Quick main tank nut plate question.

[TJ_Slice]

This is onboard/forward corner of the main tank. Is there enough room to countersink and rivet the wing skin that terminates right there? From my measurements, the edge of the rivet hole will be about 1/16” from the edge of the skin if I use a k1000 nut plate.

[Dpearson]

I’ve encountered that situation a few times. I just chopped one leg off the K1000 nut plate and used a single rivet to hold it in place. The screw + dimpled hole in cover +’countersunk hole in skin is what carries the structural loads. The rivets are just to keep the nut plates from wandering away.

[TJ_Slice]

I’ve encountered that situation a few times. I just chopped one leg off the K1000 nut plate and used a single rivet to hold it in place. The screw + dimpled hole in cover +’countersunk hole in skin is what carries the structural loads. The rivets are just to keep the nut plates from wandering away.

Thanks, I'll look at that.

[TJ_Slice]

Turns out I was worried over nothing.

[Steve W]

Can anyone refer me to where this type of installation of nut plates, cutting away material from spar flanges and skins to allow a countersunk dimpled screw hole in a tank bay skin, is acceptable under AC43 methods? Thanks
Steve

[jaredyates]

Can you say more about what you're asking here? It's kind of like saying, "where on the Chick-fil-A menu is the hamburger?" The material that we're removing in this case is not receiving any screw or rivet. 43.13 is not applicable.

[Steve W]

I will try to describe what I’ve seen some builders do to install the anchor nuts on their tank bay skins and wing inspection plates. I know I am a little off topic here but I have seen some questionable work on a number of wings I have repaired/reworked etc. One group installed the anchor nuts along the rear false spar. They cut into the spar flange with a 100* cutter for the CS -3 rivets and mounted the K1000 nuts only to the spar flange and didn’t include the rear skin. The screw holes in the skin between false spar the rear spar are opened up with a cutter to allow the dimpled tank bay skin to sit in the recess made by the cutter. Some of the false spar flange material was also removed to allow the dimpled tank bay skin to sit flush. On the other three sides of the tank bay, the overlapping skins were included with the anchor nut rivets. The number 6 screw holes were enlarged with a cutter through the skins and the ribs and front spar flanges to allow the dimpled tank bay skin holes to lay flush. I wish I had taken some pictures of this before I reworked the wings.
I guess what I’m asking here is where in AC43 is it an acceptable method to sandwich skins, rib and spar flanges, then drill out the screw holes with a cutter to allow the dimpled tank bay skin to sit flush in the assembly? I don’t think these guys are the only ones that have done this with their projects.
My understanding to do this correctly is to use countersunk anchor nuts and dimple all of the relating flanges and skins.
Where is it acceptable to cut away the flanges and skin holes enough to install a dimpled cover using a K1000 nut?
I’m looking for education here, not condemnation if I am not understanding the methods.

[jaredyates]

Hi Steve, first let me say that the best source for any structural related questions is Bob. Sometimes I do post on his behalf in the safety section but I try to make it clear when that is happening, and this definitely isn't one of those times. I would encourage you to try and reach him to discuss your concerns.

Sometimes our toddler daughter would say I was "being mood" and in my last post it might have come across that way. I do think it is important for us to be able to talk openly about things, especially when structural safety is involved. But I don't think you'll get a satisfactory answer if your approach is "tell me in the non-Bearhawk manual where it says we can do this". There are countless areas on the plane where you'll be equally disappointed. That is a general manual that attempts to fill the gaps when manufacturer or type-specific guidance does not exist.

Getting to the substance, if the work that you described was intended to fly without a tank bay cover, I would be very concerned. There would not be adequate structural connectivity between the skin and the spar. But we don't fly the plane without that tank cover in place. Once the cover is in place, it is the final, outermost layer. If the tank cover was countersunk without adequate thickness, I would similarly be very concerned. In the sandwich, the bread is the tank bay cover and the nutplate. Everything in between is held in place by the friction created by the line of screws that are spaced quite frequently. The screw is not going to pull through the nutplate, and it is not going to pull through the dimpled cover. I could definitely see it pulling through a countersunk cover, but I don't think anyone is proposing that.

I'm wondering if we are reaching the limit of productivity of this discussion without illustrations or pictures, but let me know what you think. Am I understanding the question correctly? You are concerned that countersink-shaped material has been removed from the intermediary layers of the skin, spar web, etc? Are you doubting that the tightened fuel tank bay screws are providing enough friction to hold that area together?

[Steve W]

And the edge distance left after cutting the larger holes in the flanges.
Ive never seen anything quite like it before in aviation, especially around a structured assembly. Thats all I guess.

[Bearhawk Aircraft]

This is a normal practice in most of the industry. A good example would be the main spars in almost every Vans RV. There is a large amount of counter sinking, far more than we are doing here in order to attach the fuel tanks to the forward spar. These locations are actually holding structural sections of the wing together not just simply sandwiching material to thickness like we are doing here.