13.2.0 Instrument Panel

I’ve also been working on the mechanical design for both the overhead and instrument panels. Here’s a CAD drawing of the overhead panel switches.

Here's the layout that everyone is interested in - the main instrument panel. This is going to be a Garmin G3X system panel that includes both the 10.5” and 7” screens. The equipment in the middle includes the intercom panel, COM 2 radio (COM 1 is a blind radio controlled by the G3X panel), IFR navigator and autopilot controller. The main system switches and annunciators are located between the center radio stack and the 7” panel.

There is a lot of wiring that needs to be completed for a project like this, being an all-electric system! The heart of the audio portion of the avionics is the intercom / audio panel. In my case, I’m using Garmin’s GMA 245 Audio Panel. Because there are so many connections, I decided to break it down into two separate prints (VELO-AVNCS-01 and VELO-AVNCS-02) based on the two HUGE connectors on the 245. The first interfaces the two COM radios, the GTR 20 which is a blind radio controlled via the main panel, and the GTR 200B which is basically the same radio that is panel mounted and has a full control panel on it. I decided to install at least one of the radios this way in case my main panel fails. Manual control is always a good thing in an emergency! Of course, the G3X display panels can also control this radio if I so desire.

The second drawing completes the wiring for the GMA 245 which includes power for the device as well as the rest of the headset connections. I’m also showing a speaker although I haven’t yet figured out where to mount it!!!

The drawing below (VELO-AVNCS-03) shows the CAN Bus (Controller Area Network) which Garmin uses heavily for communication between devices. The CAN Bus is also used in automobiles and other transportation devices. It was originally invented by Bosch. While wiring the CAN Bus is a bit tricky due to length limitations, etc., it really does simplify wiring between devices! The print below also includes power to some of the other LRU’s (Line Replaceable Units - as Garmin calls them).

This print also shows two GSU-25’s. These are the Air Data Sensor Units (AHRS). Only one is required; however, many people include a second as a backup. I may only install one to begin with but the print shows two of them (as well as two magentometers, the GSU-11’s). I’ll include wiring for both.

VELO-AVNCS-04 shows two systems. The first system (on the right side of the print) includes all of the engine monitoring probes for EGT and CHT. That’s pretty straight forward. The LRUs on the left side of the print are the two autopilot control servos - one for pitch and one for roll. Since this airplane does not have a standard rudder there is no need for a yaw servo.

There’s more to engine monitoring that just EGT (Exhaust Gas Temperature) and CHT (Cylinder Head Temperature) though, so once again I broke the Garmin GEA-24 EIS Interface LRU into two separate prints. This 2nd print (VELO-AVNCS-05) shows everything else. It includes probes for fuel pressure and flow, engine RPM, manifold pressure, oil pressure and temperature, and finally, fuel tank levels. For whatever reason, this unit also includes outputs for the “Master Warning” and “Master Caution” annunciators.

The last avionics print (VELO-AVNCS-06) is for the GNS 375. This unit is actually a certified unit but is required for IFR flight since it includes a WAAS-enabled GPS navigation system. It also includes a transponder with ADS-B In/Out. The GNS 375 can do a LOT of stuff. I’ll be using many of its functions, but some functions are included for interfacing to older and non-Garmin equipment. It’s a beast with two huge (62 pin and 44 pin) connectors! In order to interface it to the G3X system, another LRU called the GAD 29 is required. The two devices talk via a standard aviation ARINC bus. The GNX 375 interfaces to a lot of other equipment over standard RS-232 serial connections, including both display screens (the PFD and MFD) as well as the GSU 25 AHRS and the audio panel.

The homemade switch and annunciator panel, as well as the overhead panel, were designed some time ago. See the top of the page for the overhead panel. See the photo below for the CAD layout drawing for the switch and annunciator panel.

Depth of the panel is critical. I also needed to save room for the tons of wires coming off of the switches and annunciators that are to be installed in the panel. I also need terminal strips for some of the wiring, so I created a sub-panel for those. That’s the terminal strip panel on the right. The panel on the left is what the pilot sees. The section in the middle shows the depth of the annunciators, switches and potentiometers which will be used for controlling the brightness of various radio and avionics displays.

The actual panel is made up of two parts - one is the metal back panel and the other is the fancy plastic front panel. We’re very lucky here at the Payson airport to have a guy with a rather fancy CNC machine. After supplying him with the CAD drawings, he imported them into his CNC software and voíla, I had all of the panels cut and ready to go. How cool is that? See the photo below for what the pre-extracted pieces look like coming out of the CNC.

IMG 2398

CNC'd panels for the instrument panel and overhead panel.

Those little tabs you see need to be cut off. They have to stay in place to hold the panels in place while the rest of the cut is completed. They are thin so it’s easy to cut them with a small hacksaw. I then ran the edges against my belt sander to smooth any remaining tabs pieces.

Here’s a photo showing the front panel after it was removed from the aluminum stock piece.

IMG 2399

This is the back panel that will hold the switches and annunciators.

Eventually an identical-looking plastic cover piece gets placed over the metal backing plate. My son works at a place back in Illinois that has a high-precision laser cutter. The lettering gets backlit (kind of like in your car) so it’ll be interesting to see how this all works out!

In the photo below, I’ve flipped the panel to install some fabric to help diffuse the light from the LEDs used to backlight the lettering. Testing showed it to be quite harsh without the fabric, but one strip of peel ply, rubber cemented onto the panel, solved that problem!

IMG 2406

I'm using some peel ply to help diffuse the LED backlight intensity.

Here I’m test fitting the entire assembly. The back panels holds a bunch of terminal strips used for hooking everything together. The far back piece holds the 40-pin connector that interfaces this panel with everything else. I decided to make this a modular assembly so that it can fairly easily be removed for maintenance from the main instrument panel. Scroll to the top of this page for a CAD view of what the panel will look like.

IMG 2407

I'm test fitting the front and back panels, along with the far back panel that holds the main connector.

I still have the wiring to do, but here is the assembly with all of the annunciator lamp assemblies and switches installed. There are also three potentiometers at the bottom that interface to the Garmin electronics for adjusting the brightness of various systems. While the annunciators are just white here, they each have appropriately colored LEDs which light up if a circuit is active or if an alert occurs. The first two rows of switches/lights are tied together. In other words, when the switch below the “Main Battery” annunciator is turned on, the lamp lights up. The top two rows of lamps are pure annunciators, lighting up should a problem occur. The pushbutton at the upper left provides a “Lamp Test” function which lights most, but not all, of the annunciators. (Because of how some are wired to the associated electronics, it would be difficult to provide a “lamp test” circuit.)

All of the switches and annunciators are now installed in the panel.

The photo below shows the overhead switch panel wired and being tested. I’ve also upholstered that panel. (I had to do that before wiring.) This panel is built similarly to the main instrument panel above. It also has LED backlighting (lit up in the photo below!). I’m really pleased with how that backlighting came out! In the case of this panel, when the circuit is activated, the colored LED ring lights up. The color is dertermined by the function. All of the overhead switches are non critical, for things like lighting

IMG 2412

Testing the switches and LED backlight to make sure everything is working properly.

For the procedure I used to upholster this panel, see Section 15.2.1.

ui© John Trautschold 2018