Archive for January, 2009

Radio stack wiring

Sunday, January 25th, 2009

Today I started wiring the radio stack. For starters, here's a reference photo of how the pieces of the D-sub connector backshells for the GNS 430 and GTX 330 fit together; it took me a little while to figure it out from the drawing. And at the risk of biting the hand that feeds, I sure do wish Garmin would have used the same two-piece backshells here that they include with their audio panel. These one-piece jobs make you pass each wire through the backshell before you plug it into the connector, which means the backshell is floating around on the wiring harness the whole time you're trying to build it. It's a pain.

I found myself wishing for an easy way to temporarily pin together a bundle of wires, and in a rare burst of insight I realized that an ordinary clothespin with some self-adhesive weatherstrip on the jaws makes a pretty good little cushioned clamp. Total cost: zero dollars, since I had all the materials laying around the house.

I used the clothespin-clamps today when I was initially stringing wire bundles, then came back later and replaced them with still-temporary-but-not-quite-so-much twist ties. In this photo I'm starting on the wiring for the transponder:

Each radio has numerous power and ground pins, and in most cases you can run multiple wires into a single PIDG terminal on the other end. Here I've combined four 22AWG ground wires for one of the 430s into a single blue (14-16AWG) terminal.

The comm power input on the 430 runs off a 10-amp breaker and takes a doubled-up pair of 18-gauge power and ground wires… wow, that's a lot of current! Since 18AWG wire is way too big to insert in a standard D-sub pin, the 430 install kit includes a small handful of these special crimp sockets with heavy-gauge barrels. Since they stick out from the connector, you have to insulate them – no big deal for me, since I'm using my label printer to make labelled heat-shrink tubes for every one of these wires anyway. By the way, I crimped these suckers with my Daniels crimper – no positioner necessary, I just eyeballed it.

Following an afternoon of wiring, I now have the power and ground wires run for the audio panel, transponder, and one of the 430s. There is still a lot of wiring to be done for audio, data, and antenna wires, but this is enough to turn on three of the boxes. In the photo below you can see that I've left myself generous service loops in case I ever need to work on these connections later on down the road. I also have not bolted the connectors to the radio trays, since I don't want to have to worry about potentially bending one of the pins if I don't have to.

Apply power, and… it's alive! Of course I tested each component individually first, but this is the coolest picture.

According to the meter on my bench power supply, these three boxes running – without transmitting or doing anything strenuous – draw about three amps. That's a bit less than what I had in my power budget.

I would have wired the other 430 as well, but I ran out of both 18 and 22 gauge wire. Initially I thought that the 100-foot spools I bought would be more than enough, but now I see that there is way more wire in this project than I would have guessed. I bet I'll go through a quarter-mile of wire before all is said and done. Time for another visit from the UPS man…

Tunnel wiring

Sunday, January 18th, 2009

I spent a big chunk of today messing with two little adel clamps that I installed in the center tunnel just forward of the F-705 bulkhead:

What you see above is the finished result, but it took a long time to get there – mostly due to my own stupidity. At first I had the adel clamps attached to the seat ribs with screws and nuts that went through some convenient tooling holes, but then I realized that was dumb: once the floor panels were riveted in place, I wouldn't be able to get a wrench on the nuts. Duh.

So I took them back out, but somewhere during this process I managed to accidentally squash the insulation on a couple of the wires in the right-side bundle. That didn't sit too well with me, and I didn't feel like splicing in new sections of wire, so I ended up removing and replacing about twenty feet of wire. I also had to replace one of the replacement wires after I managed to cut it about three inches too short. So that was pretty frustrating.

Then I riveted in some nutplates for the adel clamps to screw into, which was relatively straightforward except for the usual difficulties that come from trying to work in tight spaces in the bottom of the airplane.

Oh, and the reason for having these two clamps in the tunnel in the first place is that they'll provide a way for wires to cross from one side of the tunnel to the other without interfering with the elevator pushrod. All will be made clear once I start wiring the strobes.

Fuel pump wiring

Wednesday, January 7th, 2009

Now that the holiday season is officially over, I have no excuse for not working on the airplane. Tonight after work I managed to find the time and energy to finish the wiring for the electric fuel pump. Here you can see how the power for the fuel pump comes off of the bus 1 fuse block, runs along the throttle support bracket to the fuel pump switch, then goes back up the subpanel and disappears into a snap bushing. The metal plate hanging on the throttle cable below is supposed to be there – ignore that.

On the reverse side of the subpanel, the wire heads upward, snakes through some adel clamps, and passes through the center subpanel rib via another snap bushing. By the way, all the adel clamps in these photos are currently way oversized for ease of running wires – when I'm finished wiring and I know the size of each wire bundle, I'll install correctly-sized clamps.

On the starboard side of the center rib, it continues through some more adel clamps, then takes a dive down through snap bushing in the avionics shelf. I know it seems like a circuitous route, but it's actually more direct than it appears.

From there the wire goes down the firewall, across the floor, and connects to the fuel pump via a Molex connector. A corresponding ground wire then goes back up the firewall to the ground block.

I powered up the bench supply, turned on the master, and briefly flipped on the fuel pump switch just to check the wiring – it ran! I turned it off immediately to keep it from sucking up any fod. I had previously blown out and capped the fuel lines, but given how dirty the floor of my airplane is right now, you can't be too careful.

One fact worth noting is that my fuel pump is connected to bus 1 – the "main bus" in Bob Nuckolls parlance – instead of bus 2, the "endurance bus". I would have liked to put it on bus 2, but since the fuel pump can draw up to 7 amps, I calculated that I just don't have the power budget. This means that if the main alternator fails and I turn off bus 1 to shed load, I won't have an electric fuel pump. However, the electric pump is just a backup to the engine-driven pump – the odds of seeing simultaneous failures of the main alternator and the mechanical fuel pump seem so remote as to not be worth worrying about. Besides, I will still have the option of leaving bus 1 powered up if I'm not worried about maximizing alternator-out run time… i.e. when I'm about to land anyway.