F-1040 top row early countersinking

[rdamazio]

Hi everyone!

I'm currently working on section 29 (fuse side skins) - on 29-14 step 9, it says to only countersink the bottom row of holes on the web of the F-1040 upper fuselage channels (those that are common with the skin), even though the top row was already final-drilled on 29-13 step 1. Reading ahead I can see the top holes are normally countersunk on 31-8 step 9 along with the F-1071 skin, but I'm wondering if anyone has done that early and if that caused any issues?
It seems to be much easier to do now, before primer and without the side skin interference.

Thanks

[flion]

It should not cause a major problem, but ... I think the reason they have you wait is that the initial fitting of the upper deck skin has a fair amount of tension along there and if you pre-countersink before final drilling, you risk damaging the countersunk holes when you apply clecos there. You are going a little deeper there to accommodate a dimple, which leaves less material at the edge of the hole to take those side stresses. Once dimpled, the deck skin dimples help take up that stress, so clecoing for riveting is not as risky.

That said, I don't think the risk is that great and you can do plenty of damage with clecos before countersinking if you are not careful. So just exercise normal care and you should be fine.

[rdamazio]

Thanks for the reply - that makes sense.

What I ended up doing was simply to do enough of section 31 (up to 31-8 step 4) that I could match-drill the top skin, before doing the countersink or any primer or riveting on section 29.