SB 0017 Modification

Sling Aircraft issued Service Bulletin 0017 earlier this month, indicating that the elevator torque tube may have been incorrectly assembled at the factory. The issue is that the elevator control stop plates may have been incorrectly attached with aluminum rivets, whereas stainless steel rivets are required for the shear loads.

I inspected my torque tube per the SB and confirmed both visually (dull appearance of the shop end of the rivet) and empirically (magnets readily attached to the mandrel) that my torque tube was incorrectly assembled. This post serves as I record that I have completed the modification on 3 July 2020. I made one deviation – I used 5/32″ SS rivets instead of 3.2mm rivets. The effect of this is only that the 5/32″ rivets are rated to a higher shear load (650 lbs vs. ~425 lbs), resulting in approx. +53% margin of safety.

The modification itself isn’t too bad — drill out 4 rivets, replace with 4 stronger rivets — however access to the affected component is difficult. I had to remove a few other components to get access, notably the inner seat rails and the floor closing ribs directly over the torque tube. Luckily, I hadn’t yet installed the fuselage side skins, so I had better sight lines and access than I otherwise would have with a completed fuselage. It took me about 3 hours to complete the repair.

This closing rib above the torque tube must be removed to get access; 10 rivets must be drilled out, which creates a lot of debris
The two offending rivets – aluminum rivets have a dull appearance, whereas stainless rivets are shiny
After drilling out and replacing with SS rivets
Some of the rivets for the closing rib are too close to this bushing to fit the rivet tool nosepiece, so I used an M3 locknut as a makeshift nosepiece extension

Another builder on the Sling Builders Facebook group gave me the idea that the seat rails should be more easily removable, in case future maintenance is required in this area. The seat rails are attached with 4.8mm countersunk rivets, which are not used anywhere else in the kit, so they are somewhat rare. I decided to use swap countersunk screws for the countersunk rivets. Of course, the screws need to attach to something… I was able to use locknuts for the forward portion of the seat rails, but for the portion that attaches to the spar cap of the wing spar carry-through assembly (CF-AMS-001-C-C), I had to come up with another solution. The sandwiched plates that make up the cap are too thick (6.35mm) to use rivnuts, nor is there access to secure the screws with nuts. I decided I wanted to tap threads directly into the aluminum, but the holes were drilled too large to successfully tap for M5-sized screws. So, I purchased a Helicoil kit, which are normally used to repair damaged threads, but in this case they fit perfectly into the ~5mm holes already in the spar cap. The Helicoil kit includes a special Screw Thread Insert (STI) tap, sized appropriately for an M5 helicoil spring. Once tapped, the springs are installed with a helper tool that compresses the spring. The result is perfect threaded hole for M5 screws!

Threading the hole with an STI tap
Installation tool for the coil spring
Helicoil installed, looks just like a normal thread
Screws go into a Helicoil just like any other threaded hole
There is a small tang on the end of the Helicoil that must be snapped off using a punch and a light tap with a hammer
Seat rail re-installed with countersunk M5 x 16mm SS screws

I bought the Helicoil kit from Amazon, and the screws & locknuts from McMaster. The Helicoil kit doesn’t include a drill bit (13/64″ is specified), but I really didn’t need it because the holes were already the correct size. However, I used an 5.0mm drill bit to clean up the hole prior to tapping it. The helicoil kit also doesn’t include a t-handle for taps, but I had one from another tapping kit I already owned.

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