10-23 -> 26: Revisiting Tailcone Wiring

When i originally planned and installed the wiring in the aft fuselage, I made the best assumptions on what wires i would need present - was a bit like looking into a crystal ball. I based most of my decision making on how the Van's wiring harness was made up, from the Van's Harness Schematics.

With some more research, the main difference between the Dynon servos and the Garmin servos is that the electric elevator trim is independent of the Dynon servos and controlled directly by the pilot, whereas in the Garmin servos, the pilot sends trim signals to the Garmin servo, which then sends the signal to control the elevator trim. 

The Van's harness  was designed to work with both Garmin and Dynon autopilot systems and as such was a bit of a hybrid design. The power wires for the elevator trim are run back to the tail through 2 bulkhead connectors. In the event the builder chose to use the Garmin servos, the van's harness had 2 wires which were routed from the trim motor to the right hand side of the fuselage to the right hand bulkhead connector on the forward side. They then either went from the back of this connector into the Garmin servos connector, of if using Dynon were jumpered to another 2 pins to return to the cockpit. They were just 2 plain wires. 

Thinking that this was a clunky way to do things, I found some special trim wire on the Steinair site. This was 5 wire jacketed (not shielded) and would therefore give me the 2 power wires and the 3 signal wires for the trim system. So I ran this to the aft tail for connection to the trim servo. I left this bundled up on the right hand side of the fuselage. If i decided to go with Garmin servos, i would cut this wire to to allow the power to be routed to the Garmin Servos and splice the signal wires to head to the cockpit. 

Fast forward a couple of years! and I have decided to go with Garmin exclusively. Looking and how the Garmin GSA28 servos manage trim, i discovered that they use PWM (pulse width modulation) - meaning the power wires would need to be shielded. The G3X manual shows the shield connected to airframe ground at the trim servo, and to the connector backshell at the Garmin servo. 

So i needed to remove the 5 wire jacketed cable i had installed, and i decided to replace it with a 2 wire, 22 AWG shielded wire for the power to the trim motor (and which would run from the trim motor to the Garmin servo), and a 3 wire, 22 AWG wire for the trim position (which would run straight to the panel). 

Below is an excerpt from my wiring diagram software showing how this is wired (the pink highlighted wire "SERVO ETRIM" provides power to the trim servo from the Garmin GSA28 servo). The lower wire "ETRIM POS" runs from the trim motor to the panel. 

So, this meant i needed to remove the old wiring and replace it. Originally, all the wiring ran down the left side of the tailcone. Since the trim power needed to end up at the Garmin servo, i ran the 2 wires for the trim down the right hand side of the tailcone, leaving the tail light and ELT wiring on the left side. I was then able to add a hole and a grommet to the aft deck, and terminate the trim wires in a DTM connector, which will attach to the DTM connector already on the horizontal stab (and which will mount on the underside of the vertical stab). 

I ran the trim wires down the right hand side of the tailcone in a similar fashion to how the wires on the left hand side were run.

The left hand side still has the tail lights, ELT and static.

The un-terminated wires were labelled to they didn't get mixed up. 

The 2 wires are pinned. Note the shield of the trim power wires (green wire) is terminated in a ring terminal, which will go to ground locally, once mounted to the bottom of the vertical stab.