On Aug 23, 9:52*am, Cats wrote:
On Aug 22, 9:09*am, Chris Nicholas wrote:

We had a Foka 4 at my club in the 1970s, with this sort of expanding
bolt.

My recollection is that the owners used to count the number of turns
to get the full expansion. If the number of turns was correct, and the
top bolt was visible in the right place, then the invisible lower one
must also have fully engaged with the lower lugs.

Might be a useful check in the absence of a visible sign.

(I'm not saying that this had anything to do with the recent accident
- the latter may have been caused by something different.)

It's an assumption that if the correct number of turns have been done
and the top bolt is seen to be correctly expanded that the lower bolt
has also correctly expanded. *I gather that in the US Cobra accident
there was some sort of fault in the bolt such that the top part
expanded correctly but the bottom didn't, and it is very hard to see.

Personally I am so glad I fly a relatively modern glass glider with
two horizontal main pins where the only mechanism is me pushing them
home and then crossing and latching their 'handles'.

In the US Cobra accident the taper bolt mainpin expanded correctly,
but wasn't centred due to wear in the central locating bush. Hence it
didn't fully engage in the bottom wing root fitting. This failure
could have been prevented by a visual inspection, but apparently it is
difficult to see the bottom fitting in this type of glider. I will
repeat that there is nothing intrinsically wrong with this rigging
system, as long as it is properly maintained and a visual check is
made after rigging.

Derek C