Misc Work

Time: 2.25 hrs

Did some miscellaneous tasks today:

  • Finished wiring the GMU11 CAN wiring at the pitch servo
  • Retorqued the gear bolts to 18ft lbs (24Nm)
  • Ran wire for Stratus USB charger and wired connector

Originally I had spoken to the tech at TAF about torquing the 4 main gear bolts. He had told me that they were Grade 8.8 bolts so torque them to the specified value (which was around 50Nm for a 10mm bolt). The bolts I had were actually Grade 12.9 with an even higher torque required. So I torqued them to the standard Grade 12.9 torque value and went on my way. A little while later I had noticed the Peter C. had mounted his main gear and stated that he torqued his gear bolts to 24Nm. That seemed very low to me so I emailed him and he had said that TAF told him to use this value. Well he has since verified with multiple sources at TAF SA and they confirmed that the bolts are torqued to 25Nm. With that I just backed off on the torquing of my bolts to 24Nm (18 ft lbs). Thanks Peter for verifying and letting me know.


Tied the CAN bus wiring into the pitch servo so it could continue on to the GMU. The GMU is the end of the CAN bus now so I also ha dot remove a jumper on the DB15 connector for the pitch servo.

I’m going to install the Stratus USB charger in the back of the center console. The kids are always using the USB charger when we drive on trips so I figured it would be convenient to have one in the back for them. It can also function as another USB port for me or the co-pilot as well. I will also bring the USB port on the GMA245R up onto the instrument panel. That will be the main one that I use for charging my iPad or phone.


ELT Install

Time: 4 hrs

Mounted and wired the ELT today. I chose to use an Artex 345 instead of the ACK ELT that TAF uses. The Artex seemed to be a bit easier to wire up and was a little smaller (though heavier) then the ACK. Also the Artex does’t need a battery in the remote or buzzer so thats a few less batteries to periodically replace.

I ended up mounting the ELT to the side of one of the rear fuselage ribs and was able to reuse two of the 4mm holes already in the rib. I did need to drill two new holes in the ELT mount itself since none of the assortment of holes were totally clear of the lightening holes in the rib. The wiring was straight forward. The connection only requires two wires to the remote (panel mounted) switch and a single line for GPS location information.

I was able to reuse the two outermost holes (of the three) already in the rib and then just drill two 4mm holes towards the front of the rib,

The DB15 connector all wired and heat sprinkled.the gRey wire is a jumper to activate the automatic triggering of the ELT if it senses a crash.

ELT mounted with four #8-32 Stainless Steel screws provided with the ELT and torqued to 12in-lbs. The only issue with this placement is that the rib isn’t completely straight in line with the airplane plane of travel. It’s about 4-5º off from it. The ELT manual says you need to be with 10º so it should be fine. If it’s a problem I can always shim the rear mount to bring it in line. I’m happy I didn’t have to add any holes to the fuselage skin. The manual also says that mounting on the skin isn’t recommended so another reason to not mount it that way. I did try making some cross pieces out of angle aluminum but I wasn’t able to make the bend for the attachment flange (the aluminum broke). It would have probably work if I didn’t use extruded aluminum angle, but I think mounting to the rib will work well.

The manual calls for drip loops for any wiring connecting to the ELT so there they are. I still need a few fast-on connectors to connect the buzzer. Rather then wire the buzzer directly into the connector I added a black and red 22AWG extension wires and soldered that into the connector. I’ll add fasten connectors to the extension wires and the original wires from the buzzer so it can be easily removed. I’ll clean up the wiring a bit and add a wire tie mount for the buzzer wiring in a bit.




Pondering My Electrical System

Let me start with this. I’m not an electrical engineer so if anyone can bring some clarity to this I’m glad to hear it. As I get closer to actually having to wiring the charge system using both the external and internal alternators I’m trying to pin down exactly what I need to implement as far as the charge circuit and interconnection of the internal and external alternators. As I look around for ideas and wiring examples I’m facing some conflicting information. My original drawings (available in the Resources menu) were based on what TAF provided in their electrical system manual and what Rotax shows in their 914 installation manual. I’ve found conflicting (well different) wiring diagrams from Rotax regarding the wiring of the internal alternator. I also was thinking that passing the field trigger through the VPX for the internal alternator might be a bad idea. If the VPX dies then the engine should keep running, but restarts would not be possible since the VPX would be controlling the relay that ties the Engine Circuit to the battery. I’m also unclear why some Rotax manuals and TAF manual show a relay being used to connect the “B” terminal of the integrated alternator to the battery and also a relay to control the field terminal of the external alternator to the battery (to active the alternator). I believe the field connect of the external alternator can just be done with a normal switch between the IG (field terminal) and aircraft power since it’s fairly low current. The “B” terminal of the integrated alternator could just be connected directly to the battery (or via a diode) and then the “C” (field terminal) could be switched to enable or disable the alternator/generator. So why use a relay? If I don’t need to install a relay then I’d rather not since that’s one less part to break.

