Kerry:
Well, putting in the new decal and fuel selector stop is not going to be the simple deal I thought it would be. The AD says to drill two holes in the “carpeted floorboard” (plywood floor board or floorboards) – i.e. carpet on one side and plywood on the other – so you can push the stop down to override it. It implies that the plywood has enough thickness for the holes. The carpets have been replaced in my 75 C23 and the center carpet piece that goes over the fuel selector is just a thick piece of fitted carpet with closed cell foam on the back side – no plywood. This might explain why the stop was flattened out in the first place – because the stop couldn’t be pushed down.
Should I cut a piece of plywood to back this center carpet? How thick should the plywood be? I did see a few little slivers of wood when I pulled the carpet so I guess there was wood there recently. Any helpful hints appreciated.
Technical Editor:
I don’t want to paste the entire other FAQ posting here; but if you do a BAC search on ‘floor-board replacement’, you will find some additional information on this subject.
For what it is worth, here are the original drawing specifications for the floor-board wood used in the 19/23/24/76:
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Floorboards P/N 169-530004-205, -211, -223, -565, -571 are manufactured from commercial fir plywood sheet, grade A-B, minimum 3 ply, 0.187 thick (nominal 3/16″ thick).
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Notes:
– Most original 19/23/24/76 floor-boards are found to be a quarter-inch thick, with four plies.
– The floorboards are not treated with a fire retardant, neither before nor after installation. This has been confirmed by both Beech Technical Support, and by the Plant 2 Floor Manager in Wichita (where the plywood is still being used in new Bonanzas and Barons).
Response 1:
If the current backing is working well, and you just need more “attachability”, you can put special nuts (Tee-nuts) on the back of the foam. They have a large washer with a couple of points on it, sort of like the head on a Molly wall anchor. They have a threaded barrel, which extends up into the material, so you can turn a fastener into it from the top side. They come in different diameters and barrel lengths, and are often available at hardware stores. The most common usage is to provide flush threaded metal holes in plywood and plastics.
Make sure the screw threads don’t extend below the foam and hit the aluminum on the bottom. Also put a layer of heavy Mylar packaging tape, Teflon anti-chafe tape, or aluminum foil duct tape (not the gray fabric duct tape) on the bottom, covering the fastener, so that you don’t get wear or corrosion on the aluminum floor skin. If this provides enough attachment, and you can create enough depth for the detent (perhaps with some slight bends), you’ll be OK. If you do decide to reinstall plywood, make sure you use a thin but multi-ply aircraft or marine plywood, not construction stuff. You can review Aircraft Spruce’s products to understand what you are looking for (and they sell it in small pieces).
Also, note that the SB-mandated detent is a complete waste of time and money. It was intended to make it difficult to accidentally turn the fuel selector 100 degrees or more, and shut off the fuel. The reality is that if you rotate it so it points straight forward, in between the left and right detents, it will shut off the fuel. In effect, the only purpose served by the detent spring is to make sure that if you turn off the fuel on purpose, past the detent, you’ll have a bit of a struggle getting it past the detent (in either direction).
Response 2:
Viewed from the side, Tee nuts look like an upside-down T. The washer head is the flat part, and the threaded barrel is the vertical part. You drill a hole in the wood for the barrel (snug fit if possible), put a dab of epoxy around the washer head, push the barrel into the hole, and then pull the head (with its prongs) into the wood by threading a screw (with a large washer area) in from the top side. Leave the screw tight in place until the epoxy is cured. You then have a thin piece of wood with metal screw threads in it, that essentially cannot pull out. You can use proper length screws for each need, or simply cut screws to length for each purpose, so that the screws don’t protrude below the board. If the wood is too hard for the Tee nut prongs to penetrate, you can pre-drill the contact points; and there is another version that has small holes in the washer head through which you tap in tiny brads. Either type should have the tape applied over the Tee nuts to prevent contact with the aluminum beneath.
These nuts are used to fasten things to thin wood or similar structure, for non-structural applications. Note that Tee nuts are not the same thing as the larger and heavier knurled nuts and externally threaded insert nuts that are sometimes used in composite materials and heavier woods. The Tee nuts are smaller, lighter, and easier to work with. I have seen some mechanics use anchor nuts that have been tapped out to free-running threads, with brads (and even rivets) holding them under the wood. This is not nearly as good and unobtrusive a solution as the Tee nuts.
