Major Airframe Repair Alignment Query?

How to keep the Cessna 170 flying and airworthy.

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Ryan Smith
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Major Airframe Repair Alignment Query?

Post by Ryan Smith »

Hi All,

My first foray into aircraft ownership isn’t going so well. Not totally unexpected, but I was thinking it was something that could be addressed in a couple of years with an upgrade, not needing to be addressed for airworthiness concerns.

My IA found a mess under the stabilizer when they jacked the airplane up to inspect it. Several cracks in the skin from sta. 205.812 to 228.687. The old steel tailwheel mounting bracket had several old cracks, the rudder stop blocks had a sheared rivet each sided and the sta. 228.687 bulkhead has several cracks. As this is an owner-assisted annual, they’re leaving me to my own devices to determine a solution.

My shop does not have a fuselage jig, and no shops around here do either, that I’m aware of. To be honest I’m not sure a fuselage jig even goes past the rear door posts. I have concerns about maintaining a straight aft fuselage with no warping or twisting. I realize that plumb bobs and laser levels are one’s friend, but am I well served to try to make an alignment fixture, or is it reasonable to expect that the holes align to give me something workable?

I have committed to the purchase of a tail section from an airplane from which I’ve previously purchased parts and plan to use that for my repair. I thought perhaps that utilizing the upper and lower skins from sta. 172.000 to 205.812 to help stabilize the repair further even though the tailcone reinforcement angles extend into the fuselage splice skin quite a ways forward towards the sta. 172.000 bulkhead.

I know several here have done this repair in the field in recent years.

Let me reiterate, this will be performed along side an IA of one form or another…likely the gentleman that helped my Dad restore the airplane 30 years ago, but I would like some guidance on what to expect and how to prepare so that when the time comes, I don’t have my airplane sitting for weeks or months with the tail removed or end up with a wonky repair.
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GAHorn
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Re: Major Airframe Repair Alignment Query?

Post by GAHorn »

Ryan, (Firstly, just to remind others, since a “steel” tailwheel bracket was mentioned, it is assumed we are discussing an “A” model)….

… the sta 228.687 bulkhead is the rearmost bulkhead which includes the hoiz stab aft attach, and is a common problem area of ALL Cessnas for cracks as it plays such an important part of the tail/empennage support structure. You may wish to post pics or draw a diagram to illustrate the cracked area… to get opinions on a patch/plate or doubler…versus replacement/rebuild of the entire bulkhead.

Secondly, Ryan, you may wish to consult the “Rigging Instructions” on page 6 in the Mx Library to review the symmetry checks of the airplane in order to preserve the proper correlations of the airplane symmetry/rigging.

viewtopic.php?f=41&t=8196

Station 205.812 bulkhead is identtified as Item 74, and station 228.687 bulkhead is Item 87 (or 86 if you mean the entire assembly)

Considerable discussion has occurred previously here in these Forums with regard to the attach angle and its’ mounting holes (round or oblong) (Item 88), and you may wish to revisit those also.
7E0B9C97-B5DC-425E-AAE7-81B6A3DE9C73.png
60E6207D-4424-4C3E-82EA-7251589E5561.png
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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Ryan Smith
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Re: Major Airframe Repair Alignment Query?

Post by Ryan Smith »

GAHorn wrote:Ryan, (Firstly, just to remind others, since a “steel” tailwheel bracket was mentioned, it is assumed we are discussing an “A” model)….

… the sta 228.687 bulkhead is the rearmost bulkhead which includes the hoiz stab aft attach, and is a common problem area of ALL Cessnas for cracks as it plays such an important part of the tail/empennage support structure. You may wish to post pics or draw a diagram to illustrate the cracked area… to get opinions on a patch/plate or doubler…versus replacement/rebuild of the entire bulkhead.

Secondly, Ryan, you may wish to consult the “Rigging Instructions” on page 6 in the Mx Library to review the symmetry checks of the airplane in order to preserve the proper correlations of the airplane symmetry/rigging.

viewtopic.php?f=41&t=8196

Station 205.812 bulkhead is identtified as Item 74, and station 228.687 bulkhead is Item 87 (or 86 if you mean the entire assembly)

Considerable discussion has occurred previously here in these Forums with regard to the attach angle and its’ mounting holes (round or oblong) (Item 88), and you may wish to revisit those also.
7E0B9C97-B5DC-425E-AAE7-81B6A3DE9C73.png
60E6207D-4424-4C3E-82EA-7251589E5561.png
AFAIK, there were only two tailwheel mounting brackets, the smaller steel one and the larger fish mouth bracket that was on the later B models. I guarantee without a shadow of a doubt that my airplane left the factory as a 1952 B model by year and by serial (20408) with this bracket. I was always of the impression that the bracket changed at some point in 1953 either coincident with the introduction of lady legs, or the 1954 year model. I recall seeing paperwork advertising the aluminum bracket as a service kit to upgrade one’s steel bracket.

