Originally Posted by
Dave Therhino
According to the article, it's not a fatigue issue - it's an inability to withstand limit loads issue
The article isn't entirely clear, but my reading is that there is (at least potentially)
both a fatigue issue
and a limit loads issue.
I
think what is going on is that there are two issues, bad shimming (cut too small leaving gaps) and ridges on the internal surface of the composite.
The bad shimming
on it's own might cause fatigue issues - gaps mean more movement thus more potential for fatigue. It isn't clear that they know that this is definitely a problem, yet, and if it is it may just reduce the fatigue life - more frequent inspections may be the only remedial action.
On the other hand if you have
both the bad shimming
and the rough internal surface it seems there is an immediate problem that the structure might not handle limit loads. This might be because of poor load transfer - ridge pushes shim away from composite and all the localised load goes through the ridge - or maybe the ridge means that any gap is guaranteed to be between composite and shim giving the composite space to delaminate and fail, or something else.
My guess on how this has been found is that they have found the poor shimming (and maybe some earlier-than-expected fatigue) in routine inspections and that the second issue with the rough composite may be theoretical. Since, apparently, they cut the shims custom for each airframe from scans they may well have kept the scanning data which would make identifying the affected airframes straightforward. Suspect the shimming issue is more widespread, I can't see them ending up with eight airframes affected by
both issues if it was just one bad batch of shims.