Building, testing and operating an experimental aircraft

Firewall Preparation and Fire Protection

12 Nov 2014

I did a considerable amount of research on best practices for light aircraft fire protection and carbon monoxide (CO) mitigation before we began installing the firewall accessories. The firewall penetration methods recommended in the RV-8 plans, and several of the firewall components shipped with the FWF kit would be, in my opinion, woefully inadequate for actually protecting the aircraft occupants from a fuel-fed engine compartment fire.

Firewall Test, 2000°F Firewall Test, 2000°F

Fire Protection Guidelines

Does the firewall test in the photo above look a bit extreme? It is. Consider that a fuel-fed engine compartment fire may burn at nearly 2000°F and that it may take as long as 15 minutes for a pilot to safely land a burning aircraft. You definitely don’t want any smoke or flames impinging upon the cabin during that time.

AC 20-135 AC 20-135

For a Part 23 light aircraft, the basics for demonstrating compliance with the fire protection requirements defined in the FARs can be found in Advisory Circular 20-135. This document outlines the accepted definitions of “fireproof” and “fire-resistant,” discusses fire protection principles and objectives, and suggests methods of fire protection testing.

Although experimental category aircraft are not technically bound by Part 23 safety standards, I believe the guidelines in AC 20-135 are a good benchmark to use when considering the suitability of firewall components and firewall penetration methods.

Cabin Heat Bypass Box

Our first order of business was to install the Van’s TG-10 aluminum cabin heat bypass box and its associated control cable. As with any new component, I like to do some web searches to see how others have installed things, and to get ideas.

As luck would have it, this inadvertently led me to stumble upon several great VAF threads regarding fire safety, and I immediately began to question the logic of using an aluminum fixture to seal off the largest hole in the firewall. The question was this: would Van’s aluminum heat box stand up to the Part 23 spec of 2000°F for 15 minutes?

Van's Heat Box after 15 SECONDS at 2000°F Van’s Heat Box after 15 SECONDS at 2000°F

This question brought me to Plane Innovations, LLC and their line of stainless steel heater bypass valves and firewall penetration kits. Plane Innovations has some great photos on their website of fire tests they conducted on the VG-10 aluminum heat box (pictured above) which show the aluminum turning to soup after just a few seconds of exposure to a simulated fuel-fed fire. In contrast, their stainless steel heater bypass glows red hot under similar conditions, but stands up to the heat for the full 15 minutes.

Stainless Steel Heater Bypass Stainless Steel Heater Bypass

Needless to say, I bought one. The Plane Innovations HBV-01 Heater Bypass Valve has the control arm on the engine side of the firewall, which simplified the installation of our Bowden cable and allowed us to mount the pull tab on the left-hand gear tower. This kept cable routing to a minimum.

HBV-01 Heat Box Installed HBV-01 Heat Box Installed

We had to create a custom stainless steel bracket to properly support the cable, and we decided to locate it where the brake fluid reservoir would normally go. Since we’re using the Grove pedal-mounted 067-050 reservoirs, we wouldn’t be needing to mount the Van’s firewall brake reservoir in this spot. All of the rivets and all of the seams around the bracket and the heat box were sealed with Biotherm firewall sealant.

Firewall Penetrations and Sealant

Next we turned our attention to the firewall penetration philosophy we wanted to adopt. The experimental community’s “industry standard” firewall penetration shields are described by Tony Bingelis in the EAA publication On Engines. These consist of a rubber grommet to cushion the control cable or wiring, covered by an asbestos rubber washer, and finally a stainless steel shield to deflect the bulk of the heat (pictured below).

Rubber Grommet with Stainless Cover Rubber Grommet with Stainless Cover

On paper, this seems like a reasonable congfiguration, and indeed it seems to have been used by many builders over the years. Once again, Plane Innovations put this time-tested design to the 2000°F test.

Rubber Grommets, Cabin Side, After 20 Seconds Rubber Grommets, Cabin Side, After 20 Seconds

The results are striking. The photo above depicts the cabin side of the firewall, after roughly 20 seconds of withstanding a simulated fuel-fed fire. Twenty seconds, and you’ve got flames tickling your toes. Clearly, this design doesn’t meet the spec.

(I hesitate even to mention the firewall pass-through method described in Van’s RV-8 plans, which would be even less effective than the Bingelis design in an actual fire.)

Lest the reader think that I’m a sales rep for Plane Innovations, LLC, I offer this excellent VAF thread on Firewall Sealant and Fire Safety, which features very similar firewall tests of several popular penetration methods and sealants. (Scroll down to post #38 for the good stuff.) One look at Mr. Horton’s photos and you’ll be sold on the method we chose.

Plane Innovations 1/2" Penetration Kit Plane Innovations 1/2” Penetration Kit

We ended up getting one of the Plane Innovations firewall penetration kits, which consist of a small stainless steel tube and flange, a short section of fire sleeve, and a small tube of Biotherm firewall sealant.

The idea is that the steel tube supports your wires or cables, while also being filled with fireproof sealant. The firesleeve wraps around the entire assembly, and acts as a sacrificial fire barrier to protect the materials inside. Heat from the fire causes the intumescent sealant inside the tube to char and expand, effectively plugging the firewall hole and keeping the heat, flames, and noxious fumes from entering the cabin.

Several other vendors make very similar pass-through kits that are also quite nice, and it would be trivial to make these yourself (assuming you can TIG weld stainless).

Charred, But Intact, Firesleeve Charred, But Intact, Firesleeve

Here’s an example of the charred remains of a Plane Innovations penetration kit. As you can see, the firesleeve is nearly destroyed, but the path to the cabin is protected, and that’s the whole point. You just need that 15 minutes to get the bird on the ground.

Doubletee 4200 Eyeballs Doubletee 4200 Eyeballs

Finally, for the control cable penetration points we bought three of the Doubletee Products 4200 spherical “eyeball” grommets. These all-steel grommets meet the temperature specs, and are relatively easy to install. They grip the control cables tightly (we got the .312” and didn’t enlarge the holes), and they swivel in any direction, allowing you to direct the cables as necessary.

You’ll notice we installed the eyeballs with the cavity facing the engine side of the firewall. We intend to fill these cavities with 3M Fire Barrier 2000 once the firewall setup is complete. All of the seams, cracks, edges and fasteners in the firewall will also get a coating of the 3M product or the Biotherm when all’s said and done.

Cabin Heat Control Sub-Panel

Several months ago, when I started laying out the instrument panel, I discovered the online CNC panel engraving service Front Panel Express, LLC. You simply download the free Front Panel Designer CAD software, layout your panel, and then order it with a few clicks. These guys turn out a very high-quality product, and they can do anything from simple sub-panels like I’ve pictured below, all the way up to custom engraving of a complete RV instrument panel.

Front Panel Designer Software Front Panel Designer Software

I mocked up a Cabin Heat subpanel, using features similar to others I plan to have in the cockpit, printed it out for testing in the airplane, iterated a few times to get the sizing I wanted, and ordered it in engraved aluminum with black powder coating.

Cabin Heat Control Panel Installed Cabin Heat Control Panel Installed

The finished product looks great and is installed with nutplates so it’s removable for paint touch-ups or replacement if necessary. Definitely not a cheap solution, but way better than using a simple label printer, in my opinion.

When complete, I’m hoping the cockpit layout will call to mind a military fighter aircraft with its solid components and logical, standardized, backlit switch panels (minus the backlight of course – you can’t have everything! -sigh-).

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