The minutiae of the aforementioned notwithstanding, the bracket has several cracks in it and should have been replaced decades ago with an aluminum bracket. There are several skin cracks propagating from the edge of the skin over top of the sta. 228.667 bulkhead. One rivet head on the left rudder stop block has sheared off, and both lower rivet holes of the rudder hinge have linear cracking from the rivet holes through the edge of the bracket.

I’m not terribly concerned about trammeling the stabilizer, while my attach blocks have slotted holes, there doesn’t appear to be an appreciable amount of play (unless shims are installed) to allow for any meaningful movement.

My main question relates to ensuring determining how straight the new parts/replacement is as it’s being riveted onto the airframe. Should I create a fixture that triangulates some indexing holes to verify that it’s not being installed with any warp in any axis?

I’ll search for threads on the subject. I know an A model that was based at my field that I flew some had a similar repair performed within the last year with a new owner.
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GAHorn
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Re: Major Airframe Repair Alignment Query?

Post by GAHorn »

Ryan Smith wrote:
AFAIK, there were only two tailwheel mounting brackets, the smaller steel one and the larger fish mouth bracket that was on the later B models. I guarantee without a shadow of a doubt that my airplane left the factory as a 1952 B model by year and by serial (20408) with this bracket. I was always of the impression that the bracket changed at some point in 1953 either coincident with the introduction of lady legs, or the 1954 year model. I recall seeing paperwork advertising the aluminum bracket as a service kit to upgrade one’s steel bracket.
…..
This is an example of how important it is to post information as regards Serial Numbers…(rather than Registration Numbers….and Model designations…) when asking for help. In this discussion you didn’t mention either …until this last post.
I don’t think very many of us can recall off the top of our heads Which Model is being discussed when help is solicited….unless we Post that information ..when we introduce a new topic thread.

SERIAL numbers are so very important. [edit:For example, the IPC for the A-model only shows a steel tailwheel bracket…while the IPC for the B model only shows the aluminum bracket in the tail-cone area….and shows the earlier serialed B-models tailwheel bracket in Fig 29 where the Scott 2400 tailwheel is shown. ]

BUT…we all know the IPCs are not “approved” documents and contain errors.

A LITTLE KNOWN [EDIT] is that the B-model PRIOR TO SN 20806 …. utilized the earlier 170 and 170A steel bracket.

[deleted]

The steel bracket was notorious for cracking and causing fuselage and bulkhead cracks because it had such small “purchase”… small attach area…to the fuselage which transmitted considerable shock to the tailcone.

This dubious design was quietly admitted when Cessna introduced the Service Kit SK70A52-4 for replacing the steel bracket with the aluminum bracket.

viewtopic.php?f=40&t=9442

Hope this helps.

PS: Everyone…. if you go to your Forum PROFILE by clicking on your UserName here in these Forums…..then click on “User Control Panel” at the Upper-Left of the page …..and Edit your SIGNATURE…you can include your Serial Number, Model, Number, etc …so that Each Time you post a question…anyone with answers or who attempt to help you…will immediately SEE which model and which serial you have. See my signature for an example at the bottom of this post. Again…hope this helps.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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c170b53
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Re: Major Airframe Repair Alignment Query?

Post by c170b53 »

Just my opinion; the less invasive the repair the easier, so you have to decide what can stay, what you can live with and what must go. With the more amount of structure you replace likely there will be more rivet holes that do not align and you’ll have to start replacing skins. Maybe you have to do that anyway, hard to judge without pics but again its your call to the degree of repair you’re undertaking. In terms of the very last bulkhead, it should be easy to remove and replace, you can use the stab to assist in alignment, along with measurements. Hopefully the holes will be close, other wise you will have to upsize rivets, if not close then a new skin and a bigger repair, picking up the holes at the next bulkhead. One thing that I’m curious about is whether there’s the small horizontal stiffener inside the tail-cone of an “A” as there is with the “B”, Item number 29 in fig 25 in the “B” ipc and whether that horizontal stiffener was only introduced with the fish mouth or just not well depicted in the “A” Ipc.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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GAHorn
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Re: Major Airframe Repair Alignment Query?

Post by GAHorn »

Jim, I don’t remember where exactly (this is becoming more common in my old age as shown both above and below)… but I brought that Item 29 PN 0512126 “shear bracket” to the club attention when the SK70A52-4 was first published in these Forums a couple decades ago. That SK does not even mention that shear bracket and is not included in the parts list…. I have no idea why, or how many airplanes are operating without it. Here’s the SK70A52-4 parts list:
59DC6E46-1888-4F0F-A65D-9B153E2A6C12.png
(and as you can see from the rear fuselage / tail cone illustration for the A-model above… such a shear bracket is not shown…plus, without the matching aluminum bracket for it to reinforce….and without any indication of a rivet-line… I don’t believe the A-model used one.)

