Monday, November 21, 2016

Broken Wire, Or Is It?

What an interesting annual inspection I am having this year! After last years slip into the new year (I usually started inspection at Thanksgiving), I decided to make some changes this year.
One of those changes was to start earlier and instead of planing on a long downtime and doing it all at once, I was going to split the annual up into smaller packages that could be completed in a weekend and allow the plane to usable on the following weekend.
So I did the wheel bearing service with replacing the brake linings about a month ago, utilizing my newly acquired HF Racing Jack.

Worked like a charm! No more assistance needed to get the plane on sawhorses for this task.

I also completed the engine inspection 2 weeks ago and hopefully repaired an intermittent right CHT connection by recrimping the wire. Unfortunately, the problem only shows in flight. The sensor always reads fine on the ground but goes haywire right after take-off.

Other than that, no findings firewall forward.

Now the plane is down for a longer period as I opened the tailcone bulkhead and working on the flight controls. First item was to fix a problem that occured the first time after a SkyView software update (I think it was the update to V7.0) when it finally supported the standby network wires we had put in during the build. Mine was showing as defective and later versions even allowed the problem to get pinpointed to the standby network of the roll servo (behind the bulkhead).
Having access to it now, allowed me to finally see what was causing the problem. After a lot of measuring I was surprised to find that it is a problem with the wiring harness. Somewhere between the Y-crimp at the pitch servo which extends the orange wire (standby network 2B) from the roll servo through the tunnel to the aft of the bulkhead where it connects to the roll servo, this wire is broken.
If you have built an RV-12 or seen the tunnel of a completed one, you know that replacing a wire in this over-stuffed tunnel wire bundle is a very difficult task to say the least.
As I already had worked around this issue before when adding a wire harness for the ADS-B receiver (which I put behind the bulkhead) which runs from behind the panel under the longerons on the left side all the way aft of the bulkhead. I did make this wire conduit larger than necessary for such a case as this one where I see the need to add more to it.
I will utilize this wire path to connect the 2B network wire to the roll servo. Ideally these network wires should run in pairs and get twisted to reduce the noise they might pick up on the way. I am still contemplating if I just accomodate that by running a twisted pair back to it and just not use the one wire of the original harness that does work ok.

It turned out not be a broken wire at all.

After I ran the replacement wire for the failed connection to the 2B terminal of the servo and crimped it onto a showrt stub of the orange wire that connected to the servo, I still did not have a successful continuity from the panel to the servo. This could only mean one thing!
I disconnected the spade connectors for that wire on the servo side and measured again. No connection! A closer look into the connector revealed that I must have pushed the stripped wire passed the crimp part and had crimped the connector onto the insulation of the wire.  Duh!
Measuring between panel and the stripped part of the wire confirmed that it had a good connection.
I fixed this crimping issue and hooked up the servo and the Skyview display confirmed that everything was working now:

I also confirmed that the SB that most worries me (forogt the number , but it is the one about the little bridge taking the flaperon forces where the pushrods connect to the torque tubes) is still not an issue at 178 hours.

Left side

Right side

The rest of the inspection went without any hitch and a maintenance flight confirmed that the roll servo is no longer creating warning messages about the backup network within Skyview..

We also did not have any signs of overheating the voltage regulator:

Although I wish it was running a bit cooler. Maybe I do prepare to work on a scoop for the next oil change time when I have access to the radiator side of the lower cowl.

The ignition modules were doing fine too:

She's back in service since the Day after Thanksgiving.

Quick update from about a month later:
- the recrimp of the right CHT probe connector did its job for now. The CHT indication is solid but I expect that the repeated heating and cooling of the crimp as well as the engine vibrations might require repeated attention to these connectors in the future.
- It's really nice to fly without a constantly flashing warning indicator on the Skyview!

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