GPS Replacement

This entry is part 61 of 67 in the series 13 - Electrical / Instruments

Long ago when I began this project, I decided on a glass panel. At the time, there were a number of vendors offering EFIS (Electronic Flight Information System) solutions. I decided on the Grand Rapids Technology product.  At the time, they had the HX EFIS products and an Engine Analyzer.

So I was going to have a single screen on each side of the instrument panel with the “radio stack” in the middle. This stack would have the communications/navigation radios, audio panel and GPS.

Once I started building, they released the HXr EFIS displays.  These displays support “remote” devices.  Which means the radios and audio panel are controlled through the EFIS and do not have to be located on the instrument panel. There was no IFR GPS option though so I was going to go with a Garmin 400W WAAS GPS mounted on the instrument panel.

Then a couple years ago I heard that GRT had an IFR GPS in the works. I asked them about it and was told that it was “in development” but would be ready in a year. Since I was still a couple years from needing it, I decided to go that route.

When I was time to order all the avionics equipment I placed the order.  But the GPS still wasn’t ready. So I started installing the avionics and left a spot available for the GPS.  In October of last year, I finally received the GPS!  Hooked everything up and I was good to go.

Except that I realized that I had never loaded the GPS database.  When I asked for instructions about doing that I learned that the software for the EFIS wasn’t finished. I was told November or December.

But I didn’t want to run version 1.0 software while shooting an instrument approach to minimums.

So I returned the GPS and began looking for a used Garmin 400w.

One of the glitches with this particular operation is that when I was laying everything out, I didn’t allow for a 12″ deep, panel mount GPS.  That means I have to do some rearranging.

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This is the avionics shelf behind the instrument panel. The multi-color rectangle is where I figure the panel mount GPS will be.

Obviously the audio panel is going to have to be moved. I think the cables from the hub (white cables to the left of the audio panel) will be able to be pushed down.  The pitot lines (red tubing) will have to be relocated as well.

My first plan was to remove the top shelf which currently holds the GPS, remove the trim controller and mount the audio panel just above With the audio panel out of the way, then I would just have to re-route the pitot tubes.

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So here I’ve removed the old GPS, trim controller and power stabilizer (more on that later) and relocated the audio panel above the VPX.

But there’s a problem… The audio panel cables aren’t long enough to reach to the new location.  Since I was going to need cables for the new GPS, I got in touch with Tim Hass at Approach Stack to ask if I could get an extension cable for the audio panel.  Normally, I would just replace it with a new, longer cable but the existing cable has wires running to all the headset jacks, control stick, right switch panel, etc. and I didn’t want to have to pull and reconnect all those connections.

While I was talking to Tim, I told him that I was looking for a used 400w but not having much luck and that if he knew of one to let me know. He said that he had a brand new Garmin GTN625 that he could sell me. This is basically the new, improved replacement for the discontinued 400w.  And the price was just a little more than I was finding for the old units. So I told him “sold”!

A while later, a box showed up with the new GPS, mounting hardware, cables and my new audio panel extension cables.


The audio panel cables are huge.  They are thick and they don’t bend very much. Add in the connectors and I was having trouble routing the cables so that they didn’t interfere with important stuff. So I set that problem aside and started working on getting the GPS mounted.

I decided to mount the GPS in the center of the panel directly between the two EFIS screens. But the compass was in the way.  Since I had to eliminate the power stabilizer, I needed to create a backup power source for the primary EFIS/AHRS/Magnetometer. That means I do not need a traditional whisky compass. But I do need to fill in the hole where the compass used to be. Once that was done, I had to determine how I would support the back to the GPS.

I decided to support it from above rather than build supports from the avionics shelf. So I located the center of the inside of the fuselage just aft of the canard opening. Then I used spring clamps to hold the GPS tray in place while a used structural adhesive and rivets to attach a pair of aluminum angle brackets using the tray as a guide.

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Once that cured, I made some short aluminum supports to allow the tray to sit farther forward so that it could reach the panel.

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Then I leveled the tray up and marked the panel where I would have to cut an opening for the GPS.

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After I cut the opening (which took a while because I cut it small and gradually increased the size), I had to support the tray where is met the panel. I chose to bond a couple of aluminum angle brackets to the back of the panel.

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And then just to look at it with the GPS inserted.

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Then I removed the tray, mounted it to the rear support, put the instrument panel in and attached the tray to the panel brackets.

That’s when I discovered something; There’s enough room under the tray to fit the audio panel.  By placing it there, it would be almost in the same location as before so I wouldn’t need the extension cables (and the associated routing problems).

So I pulled the GPS tray out and built a drop-down support from the GPS tray.

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Then everything goes back in.

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I rotated the wiring hub so the cables weren’t pointing straight up.

The last task is rerouting the pitot-static lines. I used the heat gun to heat up the tubes and bend them.  I’ll redo this to make it prettier later.

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Now it’s time to put everything back together and power it up.

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