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Inner z-bracket problem

It’s been a while since my last post. I’ve been away with work and I also got kinda stuck on the build. Here’s a summary of what has occurred over the past few weeks.

I tightened up all the bolts on the z-brackets using my socket set. There is no mention of a torque setting and I couldn’t get my torque wrench in between the ribs even if there was, so I just tightened them up to be “firm”. This took a surprising amount of time, probably because you can only do a quarter turn (at best) with the socket wrench due to the rib spacing (almost 2 hours for both wings).

I had great difficulty with one z-bracket on the left wing. For some reason I couldn’t get the bolts to bite into the thread on the nut plates. The best I could do was 2 out of 3 with the third not biting properly. It appeared to have a very slight misalignment which caused the bolt to be at a very slight angle. This made the bolt threads strip when you tried to force it in. After numerous attempts, new bolts and a touch of profanity, I ended up resolving this by taking the z-bracket off the spar and repeatedly screwed the bolts in and out of the nut plates. This slightly “opened” the nut plates up a little bit and eventually I was able to screw all three bolts through the main spar holes and into the nut plates.

The plans then called for the inner z-brackets to be slid in between the fuel tank baffle plate and the main spar. I slid these in and lined up the one and only hole in the z-bracket with the centre pre-drilled hole in the tank baffle plate and put a cleco through them to hold everything together.

The next step was to backdrill through the main spar pre-dilled holes into the z-bracket. Simple enough I thought, then I gave it a quick look over and found this…

I could see the rivet holes for one side of the nut plates exposed. I thought (incorrectly but I didn’t know this at the time) that the nut plates would be rivetted to the forward side of the rear flange, as was the case with the other 6 z-brackets. I checked the alignment of the other pre-drilled holes and z-brackets, which were all fine. I then double-checked the ensure that I had the correct fuel tank baffle plates on the correct wings. Again, all good. I started scrathing my head at this point and started to think I had drilled the z-brackets incorrectly which had caused some kind of misalignment. So I stopped work and went off the do some research.

Many thanks once again to Vans builder support, who advised that on the inner z-brackets the nut plates go on the other side (i.e. the trailing web of the main spar) which is the exact opposite of the other 6 z-brackets. These holes can be exposed and they actually need to be countersunk so allow the factory head of the rivets to be flush with the main spar, and allow the z-bracket to sit on top of them. Another “slap palm to forehead” moment. I ended up losing a bit of time, but I’d much rather have that than have to replace a wing spar which had been incorrectly drilled.

With that misunderstanding resolved, I got in to match drilling the z-brackets. As I was drilling through the pre-drilled holes in the main spar, I had to push back on the z-bracket against the drill to stop it lifting. First hole drilled and looking nice…

I then took my hand off the tank baffle plate. Uh ho….

As I was pushing down on the baffle plate, the z-bracket had flexed off being perpendicular with the spar and has drilled a hole off the centreline to the flange. . I thought about this for a minute or two and then thought that it wasn’t a major disaster as the hole actually isn’t being used to hold anything together, it’s purpose is to allow the bolt to pass through into the nut plate. The bolt has a washer also which should still provide enough support for the bolt head. So I straightened up the z-bracket and re-drilled. This made the hole oval, but now the bolt could pass through with everything square. Build on. I drilled all the remaining holes by holding the z-bracket flange against the main spar, making sure I wasn’t going to drill through my finger tips.

The plans then call for the nut plates to be rivetted to the main spar. There is a bit of prep work required to be able to do this. First, I had to remove the tank baffle plates to give me room to access the nut plate holes. I then drilled the nut plate rivet holes to #40 to allow easy insertion of the rivets. Next was the countersinking tool where I countersunk all the nut plate rivet holes on the forward side of the main spar. I could get to all these holes using the countersinking tool, except for the ones closest to the main spar doubler. For these, I had to remove the shroud from the countersinking tool and carefully countersink these tools without any automatic depth control. I went slow and checked the depth frequently and didn’t have any problems.

Finally, I picked out the rivets and nut plates and looked at how I could set these rivets. I grabbed my bucking bar and rivet gun and experimented with different positions and holds and rapidly came to the conclusion that this would be a 2-person job. Access is limited and it would be VERY easy to bend or split a rivet, or slide off the shop head and dent the main spar. Drilling out bent rivets from the main spar also carried the risk of elongating the rivet holes. I decided it wasn’t worth the risk to try this solo so I stopped work at this point.

Build time 4 hours.


About gstrack

Husband and father of 2. Control & Instrumentation Engineer. Flying nut. Gadget geek.


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