Friday, January 4, 2013

Modifying Stall Warning Assembly Mounting Screws

While following the adjustment procedure Van’s has in their instructions for initial setup of the stall warning micro-switch& vane, I discovered the potential for a nightmare later down the road. The stall warning assembly uses two #8 8-32 Philips head screws to secure the assembly to the two nutplates riveted onto the W-1208-R nose rib. (For those not familiar with nut plates, in a nut shell they are a threaded plate that is riveted to a structure which you don’t have access to the backside of to allow for a wrench and nut, as would be the norm.

The threads on the nut plate are purposely squished a little so a screw can’t back out on its own. However, this requires extra effort when screwing a screw into a nut plate, not to mention requiring extra pressure on the screwdriver to prevent the Philips screw head from stripping.

Even with the W-1202-L skin flexed up to allow for fair access, it was not easy to screw in the two Philips mounting screws securing the stall warning assembly. (OK, admittedly it was quite cold in the shop and my hands are used to 70 degree California winter temperatures, so a small task like this seems like a much larger chore). Frankly, once the wing is fully assembled, I can’t fathom the difficulty involved to make adjustments to the assembly through a tiny inspection hole. The odds of the switch being set perfectly at this point is rather slim, so adjustments are very likely … plus to add to the difficulty, it appears as though it will also require using the left hand if the wing is on the RV-12. Should one of the Philips screws become stripped, and that would be soooo easy, it would be a colossal bummer!
The dollar bill is placed adjacent to the inspection portal to illustrate what “fun” it will be later to make adjustments to the stall warning assembly which is nestled just aft of the leading edge of the wing where there is little room.

In an effort to alleviate the possibility for a screw head becoming stripped and to make adjustments easier, I’ve decided to use socket head cap screws (Allen head) to secure the stall warning assembly, as opposed to using the #8 Philips screws. I feel it will be much easier in such a cramped location with limited visibility to just insert an Allen wrench into the screw head and have a positive fit, substantially reducing the potential for head stripping all together. The socket head cap screws are now on order. Fortunately, the shielded wire I was waiting for was delivered this evening so forward progress on the left wing’s wiring can resume.