Cracks are bad news, right?

Remember the crack in the engine mount that failed and brought down the airliner? How about the B-52 that taxied onto the runway, applied full power and the left wing fell off! This all started as a crack near the spar after an air-refuling mishap. Would you fly a wood sailplane with cracks in th skin? No way, don't walk, run away from that puppy! Would you fly an aluminum ship with cracks in the skin? That old girls been rode hard and put away wet, right? Fear of cracks is in our DNA. Remember; Step on a crack and break your mothers back?

So now you find a crack in the skin of your fiberglass sailplane. Bad news, right? Actually no. Fiberglass sailplanes are covered with a thin layer of rock-hard gelcoat that was placed over a flexible structure. I remember the DG-400 at Minden, that had been flown extensively in wave conditions. It was literally covered with cracks. The wings had cord-wise cracks every half inch on both sides of both wings. This ship was flying regularly and was considered airworthy.

Yeah, but I got a crack coming from the corner of my spoiler box, is my wing going to fall off? Nope, when your wing skin was laid up in its mold, the fiberglass cloth wouldn't fit tightly into the corners and around the edges of your spoiler box, so filler and extra gelcoat was applied all around the spoiler box to allow the cloth to smoothly overlap the box. The corners of the box are stress concentration points and cracks will quite likely appear there. How deep do these cracks go? All the way through the gelcoat and filler, but they stop when they reach the fiberglass cloth because they are gelcoat cracks migrating IN from the rock-hard coating, NOT cracks in the fiberglass migrating OUT!

Once again, this is just my humble opinion, but it was formed after 40 years of grinding out your cracks and finding no structural issues.
:) JJ