Belly skin Wrinkles

[Indopilot]

Hello to all you holders of vast knowledge aeronautical ( have opinions anyway)
I have done a couple searches on this topic and have not come up with anything. Of course being computer challenged, that doesn't mean a whole lot.
My question is concerning the wrinkles that seem to develop behind the main gear bulkheads of 170B's. Mine is a 52 S/N 20446 In looking things over, there doesnt seem to be enough or any support besides the skin itself. Apparently Cessna realized this at some point since from the 172 on there is additional stiffning installed.
Has anyone addressed this issue and if so what was the solution? The wrinkles I am dealing with don't seem bad enough to go to the extreme hassle of replacing the belly skin, I was thinking more along the lines of doublers. Look forward to the sage advice I have come to expect from such distinguished gentlemen. Brian

[doug8082a]

I'm not familiair with belly skin wrinkles being "common" as your question seems to suggest. Sounds to me like something happened to your aircraft while in the care of a previous owner. After 52 years of service, the belly skins of my '52 are just fine as are several other 170s I've seen.

As for landing gear beef up options, there's the landing gear mod from PPONK you could investigate:

http://www.pponk.com/HTML%20PAGES/gldr.html

[zero.one.victor]

My airplane has the wrinkled belly you refer to, I assume that it came from groundloop-caused stress. Probably where yours came from, whether it's an NDH airplane or not. I think NDH just means there's no recorded HISTORY of damage, NOT that there's never been any damage!

Eric

[GAHorn]

Wrinkled belly skin in the area of the gearbox indicates unrepaird damage or possible stress due to hard landings, etc. The Pponk mod does nothing for this. It only beefs up the upper main gear leg attach points.

[Roesbery]

My current plane had wrinkles behind the left gear leg from the previous owners ground loop. My A&P IA put in a angle stiffener length wise and it has stableized the area so far, even on some pretty rough ground.

[mrpibb]

My 48 has some right behind the gear legs, however inspection of the gearboxes show no problems, I'm thinking is from a hard landing causing the gear to flex enough to wrinkle the skin. when I get some time I plan on reshooting a belly skin.

[c170b53]

My aircraft has a very small wrinkle as well behind the gearbox as well. When aircraft structure is stressed beyond design the metal will deform at the weakest point. The gearbox area will flex to a lesser degree than the skins surrounding the gearbox and thus the transferred load has caused a permenant deformation of the skin. This does not mean that the skin will fail but may cause further in service damage such as cracking should it be subjected to further excessive stresses. Likely no damage will be found in the bulkheads in this area but of course they must be inspected to ensure the damage extent. In other words I wouldn't worry until you see pulled rivets or cracks.

[DensityDog]

Indopilot, I have seen a lot of 170s with those wrinkles, probably from past ground loops, or almost-ground loops, and hard landings.
Check out older 180's and 170's, some have large skin-doublers in this area (just aft of the gearbox running aft), which are most likely reinforcing the wrinkled skin underneath.
I don't know the exact authority for those types of repairs, but it's easier than doing a new belly skin. Not pretty, though.

[Indopilot]

Its good to know that I am that I am not looking at the only two 170's with this problem. Not that I would wish this on anyone, but it tells me I am not imagining things. The second A/C is a 54 model with wrinkles worse than mine.
I have access to 185's 206's a 210, 180 and a 172 so I have been taking notes as to how that area is built in those other models. They seem to use a scalloped doubler reinforced with stringers or hat channel or both tieing into the aft bulkhead flange of the gear box assembly on the inside of the skin.
I am thinking of trying to copy what Cessna did in some manner. As far as approval I think that would be a minor repair/alteration since I am only adding to what is already there, only requiring a log book entry.
Thankfully my PMI is one of the good ones you can ask questions of and get good reliable answers from the "how can we make this work perspective"
We just drilled a wrecked 206 apart that has joggled hat channels that are good from that simular area so they might be what I will use. Does infusing 206 parts into a 170 let you go faster or carry more? Thanks for the replies, let me know if anybody has more expieriance whit this. Brian

[sharp]

I am going through a similar situation on a ground loop repair where the repair facility is requiring me to replace the skin even though their appears to be no real structural damage. I was told that doublers in this area was not allowed and the belly skin would need to be replaced in accordance with the cessna service manual. If there is any information contrary to this info it would be appreciated as this repair is running close to $ 4000.00! As a new 170 owner who had not owned the plane a week before it was ground looped ( I didn't even get the satisfaction?? of being the one to ground loop the thing ) I am just excited to get it back in the air!
Any info would help and maybe even some wheel pants!!

