Jim 59 wrote:
Some gliders have somewhat shorter airframe lives of only 3,000 hours so it clearly depends on the design of the particular airframe.
Nothing to do with the airframe design. When GRP was introduced no-one knew how it would stand up to the stresses of flying, so a 3,000 hour life limit was set. As gliders began to reach this limit, inspections indicated that there was no deterioration so limits were raised by the manufacturers, and most are now at 12,000 hours (and I expect them to be raised again once some old gliders reach that number).
Problems have arisen where the manufacturer went out of business before raising the hours limits. This happened with the Centrair Pegase, and I think most European gliding bodies had the power to raise the limit on a national basis, so did so. In the US, however, there is no such body, and of course no manufacturer to approve the raise.
I don't know of any GRP glider model which has been grounded due to running out of hours where there is some authority which has the power to raise the hours limit.
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SkyHawk's comments show how much familiarity plays a role in our perceptions of structural safety. I have many hours flying GRP gliders, rather less in wood and very few in metal. I feel most comfortable in GRP, because I think I could recognise a potential problem, less so in wood and metal because I know of structural failures in both these caused by hidden, unobservable (to me) defects. For major structural repairs to GRP I have to trust the repairer, but the UK system has meant that all repairs have to be undertaken by competent repairers, and there's no history of failures so I guess I trust the system.
SkyHawk's experience is the opposite, so he feels more comfortable with metal.
Repairs to GRP by gliding inspectors don't seem unduly costly. As an example, a bad ground loop can snap the fuselage just in front of the tail, and I think this kind of repair would normally cost £2 to £3k. GRP is pretty strong, thus minor knocks tend to cause no, or merely cosmetic, damage. If I punched my fuselage hard I'd break a knuckle but cause it no damage!
I'm not saying GRP is better than metal or wood, just that it has a very long track record for making structurally sound aircraft.