The International Cessna^® 170 Association

Hi
Just had a not so happy conversation with my AME. I'm glad he has his eyes open but not so thrilled over what he found. We're looking at rust and a white corrosion line between steel bulkhead stiffeners (all 4 of them) and aluminum bulkheads. see attached photo. We also have some rivets starting to smoke in the gearbox area on the front skin line between the gear legs. He suggested while we are repairing gearbox area to do pponk mod right away. Just looking for any good advice you all would have over these issues
Thanks
Kim

Sounds as if you've got a good inspector/mechanic.

OMG (that's oh my gosh) corrosion in a 65 year old plane? (round numbers to go with the round tail)!
Get some non aggressive ( the stuff approved for such uses by the Kansas City Chiefs) scotch brite and gently remove the powder, vacuum up the dust and dirt. I wouldn't try to make stuff shiny especially the steel stiffeners as all you will do is rub iron oxides into the alum. Bulkhead material. What your doing is cleaning enough to see if the surface corrosion is more than surface. After the powder is cleaned up I would apply my (your) favourite corrosion inhibitor to the parts and you'll be able to see if there exists further defects. if there's no cracks, bulges or buckling, you shouldn't be sad
The smoking rivets issue is likely engine oil flowing aft along the lower skin into the lap joint and finding its way out of that joint. You'll have to clean inside and have a close visual inspection of the forward bulkhead lower flange to ensure its integrity. Then determine if in fact there is movement between the parts and or if the rivets are loose or sheared. If required determine which ones need replacing, which again is nothing that should make you that sad if there's nothing big going on.
In my opinion your airplane has obviously been operating in a non dry environment. Your AME likely is used to working on newer aircraft and may want your airplane to look pristine. Ain't going to happen so I'd try to arrest the damage and monitor the area closely during annuals for signs that the damage is multiplying and the total rebuild can then be left to a museum to do after your gone.
I'd would keep flying it and would have no qualms about it. I 'm sure you have your comfort zone and the same goes for your AME as he may think differently than this AME and thats of course is his privilege.

Jim...YOU are the MAN when it comes to corrosion evaluation/treatment...no question about it.

But I was thinkin' if his inspector was looking at smoking rivets in person....they must be smoking rivets...not oily/dirty airplane..... as an A&P/IA "worth his salt" would likely know the difference. Of course, you and I do both know some guys with those tickets who aren't qualified to balance a lawn mower blade, so maybe his situation ain't as bad as described....

I was just thinkin' about how a picture don't always show everything that can be seen with the actual eye...
(How bout it? Got some pics of thsoe "smoking" rivets?)

If it COULD be cleaned up and treated (maybe with a little acid-wash, nuetralizing and then alodine, too? What do you think?) with a barrier-chemical such as ACF-50/Corrosion-X/etc., then if it's otherwise still sound, certainly avoiding a teardown/rebuild might be possible.

George somehow I know when your having fun with me Again Hopefully this owner will be able to keep enjoying his airplane.
Your also correct, I haven't seen his rivet issue but I can't imagine why the rivets would work loose in this area unless there's an underlying reason such as a bad repair or a crack that runs along the rivet line on the inside flange.
For all of us that try to suggest, suggestions to problems posed, being there would make things somewhat easier. Making a bad call is possible and possibly embarrassing when you don't have all the info but trying to be helpful is the goal. And in that regard George, your contributions and parts knowledge far exceeds my efforts and for that matter the efforts of many others.
No doubt your hillbilly brother will argue against that last sentence and I'll be disappointed if he doesn't!

Good morning
Thanks for replies. Yes there are a few loose rivets my AME wants to stay on top of. He's concerned about the "white powder" on the line between the steel strap and aluminum and thought we should possibly remove stiffener and clean it all up to make sure that area is still structurally sound but said I should check with you guys about the best procedure for dealing with this. I'm currently over in Saskatoon with my wife and her parents as mother just had hip replaced Monday, so can't really take pictures. Thanks for replies
Kim

Hi Kimball,

I would agree scotch brite is a great corrosion cleaner especially the little roloc pads you can use on an small pnewmatic angle grinder. The roloc pads are about 1.5 inch diameter. I like the dupont vari-prime which in a spot like that you can just brush it on. My airplane also had a case of the smoking rivets on the external skin just under the gear leg. There were about 5 of them. If they are the same place mine were you would need to remove the gear leg to buck the rivets. I did mine when I did the P-Ponk mod. I built a cradel to set the fuselage upon when I did the mod; two cradels one for front and back to keep the plane fairly level. My airplane sat out at least 10 to 15 years of its life and it gets pretty windy around here especially during the winter months. I think that's where a lot of the smoking rivet issues came from. I lost a lot of sleep when it had to set out 3 of the 27 years I've owned it. Good luck, hope your corrosion is not too serious.

