Alternate air screw backing out

[Carl Froehlich]

Two local RVs reported the screw that provides the pivot point for the engine alternate air door had backed out. The plans call for a #8 nutplate to hold this screw.

I did away with the nutplate and #8 screw opting for a AN3-4 bolt, castle nut and cotter pin.

For you vertical induction guys this is a simple back fit. For a snorkel install, careful positioning of the door flange allows for access to the bolt.

Carl

[Ralph Inkster]

MANY times I have run into planes where the base plate had worked loose if it was riveted to the fiberglass per plans. Especially in the vertical filter, not so much in the horiz filter arrangement. Imagine where those loosened or missing rivets will go.
So others don't repeat the anguish I experienced (2 cyl overhauls!), I strongly suggest an alternate mounting technique.
I fashion a .032 backing plate ring with a series of #08 nutplates and a #3 nutplate for the swivel. I dimple the valve base plate for #8 AN507 screws. In mounting, the fiberglass is sandwiched between the base & backing plate, and screwed down with PRC gluing and sealing the whole thing. The AN3 (ANH3?) with drilled head is used for the swivel & safety wired so it will never work loose. Have not seen any further failure of this type in any plane I'd modified this way.