Thanks
Phil

[punkin170b]

Yep, we've got those wrinkles too - they've been there since well before we acquired the airplane. There are groundloop repairs documented in the logs, and would seem to explain the wrinkles. The belly skin was not replaced (although it should have been). The wrinkles were one of our bargaining chips in getting the price down - although we're gonna pay for the repairs now...

We're getting ready to reskin section of the belly just aft of the gear legs, as we're about to strip & paint the airplane. I am interested to know about any strengthening modifications that have been done and approved as well. We enjoy the backcountry although we are going to be a little less aggressive after all the restoration is done!

Our airplane has the PPonk modification with 180 gear legs. We can't actually find when it was done in the Canadian logs, so we don't know if it was modified before or after the ground loop incident. Yet another mystery to resolve and document...

Matt

[GAHorn]

Ok, Guys (and Gals)...Cessna has specific instructions about this exact type of damage. Local opinions and hopeful/wishful thinking doesn't count.
Everyone pull out their Service Manuals (100 Series prior to 1962) and take a look at section 19, "Structural Repair".
Read para. 19.43 and the "NOTE" following on pg 19-27 where it states:
"Wrinkles occurring in the lower skin of the main landing gear bukhead areas should not be considered negligible..."
Then read para. 19-52 "Landing Gear Bulkhead Assemblies" and para. 19-53 "Replacement of Portions of Skin Panels" (regarding the same landing gear bulkhead areas) where it states:
"In case metal skin is damaged extensively," (this means other than negligible-ed.) "...repairs should be made by replacing an entire sheet panel from one structural member to the next. The repair seams should be made to lie along stiffening members, bulkheads, stringers and the like. Each seam should be made exactly the same in regard to rivet size, spacing, and rivet pattern as the parallel manufactured seams at the edges of the original sheet."

Sorry to disturb anyone's peace of mind on this commonly found area of damage...but notice that insertion of patches, doublers, and stringers are not part of the repair instructions. It must be repaired in accordance with the structural repair manual. Patches, stringers, doublers, etc. are all short-cuts which do not meet the mfr's repair instructions. Any other method requires an engineering/approval basis AND...a Form 337! This is not a minor repair.
It is structural damage that should be properly addressed.

[c170b53]

SRM'S are standard repairs, all others repairs require paperwork agreed. Modern SRM's clearly define damage, I'm not sure the 100 series does define a wave verses a wrinkle verses a crease. Regardless all SRM's contain weasel words such as "should" and are open to interpretation. Its still up to the interpretation of the documentation and the assessment of each individual aircraft's damage to ascertain whether a repair is required. Its obvious that many aircraft have suffered minor damage which has not progressed into further degradation of the structure over a significant time frame. Thats why there are inspection programs, to periodically assess known damage and detect hidden damage. Obviously progression of the damage into metal failure such as cracks and pulled or sheared fasteners requires repair.
And yes George is right, the above is my opinion. Your I.A. and another I.A. may or may not agree, just as one's experience or judgement may lead them to insist on immediate major repair, another may choose a different approach.

[punkin170b]

In our case, the belly skin damage on our airplane has been inspected by 3 different IAs since purchase almost 5 years ago, and it has been passed as "ugly but airworthy" every time. We are doing the repairs for peace of mind, the fact that it "should" be done, and aesthetic reasons prior to paint. We don't have any cracks, sheared fasteners, or other visible or degrading structural problems. We'll see what we find when we open her up. I do think that the repair "should" have been done years ago when they had the gear box opened up for repairs after a ground loop.

Once again, opinions and interpretations vary greatly, all can be respected; but who's to say which one is exactly "right"?

Matt

[GAHorn]

c170b53...Quite right you are! Opinons of our professional repairmen are what we correctly rely on. I certainly don't mean to imply that long-distance opinions of never actually seen damage can be properly assessed. I only wanted to mention that there ARE resource documentations we can rely upon and ....of which our repairmen should avail themselves.
When I inquired of a FSDO MX inspector about the term "should" (regarding a Beechcraft MX manual statement that a cracked windshield should be replaced prior to furhter flight)...he responded (on letterhead) that: "...The root word of Should is...SHALL!"
I was convinced.