEze, Long-EZ and other light composite aircraft because it saves two to three pounds over the stainless or galvanized sheet normally installed.
Although FIBERFRAX is flexible and considerably lighter than steel, it is a ceramic material and is thus fragile. This may introduce in-service problems unless the material is somehow protected from abrasion and abuse.
The technique for using it seems to have been standardized as follows: The layer of asbestos cloth ordinarily used under the stainless steel firewall sheet in wood or composite aircraft may be omitted. Instead, a layer of FIBERFRAX is attached to the firewall and overlaid with a .016 inch sheet of 2024 T3 aluminum to protect it. Although aluminum is deficient when used alone as a firewall material, its use with FIBERFRAX is acceptable because it is really the FIBERFRAX that provides the fireproof barrier.
Firewalls in Welded Steel Frames
Before installing a firewall on a welded fuselage, you should seal the open ends of the longerons. This helps keep out rust-inducing moisture which can be very destructive over the years. Weld small metal washers or discs of .032 inch steel across each of the open ends.
Your firewall will undoubtedly consist of a single sheet of metal with nothing behind it for support except perhaps a diagonal tube welded across the front of the tubular frame of the fuselage. A large, flat expanse of thin metal like the firewall is always likely to vibrate (called the "oil-canning effect") noisily in flight. Firewall metal in this type of installation is, therefore, often made more rigid by embossing random beads or ridges across the sheet. This inhibits vibration by stiffening the metal and it virtually eliminates oil-canning noises.
At aircraft factories, technicians form the beads in hydraulic presses, but we amateur builders must usually hammer ours by hand using a wood dowel and a back-up block with recessed groove in it.
The firewall can be held in place initially by the four bolts securing the engine mount to the fuselage. Sometimes small tabs are welded to the firewall bulkhead and sheet metal screws, rivets or bolts are added to secure the firewall. Do not go overboard in this regard, however, because the firewall will take on greater security and rigidity as construction progresses. In addition, mounting the fuselage side skins will completely stabilize and immobilize the firewall.
Since there is no back-up structure behind the firewall metal in this type of installation, it will be necessary for you to reinforce parts of it before mounting heavy objects such as a battery, header tank, oil cooler or oil tank. If you don’t do this, you can expect to eventually find localized cracks that result from vibration.
In All-Metal Fuselages
The firewall sheet in an all-metal aluminum fuselage is ordinarily riveted to what comprises the engine compartment bulkhead. In effect, it becomes an integral part of the fuselage.
In this type of structure, as in welded-tube type, the firewall metal has the same kind of oil-canning tendency. Heavy objects require localized reinforcement.
In Wood and Composite Aircraft
An all-wood or composite aircraft is ordinarily constructed with a built-in engine compartment bulkhead to which protective firewall metal must be attached. The bulkhead, although structural in nature, is not fireproof because it is commonly made of wood overlaid with plywood.
Before a firewall is installed in a wood or composite aircraft, I recommend that the firewall bulkhead be overlaid with a protective heat insulating layer of asbestos or FIBERFRAX paper.
Caution: Asbestos has developed a bad reputation and, according to federally funded studies, constitutes a health hazard. Because of its bad press, asbestos sheet is very difficult to obtain. However, you can still find it in some plumbing shops and hardware stores. As a precaution, avoid creating asbestos dust when cutting and handling the stuff. And, always wash your hands afterword. You may also want to avoid a situation where air passing over the asbestos might enter the cockpit.
If you choose to use FIBERFRAX in your wood or composite aircraft, don’t waste time by attempting to bed down the material with epoxy as you would fiberglass. At best, wetting it all out would only add unnecessary weight. Instead, affix the FIBERFRAX sheet with a bead of silicone rubber laid around the periphery of the bulkhead. That should be sufficient to hold it in position until the firewall metal is installed with a few sheet metal screws. Use only enough screws to temporarily hold the firewall in place. Each firewall accessory and unit added will contribute to its security. You can always add fasteners later wherever needed to complete the job.
Being supported by a solid bulkhead, the firewall metal will not vibrate and oil-can as it would in an all-metal or welded steel tube installation so no particular pre-installation forming of reinforcing ridges or beads need be made.