Back Seat Attachment Question

[Robert Eilers]

I pulled my back seat in anticipation of flying into Minam River Lodge in July. I discovered the back seat had no bolts in the rear seat brackets (depicted on page 56 of the parts manual item #3 - An3-10A. I also noticed, studying the parts manual that a AN365-428 Nut is depicted for the rear attach point (page 56 item #5). My 170 seems to have some sort of nut plate revited into the floor for the rear attach point and the bolts for both front and rear attach points are something other than AN hardware. Basically, my rear seat is held in place by a front attach point and a rear attach point, the side bracketts are not in use. Page 56 depicts a AN3-10A bolt inserted at the side brackett location. Looking at the parts manual description I cannot comtemplate how the hell you would install the AN3-10A bolts, and afterward remove the rear seat .

[GAHorn]

Item 3 (AN3-10A) bolts, one on each side, bolts the seat to the side brackets. Those brackets have riv-nuts already installed in them, and the illustration has the bolt reversed incorrectly. The bolt actually points outboard, and slips thru the seat frame and screws into the brackets, just like that rear seat attachment does. (the floor has a riv-nut beneath it also.)
Before you install the seat, remove items 4 and use clevis's and pins to attach the seat back to the seat bottom. Install the clevis's with the heads inboard and the pins outboard. That way you can remove the seat back very easily, and later remove the seat much easier as well. (minor alteration)

[DaveF]

Item 3 (AN3-10A) bolts, one on each side, bolts the seat to the side brackets. Those brackets have riv-nuts already installed in them ...

On my airplane the captive nuts in the side brackets are stripped, and I need to replace them. Are you sure they're rivnuts and not nutplates? Looks to me like the steel threads start behind the aluminum bracket material. I also think I see the manufactured heads of some flat head rivets holding a nutplate in place on the back side. But I don't know.

Anyone on the forum know for certain what type of fastener is in there? Has anyone done this repair?

[GAHorn]

Dave, I apologize for being so careless with the terminology... Here's the picture I just took of the bracket so everyone can see exactly what is in there...it's actually a different fastener that's like a self-locking nut on top of a pedestal mounted to a nutplate: (My camera battery died, and i'll upload a side-view when I get it re-charged.):

click to ENLARGE

If yours is stripped-out, it might be easier to drill it out, let it fall into the empty cavity, and install a genuine rivnut, or alternatively, cut a 1-inch or so hole in the bracket down lower to fish-out the drilled-out old nutplate, and to hold up a new one with your finger, to be blind-rivetted into place. You could then fill the hole with a snap-cover/wilkie-button.

Here's that other side-view pic:

[DaveF]

Great pictures, thanks!

My mechanic wants to stick with nutplates in this application, so I'm going to drill the rivets and pull the side brackets out. Mine's not a rat plane, but neither is it so nice that a few touched-up rivets on the outside will seriously detract. The rivets will be easy to buck, too.

[170C]

George, there was an article written by C170 member John Gileno #1776 CT describing a method of replacing the nutplates being discussed here that does not require drilling out rivets or cutting holes in the bracket. I have a printed copy of the article that I would be glad to send to someone or to you to post on the forum. I don't know how to do so myself or I would do the posting. I gave a copy to the IA who was doing my mechanical work several years ago and he used the method described to replace the two nutplates in my plane. I don't recall if the article was in the 170 News or was on this forum, but possibly others reading this will recall it and can post it.

[DaveF]

Frank,

I was planning to start work on this tomorrow morning. Can you fax me the article? Can you take a digital photo and email it? How about typing up a summary of the method and posting it here? Did they drill out the old nutplate and then cleverly fish the new one down, or did it require some kind of modification of the side bracket like George suggested?

Dave

[170C]

Run a piece of safety wire through the bolt hole (in the nutplate) & up through the space between the top of the bracket and fugelage. This permits retrival of the old nutplate. They twisted a loop in the lower end of the wire so the nut wouln't fall off. Drill out the rivets that hold the nutplate. then pull the old nutplate up through the slot beteen the bracket & the fugelage. (You might have to file the slot some to permit the nutplate to pass through and for the new one to go back inside the bracket.)
Thread the rivet holes in the new cornernut/nutplate with a 6-32 tap. Drill out the rifet holes in brackets with a #28 drill so 6-32 screws will pass.
Insert a piece of safety wire in each enlarged rivet hole in the bracket and push them up throught the slot between the bracket and fuselage. Place the nutplate onto the wires and let slide down into place insied the bracket. Therad a 10-32 bolt through bracket into the nutplate so you can maneuver it into place and secure it with two 6-32 screws. Nutplates are MS 21081-3 or 22NA5-02.
I used the term "nutplate" where the original word "corner nut" was used. Good luck. If you have a question I will have the data with me and you can call me anytime after 8:00 CDST <edited ph no, mod>
Good luck!

