I had some questions about this SB and received the answers from TAF that I’m allowed to share.
1. What is the exact number of rivets?
The total number of rivets on the photo that shows the fretting is 41 rivets on the top and 17 on the bottom (excluding leading edge / fuel tank):
As illustrated below, on the Sling 2, the rivets must be replaced on the Leading edge (12 rivets) and trailing edge (25 rivets) on the upper wing spar.
On the underside of the Sling 2 wing, the rivets to be replaced are 14 on the leading edge and 27 on the trailing edge.
Hence, the total number of rivets, for both wings = [(12 + 25) + (14 + 27)] x 2 à156 (wings only).
The number of rivets along the centre spar carry-through under the fuselage à62 (centre spar only).
This totals: (156 + 62) à218 rivets (Sling 2), just under the number of rivets recommended in the Service Bulletin (ie 230).
Figure 1 in the Service Bulletin was taken from a Sling 2 with time-in-service in excess of 2000 hours.
It was inserted to demonstrate fretting in aircraft so that one could identify fretting following visual inspection of their aircraft, rather than being a guide to the number of rivets to be replaced.
In addition, Figure 2 and 3 in the Service Bulletin were used to show the regions affected by fretting, not as a guide to the number of rivets to be replaced.
2. How to prevent calvanic corrosion due to the dissimilar metals?
See also EAA – Corrosion Control – Galvanic Table
We have not seen galvanic corrosion on the stainless rivets even on old Slings. On some a/c we assemble we put tank-sealant onto the rivet and set it.
Also the EAA article mentions sealant conform MIL-S-8802, which is tank-sealant CS3204B.
I will follow this procedure:
– prepare the aluminium skin with Alodine Touch-N-Prep
– spray a light layer of 2k Epoxy primer on the skin and into the rivet hole
– add some tank-sealant CS3204B around the rivet and set it wet
It was supposed to be one sentence referring to the TSi only, so note 2 should read for TSi only.
All is clear now, I have received the rivets and will work on this in the upcoming days and post it here on my blog.