Highlander Butt Rib Reinforcements

[SheepdogRD]

The wood butt ribs on the Highlander fuselage should be strong enough as designed. But a few users of older Highlanders are reporting that negative pressure at the top of the turtledeck has caused the ribs to fail, and at least one has reported loss of the sunroof in flight. In that case, the capstrips were ripped off the ribs.

You can prevent this on your Highlander by installing a reinforcement on each butt rib. These reinforcements fasten the capstrip -- and the sunroof -- to the rib web.

You’ll need a fluting tool. I used Aircraft Spruce Part# 12-01782; it’s about $35 in 2025.

You’ll need two strips of .032 aluminum, 1-1/8” x 30”. If you haven’t used a fluting tool before, you might make a couple of extra pieces for practice. You’re going to bend the strips into angles, and then use the fluting tool to make the angles curve to fit the underside of the capstrips. The amount of curve you get will depend on how close together you space the flutes. Some experimenting produced the spacing listed here.

First, smooth the edges of your strips; it’ll be a lot more work if you wait and try to smooth fluted surfaces. You’ll be bending the strips into angles, with one leg 5/8” and the other 1/2”.

Before bending the angles, mark your bend points. Put the strips on your work surface with the 1/2” sides next to each other. Starting at one end, mark both of the 1/2” sides at 2”, 3”, 4”, 5”, 6”, 7”, 8-1/2”, 10”, 13”, 16”, 19”, 22”, and 25”. The marked pieces should be mirror images.

Bend the strips into angles. The 1/2” side gets fluted.

Now use your fluting tool to create a flute at each of the marks. If you haven’t used a fluting tool before, practice on some scrap so you see how much force it takes to get the bend you want. Be sure your flutes are on the insides of the angles so the reinforcements fit smoothly against your butt ribs.

You probably won’t need to get the maximum flute depth, but flute deeply enough that the material bends a little. They should look roughly like this when you’re done:

As Bent.jpg

When you fit your reinforcements to the plane, you can adjust the flutes for additional depth if it’s needed. Or, if your flutes are a bit deep, you can pull the part a little flatter where necessary.

Set the reinforcements so they’re just far enough back on the rib that they will clear the front spar. Add to your flute depth -- or flatten slightly -- as necessary to get the reinforcements to fit nicely under the capstrips.

If you’ve already drilled your butt ribs for fasteners, match those holes in the reinforcements. If you haven’t, consider using nutplates installed on the underside of the reinforcements (I used 8-32 nutplates, Aircraft Spruce Part# K1000-08). Your sunroof fasteners start 1” back of the front edge, and run every 3” to the back. The final fastener should be 1” from the back of the rib.

With the capstrips and reinforcements drilled (and nutplates installed, if you use them), trim the back of the reinforcements to fit flush with the back of the capstrips. You’ll need to trim the web of the reinforcements to fit around the openings cut in the rib webs.

Standard 1/8” clecos work fine on 8-32 nutplates. With the reinforcements cleco’d in place on the capstrips, mark for web attachment holes between the flutes. You can use 1/8” rivets -- spaced every 4” or so -- in the flat areas between the flutes. Drill the holes in the reinforcements, and match drill the butt ribs. You’ll probably need longer rivets, since the rib webs are 1/4” thick.

Fit Check.jpg

When your reinforcements are painted, rivet them to the rib webs. Now your capstrips will hold that sunroof solidly, and they’ll hardly be noticeable:

Ready to Install.jpg

[Fozz]

NICE! are you using any bonding beyond the screws i.e. Epoxy?

[SheepdogRD]

I didn't use any other bonding. I figured eight rivets through the 1/4" web would be stout enough to keep the reinforcement -- and thus the sunroof -- attached.

I had originally figured the use of nutplates under the capstrips would spread the load further than just using rivets to hold the sunroof. But, when I heard about a plane that lost the capstrips, too, I was motivated to build these.

[KevinC]

Thank you, Richard. This has been on my list for years after talking with one who lost the “sunroof” in flight. I’m retired now, so have the time to get it done!

K

[Tralika]

DSC02896.JPG

I noticed the web of your butt rib is solid, mine is cut out like the rest of the wing ribs. I wonder when this change was made and if that has any effect on the failures. My kit was produced in 2009. When was yours made? This is the first I've heard of the cap strip/greenhouse failure.

[SheepdogRD]

Mine is a 2011 kit. The butt ribs were originally like yours, but I switched to aluminum tanks, and got new flat-bottomed butt ribs when I did that. I think the change came along with the aluminum tanks.

[Tralika]

That makes sense, the aluminum tanks have the sight gauge which wouldn't work the the original rib webs.