[DaveF]

Thanks Frank. That's good info and it sounds like it should work. Interesting that the procedure calls for a corner nut but George's picture is a standard two-leg. Not sure which is in my airplane. I'll have to countersink the holes in the bracket for the 6-32s so the heads end up flush. Pictures to follow ...

[Karl Towle]

Proper rear seat fastening obviously affects "crash worthiness" since the rear seat belts attach to the seat. If I may, I'd like to extend this topic to the fore and aft "nut plates" in the floor. Specifically, the stripped aft nut plate can be felt through a port side inspection hole. Unfortunately there is a doubler on top of the floor, totally covering access to the flush rivets holding he nut plate under the floor. Does anyone have any good suggestions on how to replace this particular nut plate, short of installing a new inspection plate in the belly of the airplane? I could adapt the 6-32 screw/nut combination that was suggested for the sides, but was wondering if anyone had anything else.

[blueldr]

Don't even think of putting an inspection hole and plate in the belly skin. Put one on the floor of the baggage compartment.
Use a small hole saw to cut through the floor doubler to expose the rivets on the plate nut.
Drill out the rivets and remove the plate nut.
Make a plug for the doubler hole and fasten it in with the rivets on the new plate nut.

[n2582d]

If I may, I'd like to extend this topic to the fore and aft "nut plates" in the floor. Specifically, the stripped aft nut plate can be felt through a port side inspection hole. Unfortunately there is a doubler on top of the floor, totally covering access to the flush rivets holding he nut plate under the floor. Does anyone have any good suggestions on how to replace this particular nut plate, short of installing a new inspection plate in the belly of the airplane? I could adapt the 6-32 screw/nut combination that was suggested for the sides, but was wondering if anyone had anything else.

Here's what I would try. Buy a K1000-4 or AN366F-428A nutplate and install a short AN4 bolt in it. Cut the head of the bolt off and use the nutplate / headless bolt as a guide for drilling out the stripped out nutplate. Draw a fine straight line between the rivets on the doubler and then place the nutplate upside down over the existing stripped out nutplate with the mounting holes centered on the line you drew. Now, (if you're lucky), you will drill out the two rivets holding the stripped out nutplate. The holes on mine were one inch apart. Finally countersink the doubler and install the new nutplate.

[Karl Towle]

I was only kidding about cutting an inspection hole in the belly.

I like the ideas. I'm thinking of starting with #2, and then moving onto #1 if needed. I will try cutting off the shop head of the port side rivet under the floor, and then drilling up from beneath using an angle drill. If successful, then I can use the new nut plate as a template to drill from above, as suggested. I'll post when I've had a chance to try it. Thanks all!

[DaveF]

So it turns out that the center aft nutplate is riveted to the stringer, and not through the floor skin. That means even if you remove the doubler plate you still don't have direct access to the attachment rivets and still have to blind-drill them out to remove the nutplate. I wish I'd known that before I removed the doubler!

Also, beware that the parts manual is wrong about the aft bolt. Figure 31 shows it as an AN4 with a non-captive AN365-428 nut underneath. When I removed the nutplate, which I'm pretty sure is original, it turned out to be an AN3. Now I'm not sure whether to replace it with an AN3 or AN4.

Here's the doubler removed and holes drilled to remove the hidden nutplate.

Now here's a picture of the old nutplate in place, shot from below.

Here's the nutplate, removed and with an AN3 bolt threaded in, sitting on top of figure 31 from the parts manual.

[GAHorn]

...Also, beware that the parts manual is wrong about the aft bolt. Figure 31 shows it as an AN4 with a non-captive AN365-428 nut underneath. When I removed the nutplate, which I'm pretty sure is original, it turned out to be an AN3. Now I'm not sure whether to replace it with an AN3 or AN4....

Yes, there is obviously an error in Fig. 31. Notice they label that item as No. 5 (no textual description of that part), while the nut holding the seat-back to the seat-bottom is also labelled No. 5 , "attach seat back to seat bottom".

It's bluEldr's opinion this is likely one of the few pages in the IPC which is FAA-approved.