This Tee nut usage is unrelated to the fasteners used to hold down the floorboard itself to the underlying aluminum skin and structure. These planes all came with sheetmetal screws for this purpose. They are now old enough that many or most of the original holes have gone though a series of larger size screws, until the original positions were abandoned and new holes drilled and used. I’ve seen a couple that looked like pincushions under the floorboards, where mechanics just kept punching in new holes over the years. If you are capable and willing to do proper and legal work, installing clip-nuts or riveting in anchor nuts will enable you to provide permanent threads for machine screws, to hold down your floorboards.
Response 3:
The plywood floorboards are there to prevent damage to the thin and relatively fragile aluminum floor skin and formers/stringers. This sub-structure cannot withstand much point-loading without serious damage. In many of the planes, when you pull the floorboards you’ll find badly dished-in floor skins as well as bent structures. No one should be in the plane, even during inspections, without at least something temporary under them to spread the load.
The original Beech design intent was to use floorboards that could withstand a 110-pound woman wearing high-heel shoes, according to one of the original designers (same with the Bonanza). This is, in part, why you need to use the very high grade plywoods, that have more plies for a given thickness of wood. Some premium quarter-inch plywood has seven or more plies. This also enables you to get adequate strength with the minimum thickness. Using thicker, cheaper plywood can easily add 20 to 30 pounds to the overall weight. May not sound like much, but it is the equal of a lot of travel bags, and you’ll be paying to haul it around “forever”. BTW, I was also told that the original plywood was not rated as being waterproof, but it seems like a wise idea. When using thin plywood, it is best to use Tee nuts applied to the back, if you need to fasten something to the plywood alone.
For much more commentary on this, visit BAC and do a search on plywood. Then select the MusketeerMail Archive, check both search modes, and search on plywood.
Response 4:
Like any other material, plywood can be (and is) structural when the right kind is used in the right place. It is used for wing and fuselage skins, as wind-racking bracing in the corners of wood-framed construction, as “skin bracing” in the form of decking on trussed roofs, and as “spar webs” in structural construction trusses, and as hull skins and decks on some boats (just a few examples). As is also commonly the case with most materials, using the wrong kind or size will result in a failure to meet the requirements of the application.
In the case of our floorboards, I have been told by retired Beech engineers that the floorboards are not structural, only protective. This is illustrated by the fact that most of the boards are secured only by a sheetmetal screw in each corner; not even remotely a structural fastening. While the plane technically has to have “parts replaced with the same certified parts”, there are exceptions to this in the case of “standard materials and hardware”; particularly when a TSO is involved.
Personal opinion is that no one is ever going to care whether you have high-strength aluminum honeycomb under your carpet (if you can afford it), or high-strength plywood (whether aircraft or marine). Having said that, I’d like to think that an A&P-IA would question the presence of half-inch thick, three-ply, knot-holed CDX from Home Depot that weighs three times as much as the original boards (if they see them during an Annual). Though perhaps he’d be satisfied if shown a current W&B that followed the installation of the new, much heavier wood.
Response 5:
While good from a dissimilar metals standpoint, I’d be a little concerned that using aluminum floorboard screws might lead to a breakage problem due to them being so soft. You would also need to make sure you got aluminum sheetmetal screws as opposed to aluminum wood screws. You don’t want to use screws with tapered threads, as they will distort the receiving holes in the sheetmetal.
If you put a little dab of LPS2 or LPS3 on each screw hole before setting the floorboard, you should have no problems with stainless steel (or cad-plate) screws. If the interior is staying so damp that the screw type triggers corrosion, you’ll have far larger problems to deal with than the screws.
Regarding washers, the ones that I think look the best are the small recessed trim screw washers intended for use in soft or dimpled materials, and with an oval-head or flat-head screw. I believe that the correct number is NAS390, flush-type upholstery washers, as opposed to the more commonly-seen NAS391 cup-type upholstery washer (which sits on the surface and takes a countersunk-type screw). The flush type dimpled washer gets drawn down into the surface a bit (especially in carpet), where it is less obtrusive, while the slightly roll-flanged washer spreads the load (and prevents damage from the turning screw). Neither of these are the same as the Tinnerman-type, large-flange washer, that has the center dimple surrounded by a wide flat flange. The Tinnermans (in stainless steel) are terrific for plastic fairings (using countersunk-type screws), where they spread out the load and provide excellent part retention, while preventing wrinkling along the edge. Both the Tinnermans and the standard rolled-rim trim washers stand out a bit more than I like, when used on most interior parts.