Also, I edited my previous comment re: the steel bracket in the B-model, in that the B-model IPC actually does mention the steel bracket …just not where previously expected… Instead of the tail-cone illustration where the tailwheel bracket is addressed… it’s shown in Fig 29 where the small Scott 2400 tailwheel that no one has is shown. My bad. :?
D1B4796E-BF2E-42BF-9C27-DF39B7DB11E7.png
'53 B-model N146YS SN:25713
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An originality nut (mostly) for the right reasons. ;)
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c170b53
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Re: Major Airframe Repair Alignment Query?

Post by c170b53 »

Sorry George I have the same affliction, Looking at the IPC’s, I now see that the A has the shear bracket as well just not very well depicted (A ipc pg 22 item 94) thus the tailwheel bracket (regardless of bracket type) top rivet line picks up the tailwheel reinforcing angles and that tailwheel mounting shear bracket.
So likely repair would be to drill out old style bracket, remove aft bulkhead, install serviceable aft bulkhead and see whether the bulkhead can be installed based on the fit and alignment of the majority of the rivet holes or whether a new skin is required and whole lot rivet removal. I would think the reinforcing angles along with the vertical and horizontal bulk heads would provide a good guide to position the skin on to the bulkhead forward of the aft bulkhead. Making a jig to fix the position of the stab attachment angle wouldn’t hurt either if a new aft skin is fabricated.
Del probably has a better method.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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GAHorn
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Re: Major Airframe Repair Alignment Query?

Post by GAHorn »

c170b53 wrote:Sorry George I have the same affliction, Looking at the IPC’s, I now see that the A has the shear bracket as well just not very well depicted (A ipc pg 22 item 94) thus the tailwheel bracket (regardless of bracket type) top rivet line picks up the tailwheel reinforcing angles and that tailwheel mounting shear bracket.
So likely repair would be to drill out old style bracket, remove aft bulkhead, install serviceable aft bulkhead and see whether the bulkhead can be installed based on the fit and alignment of the majority of the rivet holes or whether a new skin is required and whole lot rivet removal. I would think the reinforcing angles along with the vertical and horizontal bulk heads would provide a good guide to position the skin on to the bulkhead forward of the aft bulkhead. Making a jig to fix the position of the stab attachment angle wouldn’t hurt either if a new aft skin is fabricated.
Del probably has a better method.

I think you mean FIG 22…not “pg 22”. …and I see what you mean. What a sorry illustration of that part in the A-model IPC…as it doesn’t really depict it at all. (appears to me to be illustrating the item 94 posted TWICE…. PN 0512118-7 “horizontal bulkhead”) .
5F12A8FF-1C1D-4902-905F-68AA6931DA22.png
'53 B-model N146YS SN:25713
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An originality nut (mostly) for the right reasons. ;)
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cessna170bdriver
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Re: Major Airframe Repair Alignment Query?

Post by cessna170bdriver »

Ryan, you are correct that the factory fuselage jig ends at the rear door posts. To get my aft fuselage straight Del constructed a fixture around a known good fuselage (Paul’s, I think), then used it to build mine. You might be able to construct such a fixture around your fuselage before you take it apart.
9FF242C1-57C9-4E1B-8EAF-BB1074777576.jpeg
Miles

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GAHorn
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Re: Major Airframe Repair Alignment Query?

Post by GAHorn »

cessna170bdriver wrote:Ryan, you are correct that the factory fuselage jig ends at the rear door posts. To get my aft fuselage straight Del constructed a fixture around a known good fuselage (Paul’s, I think), then used it to build mine. You might be able to construct such a fixture around your fuselage before you take it apart.
9FF242C1-57C9-4E1B-8EAF-BB1074777576.jpeg

Ya gotta love that guy! Del does amazing work!
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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c170b53
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Re: Major Airframe Repair Alignment Query?

Post by c170b53 »

My comments were aimed at using a used aft bulkhead, hope Ryan can find a new part that would make things much easier.
Looking at the pic’s from Mena looks like a new bulkhead from a impressive secret stash somewhere. :D
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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Ryan Smith
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Re: Major Airframe Repair Alignment Query?

Post by Ryan Smith »

At the behest of quite a few people, I have begun sourcing new parts for a repair rather than grafting on a new tail.

That being said, I am on the hunt for a NS or NOS rudder hinge 0512109. Anyone know of the existence of one?

Thank you!

Ryan
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