The International Cessna 170 Association

I've purchased a set of BAS seatbelts and am prepping to install them soon. I have a question about the option for installing the nutplates in an A model (the B models already have the nutplates). BAS has two options, one is drilling a single hole in the bottom of the carry through spar and inserting the nut plates that way or the second option is to drill two holes in the top skin directly above the nutplate locations. If you've done the install, what method did you choose? If you have not, I'm open to your opinion.

Thanks,
Doug

The factory drilled a hole in the bottom of the hat section with no holes in the top skin.

The IA who installed my BAS shoulder harnesses drilled the bottom of the hat section, like the factory.

I would never drill holes in the top skin if there is another option, which there is.

Agree with Richard. Holes in the top skin are an invitation to water ingress.

There were already 5 holes in my top skin from current and old antennas. My A&P elected to drill the top holes. We bore scoped the spar piece, Corrosion X'd it and sealed it all back up.

Two current antenna located rear cabin top, and the big square hole from the VOR mast. A Winky sealed hole in front of that and an old Loran antenna hole above the co-pilots shoulder mount. That one was also plugged and sealed.

The interior is a current project for this year, thus the horrible mess waiting to be cleaned up.

But I am truly loving the BAS belts. Very well done, and I opted for the permanently attached shoulder belts which make donning and doffing particularly easy.

I've not installed the nut plates, or the BAS but I did install the Wag Aero STC,d attachment on which I installed AMSafe inertia reels. So I've done some work on the center carry through spar. While I was doing that work I actually stopped to contemplate installing the factory nut plates and how I'd go about it without drilling the skin. I thought it was very possible. As I'd already bought the Wag mount, that is what I installed. I wouldn't necessarily do that again.

So if I were you I'd exhaust all effort to install the nut plates without drilling the skin cause you can't undrill the skin once you do it and wished you didn't

You have me wondering how so many others have installed BAS without drilling the skin.

‘98C, being a1955 B-model, already had the nutplates in the rear spar, but the BAS installation required two more structural screws through the roof panel for each seat. We put them in in 1995, and they haven’t been an issue.

Thank you everyone for the reply's... keep them coming if you can.

I didn't want to 'lead the witness' with my preference, but I was leaning towards the single hole in the bottom of the carry through spar. The idea of a leak that would allow water to sit in the bottom of the carry through spar is what scares me about the holes in the skin, although the holes in the skin seem more minor with regards to the structural aspect. At this time I plan on the hole in the bottom of the carry through.

I also just saw this picture someone posted on Facebook of the nutplate in a 170B... I'm not sure why it surprised me, but I expected the nutplate to be oriented 90 degrees from this. I'll have to pull out the STC instructions again and look for that detail.

cessna170bdriver wrote:‘98C, being a1955 B-model, already had the nutplates in the rear spar, but the BAS installation required two more structural screws through the roof panel for each seat. We put them in in 1995, and they haven’t been an issue.

Yeah, those I understand, I was just leery of adding holes above the carry through and then having to seal them. If the holes you mention leak, then it would leak on the headliner, not hidden inside the spar. Glad to hear that it hasn't been an issue for those.

Doug

dstates wrote:
I also just saw this picture someone posted on Facebook of the nutplate in a 170B... I'm not sure why it surprised me, but I expected the nutplate to be oriented 90 degrees from this.

Cessna_factory_nutplate_170B.jpg

This little nutplate seems so undersized...until you recognize it places the mounting screw in Shear... and the appropriate screw has a very high strength rating.... it’s not common grade 2 hardware.

For those curious about the full installation instructs for the BAS, I’ll upload the instruct-sheet to the Mx Library in the Fuselage section.

Doug,
It would be a lot easier to just follow the installation instructions in the STC but I understand your reluctance to drill holes in the roof or carry through. If you're set on not adding any holes to your roof or carry through spar here's one possible way to install those nutplates. Drill a AD470-6 rivet to fit a fine wire or cable. Thread the rivet with a 10-32 die and then crimp the wire or cable in the rivet. Finally, cut the rivet as short as possible. Remove the fuel tank cover on one side. On the butt rib (p/n 0523507) there is a small gap that you can squeeze the nutplate through. Above the wing attach block there is about a 1/2" gap. Thread the cable through the mounting hole and out the fuselage above the rear spar attach block. Attach the nutplate and pull the assembly back into position.

I had a nice surprise today as I opened up the headliner to get ready to start the shoulder belt install I found that the rear carry through spar on my 170A already had the holes for the nut plates, just not the nut plates. That will definitely make it easier.

For those hoping the holes are already in their spar, my 170A is a 1951 with serial number 20118.

Here is a picture of the holes for the nut plate(in red box). You can also see another hole that actually does have a nut plate installed further outboard. Anyone know what that would be used for?
Doug

Your serial is not at the very-end...but close to the end of A-model production. I wonder if Cessna had not already accommodated it for the cross-chest shoulder harness available as an option shown in the B-model IPC. ...or perhaps used a carry-thru already in-production for the B-model...?

Or perhaps a repair has been made on your A-model which incorporated a replacement rear-spar carry-through...?? Stranger things have been known to occur... (We know of an A-model in which the right wing rear spar was actually a B-model spar .... having been installed on a hangar-floor (no wing jig) ...which helped explain why it refused to fly hands-off for decades until the owner asked Del Lehman to look at it.)

IPC for the B-model shows it as Fig 24, Item 7, nutplate...for shoulder harness.

The IPC image shows cross shoulder belts. The Cessna installed nut plates on my first 170 where directly centered behind each seat and would be inappropriate for a cross shoulder belt. I wonder if the Cessna optional belts were cross shoulder or is the IPC image incorrect.

I don't see why there is so much concern about drilling holes in the roof. If you hadn't noticed these aircraft come with LOTS of holes drilled in the roof, you just have to fill them up with something (rivet, bolt, etc,) and everything's fine.

Bruce Fenstermacher wrote:The IPC image shows cross shoulder belts. The Cessna installed nut plates on my first 170 where directly centered behind each seat and would be inappropriate for a cross shoulder belt. I wonder if the Cessna optional belts were cross shoulder or is the IPC image incorrect.

The cross-chest harness was a “kit” which could be ordered. Later, Cessna installed the centered nutplates for the “Y” (H) type harness.