Custom vertical stabiliser mounting washer sheet

The stainless steel rivets holding the nutplates to RF-RIB-901 are very slightly proud of the surface, about 0.2mm. This is owing to the thinness of RF-RIB-901 which prevented a full depth countersink of the nutplate attachment holes without significant broadening of the holes. However the vertical stabiliser channel is bolted directly against the rib surface with multiple AN3 bolts. Unmodified, the clamping load will be transferred to these slightly proud rivet heads, with consequent stress riser creation, flexure and possible loss of bolt preload.

The fix for this is to create an extended "washer" sheet of 0.4mm 6061-T6 containing clearance holes for these rivet heads, alsong with all the other protrusions.

It would have been much better to use squeezed solid rivets, with dimpled rib holes and nutplates. This is true here and in numerous other places in the design.

The effect of this modification is to move the vertical stabiliser aft by something like 0.4mm, maybe less. This is thought to be insignificant. However the 4mm rivnuts of the vertical stabiliser are moved aft by this amount, so the matching holes in the rear fuselage skin - which are currently undersized and slightly out of position - will have to be created with this shift in mind.