On Monday, October 21, 2019 at 3:37:59 PM UTC-4, wrote:
On Monday, October 21, 2019 at 4:15:52 AM UTC-4, Charlie Quebec wrote:
The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

The AN is an interesting read--and an alarming one given the warning that:

"Irrespective of the initial cause of the gel coat cracking, all gel coat cracks can, given time:
...
(4) Travel completely through the skin layers of wings, tailplanes, fins, rudders, elevators, flap and ailerons.
(5) Transfer into and through the foam layers of sandwich skins.
(6) Transfer from skin layers into spar caps."

However, I'm not sure I understand the basis for these warnings given a statement earlier in the AN that:

"It must be noted that at June 1987 the "cracking" seems restricted to the epoxy resin in the skins, the question being - how long can cracked gel coat be left before it will induce glass or carbon fibre breakdown?"

That's the real question, apparently, but one the AN seems to sidestep. Has there been more study since 1987 then to validate the claims made above?

Chip Bearden
JB

I can see no basis to an expectation that a crack propagating into an outer skin would continue through a core foam and then start again through inner skin.
I've seen imprinting of gelcoat cracks in the outer layer of skin laminate, but have not seen one that progressed beyond the outer(usually 110) layer.
The real message is that if the glider is in gelcoat failure(deep cracking, cupping, peeling) real attention needs to be paid.

FWIW
UH