Now I if you don't like the word "likely" when applied to this particular
process then you should not fly any composite glider because such assumptions,
based on tests and real world experience, are used through the construction
process. 100% inspection of every bonded joint is impossible given the
constraints of manufacturing of gliders as it is done now. Perhaps these
incidents will cause the LBA and manufactures to re-think the inspection
standards. And likely raise the price of an already costly toy.
Robert Mudd
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W. Edwards Deming the father of statistical process control showed
that a 100% inspection system will regularly miss 20% of the defects.
The goal of manufacturing and design should be to develop a process
which minimizes the likelyhood of defects, especially catastrophic
defects.
The DG website has a very detailed explanation of how a wing is
constructed and I assume the technique is similar for all German
designed gliders. Spar CAPS are formed with carbon fibre rovings and
are inspected in minute detail for voids and any cap which fails
inspection is immediately cut in half to preclude its inadvertant use.
As the upper and lower wing skins are moulded, the spar caps are
glued in place so that they are well bonded to the skin. So far so
good, the top and bottom of an I-beam shaped spar are well built and
in place. Next the shear web, which is also well built and inspected
is glued into the bottom part of the spar cap. Gravity holds the glue
in place and the surface is well lighted and clean so the craftsmen
and inspectors can see what is happening. The wing spar is now an
upside down T shape. Later on in the process the top surface of the
wing is glued to the bottom half forming both a wing and a complete
spar. It can be SEEN that the exterior surfaces of the wing are well
mated because epoxy oozes out of the joints. It can only be ASSUMED
that the upper spar cap is well bonded to the shear web because the
glue joints cannot be seen. Assuming you have a good spar because of
German craftmanship works because German craftsmen are quite good, but
it is not a well designed manufacturing process. Building a complete
spar outside of the wing and then bonding it to the upper and lower
surfaces strikes me as a process much less likely to lead to a
catastrophic failure. In the case of the Duo Discus and probably the
Discus CS, failure of the spar is more of a process defect than an
inspection defect.
The price of these costly toys may indeed go up but something bad has
just happened to the value of Shemp Hirth products.