Oh yeah, it was a good day. Not only did it not require the use of an AK-47, I got a lot of momentum going on firewall-forward wiring. The overall schematic is still rather nebulous, but it’s based more or less on Bob Nuckolls’s Aeroelectric Connection, drawing Z11. In this scenario, there’s a main bus, an endurance bus, and a small always-hot bus. I’m toying with the idea of deleting the always-hot bus and just putting in a switch for the alternate e-bus feed because quite honestly, an always-hot bus is an excellent opportunity to drain every last molecular twitch out of an otherwise healthy battery.
Also, with the acquisition of a Garmin GNS430W, my avionics stack is now complete. EFIS, audio panel, transponder, nav/comm/gps, done. Good god, I’d love a cigarette right about now… But the upshot of this is that the the electrical picture is now complete. I have to provide power for these devices, plus the various other implements of flight, namely trim, strobes, lights, and autopilot servos. How does that work? Common wisdom is to start at the battery and work your way back. Instead, I ran loads for lighting and strobes, and today I ran starter and e-bus feed, and they’ll collide behind the panel in a Gotterdammerung of switchgear and fast-on tabs.
So the next step is, how to get the electrons from where they are to where they need to be? I had a few simple rules, gleaned from the Aeroelectric Connection and the mighty oracle of Van’s Air Force, to wit:
1. Thou shalt not run thy strobe cables alongside thy data cables.
2. Thou shalt not run thy data cables alongside thy power wires
3. Thou shalt provide ample room to service thy components after the holy top deck skin is on.
4. Thou shalt not run thy wires below tubes which carry fuel, for the drips from leaks onto that which arcs may beget the inferno.
5. Thou shalt not allow breath or light to pass between cabin and engine compartment.
6. Thou shalt not expose thy wires to sharp metal edges.
The first step was to figure out where to make holes in the firewall for pass-through of electrical cable and sensor data.
A little off from my original guesstimate, but this’ll do. A 1-inch hole accommodates the SafeAir1 firewall Passthrough, a stainless steel gizmowith a rounded outlet to let wires exit in any direction without chafing on one side, and get sealed with fireproof tape and goop on the other.
After a little cleanup, it looks OK.
My original estimate for the size of the MGL RDAC engine monitor module was way off, and MGL doesn’t actually publish the dimensions of the unit in the installation guide, which is problematic. So I scoured the newly-minted mglavionicsusers.org forum and found the answer I needed, then made this ghetto-ass mockup from the battery box packaging and a roll of blue masking tape. This made me reconsider the location for the data wire hole. In theory, there should only be one data wire going aft, that of the RDAC itself. All the engine probes and sensors should go from the engine to the RDAC.
How do you make a meal out of stainless steel? Chew slowly. Everybody gets all weird about stainless steel, and true, it’s a whore to work with, but remember, if you can scratch it, you can cut it. This is a 1″ hole saw, about three bucks from B&B hardware. The trick is to dunk the end of it in Boelube and go SLOWLY. Make your pilot hole with a #40, then move up to 1/4″, which is the size of the hole saw’s pilot bit. This one’s so dull it won’t go through warm cheese, but it serves as a good guide for the hole saw. Then, if the teeth of the hole saw are sharp at all, you should be able to grind your way through the firewall fairly easily. Keep it from heating up. If it starts to smoke, put more Boelube on it. The amazing torque of this Makita cordless drill is also helpful. It’s relentless. If you’re working above the battery, cover the battery with a sheet of plastic or something. You don’t want stainless steel chips grinding away between your battery and the firewall.
For added fireproofing, I used a bead of my leftover Fire Barrier 2000 around the FPT (firewall pass through) flange, just to seal the deal. Not that it’s very necessary; you get pretty much an airtight lock when the two halves are screwed together with the firewall between them.
And there you have it. Starter load wire, main bus feed, and e-bus feed, all going through the firewall just like they’re supposed to. What this photo doesn’t show is the firesleeve I forgot to put on the outside of the FPT before I ran the wires through it. It also doesn’t show the master contactor load wire I put in shortly after. I put the fire sleeve on and clamped it down with one of the hose clamps provided in the kit.
Since I was feeling inordinately proud of myself, I figured I throw the engine mount isolators on there. Still not sure how they go, I’ll have to check, but the red bolt protection nipples are a nice touch.
Up in the corner next to the VA-168 manifold, you can see the second FPT. This will carry a data cable from the RDAC back to the EFIS, with room for future additions should I wish to take my life in my hands with some sort of electronic ignition.
And here I am, sitting in the focus of the Dynafocal brainprobe. Maybe I can infuse it with some of my own sentience, such as it is.
I just got off the phone with my cousin, Navy SEAL and former SDV electronics tech, who assures me, despite my misgivings, that the switchgear on the panel and the power routing is not a problem. Nor is anything else. Where it gets tricky is the audio wiring, where impedances must be matched and other arcane spells must be cast. I’m going to bring him out here from Yuma for the hard stuff, I think.