Also all the diagrams I’ve seen show no concern that the external and internal alternators all tie to the main aircraft power bus. From some research I found this is a big No No. You want to keep the alternators completely independent if both are on at the same time. This is different then a primary and backup alternator situation. The internal alternator should only be used to drive the main fuel pump, the turbo servo and other engine components. While the external alternator is used to drive all the avionics and the aux fuel pump. Of course the catch is that the battery needs to connect to both the Aircraft Power Bus and the Engine Bus so I would think you’d need to use a diode to connect the battery and internal alternator so the alternator doesn’t feed into the main aircraft power bus, but no diagrams show this.

NOTE (12/10/2017): Found this little nugget in the Rotax 914 Installation Manual which explains a bit about the reason to use a relay to switch the connection between the internal alternator and the battery. From the Rotax 914 Installation Manual:

“Never sever connection between terminal C and +B of regulator (e.g. by removal of a fuse) while the engine is running. Overvoltage and regulator damage can occur. During engine stop break circuit between battery and terminal C to avoid discharge of battery!”


This diagram (from Rotax) makes sense to me. When the master solenoid (19) is closed then the internal alternator (5,6) is enabled via terminal “C” connecting to the battery. Plus the engine components have battery power for starting. The external alternator (10,11,12) is enabled by the double pole master switch (16) which directly connects the battery to the IG (field) terminal. The only thing missing is isolation of the integrated alternator form the main aircraft power bus.

And here’s a similar digram from Rotax. The difference is the use of a relay (58) that is closed when the master solenoid (38,39) is closed and connects the B and Field terminals of the internal alternator to the battery. I also just notice that the main fuel pump was added to that part of the circuit as well. But still that doesn’t explain why you use a relay instead of just allowing the master solenoid to supply battery power to these devices directly. I always thought that the reason to use a relay is to switch a high current load with a load current load. In this case you’re not really doing that and it’s not adding any isolation between the integrated alternator and the rest of the aircraft power.



TOGA Button… Work in Progress

I thought I’d share this idea to see if I can get some feedback from other builders. I found a  DPDT (couldn’t find a DPST) switch small enough to fit nicely in the end of the throttle handle. I just had to remove some of the threads on the switch and will probably use some RTV to hold it in place. I think this would be a very convenient place for the Go Around button so I’m hoping to get this to work. The other idea was to use a button on the control stick, but since a DPST switch is needed (As stated by Garmin) and all the switches in the stick grip are SPST, then I’d have to use a relay and I’d like to avoid that if possible. So with the switch able to fit in the throttle handle I just need to figure out a nice way to route the wiring. I need to run 3 #22 wires so it’s not too large of a bundle. I know I can drill out the center of the throttle handle a bit and get the wire to the throttle arm pretty easy. The hard part is running the wiring down the throttle arm and into the center console. I’m thinking maybe down the back of the arm using a snap on plastic U shaped cover or maybe through a small aluminum tube that’s epoxied onto the arm. Anyways if anyone has any ideas I’d be glad to hear.

The switch fits nice into the bolt hole in the throttle handle and is short enough to allow for the wires to get soldered as well. I purchased the switch from DigiKey, it’s an NKK LP0125CCKW01F (Digikey #360-2524-ND). I believe there’s also red and grey. Now I just need to figure out how to run the 3 #22 wires to it. Fortunately I have an extra throttle handle and arm for hacking…. I mean testing.

More Connector Wiring

Time 2.5 hrs

Well the heat shrink labels came today so I was able to finish the connectors that I had stated to wire up the day before. 

Control stick wires labeled and pinned. 

Control stick pins in the connector. 

