Best Practices - Skin Repair of Trimout Over Circumferential Splice

[LiftDivergence]

Hi all,

A few months ago I saw an post to the "Aircraft Structures" group on LinkedIn. I'm not going to link to to the post because I don't want to link to external sites in the forum. But this is a public group on LinkedIn with ~27,000 members where various aircraft structures are discussed, usually structural repairs.

The post in question was showing a repair for an aft fuselage skin belt loader impact on a circumferential splice. A large area of the skin was pretty severely dented and buckled in. They trimmed out a large area both skin panels across the splice. I have attached some pictures of the trimout and also the completed repair (external and internal).

What immediately struck me about the repair was their choice to design it as a "split patch". Effectively, one doubler for each skin panel with no continuity between them (other than the replaced production internal splice strap).

In my experience, I would have expected a repair for this damage would either have a continuous doubler over the splice, or a tripler layer over the split doublers. This comes both from familiarity with SRM repairs for several different makes/models of aircraft where a trimout is made on a skin splice, and from analysis experience of reinforcements spanning damage across splices.

There is some discussion on the post pointing this out. However, the individual who posted the repair seemed adamant that this was correct.

I'm curious what people think of the pros & cons of this design. I have my opinion of course, but I'm curious what other peoples experiences are.

 

[rb1957]

Opinions are like ...

If I had a to patch over a splice strap, I would probably (depending on specifics blah, blah, blah ...) combine the splice strap into the repair dbr ...

cut out the damage (or cancer as I'll often call it),
rebuild frames and stringers,
then add a large repair dblr over the cut-out and splice into the external splice strap (and this could be simple butt joint or a small splice strap).

To say a particular repair is "right" (which implies all others are "wrong") is IMHO pedantic ... so long as the repair considers all the necessary issues (particularly DTA) then all should be "good". Sure some repairs will be lighter than others, but is that Really significant ? Sure, it can be, but often isn't. Sure there are badly designed repairs ... because they haven't considered some aspect or another (like super heavy repair doublers, attracting load, making the "repair" worse).

We had an operator do a repair on something we'd built (many years before) under their own paperwork, so not my problem. But I looked at it and was pretty horrified ... they had rebuilt the OEM structure (that had cracked) precisely without thinking/asking and it was pretty "wimpy" ('cause that suited the OEM's environment). Making the repair match exactly the OEM structure (like splitting the repair dblr into two pieces to match the OEM skin panels) is not IMHO the right thing to do (by default).

 

[3DDave]

Is this correct- that the original skin had a discontinuity over the middle of the internal splice strap and that the repair has the same discontinuity over the same section internal splice strap replacement?

Is it possible the patches were removed from out-of-service aircraft rather than having custom formed replacements?

A single piece would also be stiffer across the splice than the surrounding material, though I suppose the doublers already are.

It doesn't look like there's any fundamental problem with the patch.