Replacement Rib 401 Finally Received

I write this post in the midst of the COVID-19 crisis. Life is changing so fast, and yet so many things remain the same… it’s a confusing time for sure. Social distancing is now a house-hold term, whereas this used to be a concept familiar only to geeks and introverts (I qualify a little bit for both!). My airplane project is one thing that helps remind me of our normal lives, and keeps my mind off the constant barrage of bad news, so I’ll continue to work on it and post updates here.

After a long wait, I finally received a replacement rib 401 for the rear fuselage! To ensure I received the correct physical part (RF-RIB-401-R-C-), I ordered both the left and right side parts. Glad I did, because even though I needed a right-side part, for some reason the factory is marking these parts reversed (i.e. left- and right-side are swapped). I’ve alerted TAF.

Hooray! Replacement rib 401

After priming the part and starting to install it, I didn’t like the fit of these ribs in the rest of the rib 4 assembly. The lower ribs (RF-402-R/L, green in the drawing below) are formed into a small c-channel where they meet up with rib 401 (yellow in the drawing), and intuitively I would expect rib 401 to go into this channel. However, the KAI doesn’t show it this way, and neither do factory photos that I’ve seen; they show rib 401 attaching outside of the channel. I assembled the ribs per the manual, but as I expected, the fit isn’t great, as you can see in the two photos after the drawing.

Assembled view of Rear Fuselage Rib #4
The arrow points the c-channel in RF-RIB-402. There is noticeable interference between the parts on the inside corner
Noticeable interference on the outside corner as well

To resolve the fit, I would either need to elongate the rivet holes, or try to reshape the angle and/or c-channel. I decided it makes a lot more sense to assemble the rib for best fit, placing the upper rib inside the c-channel. As shown in the next few photos, the fit is a lot better – in fact, it lines up perfectly, so I can’t help but wonder if this is how it’s supposed to fit. I found this to be the case for all of the rear fuselage ribs, so the other ribs have been assembled this way as well. From a structural perspective, I don’t see that there will be any impact, but I’m going to check with the factory to be sure.

UPDATE 3/25: I received a reply from TAF technical support yesterday: no structural issue. The full response:

“Fitting the upper rib inside the lower rib will not give you any structural issues.
You might find a bit of misalignment with the top skins when you fit them.
This will be a very small amount and the rib is flexible enough to fit clecos and install the rivets.”

After putting the upper rib inside the c-channel of the lower rib – perfect fit
Inside view of the 401/402 join – again, perfect fit now

With this rib now installed, I am moving on with skin attachment. The manual suggests putting clecos in every other hole, but that’s putting a serious demand on my supply of clecos!

Attaching the rear skins

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