Just a couple of thoughts here. I wouldn't recommend cleaning too aggressively, as I'd worry that the acid etch or alodine would end up in various crevices or deep pits, exacerbating the issue. You'd want to ensure that both products were neutralized and removed with plenty of water whenever you utilize these products and that wouldn't be easy in this area.
As for the rotary discs, be wary of the amount of heat that can be generated, especially the aggressive grades. The heat can ge sufficient to alter the properties of the metal when using an angle pneumatic die grinder.

we had the same problem on a 170b that was used for float plane operation.However the steel supports were very well corroded and the aluminium supporting structure had to be replaced also.Thsi aircraft was in pristine condition and it was a Hugh shock to the owner. the steel supporting structure had to be manufactured in house (still waiting on cessna to get back to us regarding this problem). if you have white "dust"between the steel and the aluminium you have PROBLEMS. So do not be afraid to look deeper.on reassembly Mastonix was used between I the steel and the aluminium.

Is there nothing then, from the factory between the two to prevent this dissimilar metal corrosion? If not they can all be expected to have corrosion.

ghostflyer wrote:we had the same problem on a 170b that was used for float plane operation. ... on reassembly Mastonix (sic) was used between I the steel and the aluminium.

I'd never heard of Mastinox until now. While it sounds like a good way to prevent corrosion, the little reading I've done about it online seems to indicate that it should be handled with caution. In a comparison between Mastinox and Cor-Ban 27L the Coast Guard said, "Based on the examination results of this evaluation, H-60 Prime Unit recommends H-60 fleetwide implementation of Cor-Ban 27L. Cor-Ban 27L performed as well as Mastinox in the various applications examined during this evaluation. As Mastinox is a known Carcinogen and a suitable replacement has been identified, it should be immediately removed from H-60 fleetwide use." I found that conclusion here. FredMa, you seem to be a fan of Cor-Ban. Is it any less dangerous to use than Mastinox? How thick is this stuff applied? Wouldn't rivet strength be compromised if this stuff was much thicker than a coat of paint?

We have been using Mastonix for many years with success and do note the dangers of using it. However it is applied using gloves and carefull application and disposal of utensils. A thin smear [3 thou] thick is all needed. The two parts are then joined together and the excess is wiped away. the area is then coated in at least 2 types of primers and a 2 pak final coating over the top to seal the 2 parts. Just finished the annual on a Piper cherokee 180hp fixed gear, the undercarriage legs mounting to the spar area was Mastonix about 5 years ago after finding heaps of corrosion in this area . No corrosion was detected . This is a very corrosion prone area.

Something I should have added , after reassembly this time the mastonix had hardened a little but seamed ok to reassemble ,however we sprayed liberally Ardrox
AV 8 and this provides a hard dark colored protective coating to all the exposed parts. This has been a very successful product. Have used LPS-3 for years but it does dry out ,but the AV8 doesnt .

On my 52B I was going let Corrosion X pentrate between the two surfaces at all 4 locations. All floors are off so its easy access. All mine look fine. Are all 4 corroded. If only 1 or 2 locations could have been a water leak that ran down the door post over time. C195s had a problem with the root vent scat hose wire rubbing thru and rain water running down the post internal.I wonder if they looked in the inspection holes in the floor above the gear to see how the box looked inside.
Mike

Cor Ban 27L contains no toxic substances. It is the consistency of grease and does not harden. It isn't like a coat of paint, it would simply squeeze out during assembly as would mastinox, which contains chromates. I hate Mastinox simply because it is so messy and the color is puke yellow. My jeans end up with bright yellow spots all over them and you can't get the stuff off. Wearing latex gloves does help keep it off your hands but it seems to get everywhere else too. I guess it works fine, just don't see a reason to use it when there is a non toxic more user friendly substance that is just as good.