And finally the TE mini CPC connector mounted under the control stick. The gribs will have the female side of the connector on the pigtail. If I ever have to remove a control stick then I just de-pin the connector and pull the pigtail wires back through the control stick. 

I also wired up the connectors and VOR coax on the tail cone. 

Connector Wiring

Time: 2 hrs

While the epoxy was drying on the vertical stabilizer I started wiring up a few connectors. I decided to use TE mini CPC connectors. The connectors are sealed and lock together. They come in a few pin arrangements but so far I’m just using some 4 pin ones in the VS, 5 pin for the pitch trim and flaps, and 9 pin for the control sticks. I bought these from online-components.com. Their site looks pretty minimal and I was a bit hesitant to order from them, but the items arrived on time and it was exactly what I ordered. Their prices were also much better then Digikey or Mouser (with the exception of the pins… Mouser had the best price for those). This stuff adds up quick so it’s always good to shop around. I was hoping I could finish up most of these connectors, but I ran out of heat shrink labels so I need to wait on putting on the connectors until I can label the wires. 

Some pins done on the tail cone. These are for the tail strobe.

I decided to put in a connector between the VS and the rudder so I can take the rudder off if ever needed. 

All connectors done for the vertical stabilizer/rudder. I put in a female coax for the VOR. 

Put pins on the control stick wires. I’ll be using a 9 pin panel mount connector that will mount under the control stick. The other side will connect to the pigtails of the control stick grips. 

GMU11 Part 2

Time: 3 hrs

Things done today:

  • Removed old cabling for GMU22
  • Ran new #22-3 between GMU11 location and instrument panel
  • Ran CANbus cable between GMU11 and pitch servo
  • De-pinned CANbus pins from pitch servo connector
  • Pinned and installed 9 pin connector for GMU11

Today I did some prep work for the GMU11 even though I don’t have the new mount yet. The factory said they should be shipping them by the end of the week and sent me a drawing of where it will be mounted.

TAF photo of the new GMU11 mount and location in the rear fuselage. It should be doable from the access hole in the bottom.

I ran the power and CANbus wire for the GMU up the left center longeron. I probably would have picked a different route if I had more access to run the wire. I think this was the  best way for my situation since I can get to most of the longeron to drill and install cable ties. The cable splits here near the pitch servo. The #22-3 (power1&2 and ground) go to the instrument panel and the CANbus wire runs to the pitch servo connector to tie into the CANbus there. I ran a single bundle back from here to the GMU. I used a multicore wire wire for the power and ground rather then single wires just because the Garmin manual said to do this to cut down on any EMF.

Note (9-3-2017): Just realized that I should have used #20-3 wire since I’m tying this into a 5A breaker. Well the device isn’t going to pull nearly enough to heat up the wires (it only pulls 0.1A), but the wire would technically fry before the breaker pops. So I think I may move it to a smaller breaker (it was on a breaker shared by other Garmin stuff) and then it should be fine. Also it’s really hard to find #20-3 cabling so I probably would have had to use #18 which is really overkill.

The one problem with this route is the wire gets a bit close to the pitch servo. The tie in the upper right of the photo helps keep it away from the servo. I didn’t shrink the heat shrink here yet because I may have to adjust the split point a bit once I get the GMu installed. I wanted to leave enough cable on the GMU side so I could put the connector on.

Cables on the GMU side with pins and labels.

Connector almost done. I put silicon tape where the cable strain relief is to protect the wire and add some resistance. One important note is that the metal bar part of the restraining clamp has the raised part facing away from the wire. If you flip it over then it can eventually damage the wire.

All done, just need to get a stainless steel 8-32 screw and lock washer to attach the shield of the CANbus. I didn’t put a drain wire on the other multicore wire because you really should only connect the shield at one end of a shielded cable to minimize ground loop issues. The CANbus is one except to that rule. It would probably be OK to ground the shield for the other wire, but just keeping consistent. Also the terminator will be installed here as well since this will be the end of the CANbus. The GMU11 uses an external adapter for termination rather then a jumper.

NOTE: The ring terminal is a Raychem B-106-1401, I believe I ordered it from Mouser Electronics. It’s nice because it’s a crimp connector, but also is heatshinkable. This saves a step of crimping the connector and then adding heatshrink over the end of the connector.

Now just need to wait for the new mount and get that riveted in.