Rebuilding airframe, what to do with corrosion.

sanships · Post by **sanships** » Sun Nov 16, 2003 7:29 am

Have started a total rebuild of the my aircraft. After 51 years, and mainly farm use, it is having it's first restoration. After paint removal, corrosion was found in fuselage, mainly in the joints of the outer skins. This prompted the total removal of all skins for corrosion proofing and repair. In the process, all electrical, fuel, vent and hydraulic lines will be renewed as required.

Are there any specific areas I should be looking for during the rebuild that should be mandatory? Any upgrades I should do? There was heavy corrosion on the firewall where the battery was, what can I do to fix this? can this be patched? Is it structural?

In general, the airframe is in pretty good shape for it's age and usage. The shop says it will be no problem to repair the damaged parts. It's only the parts mentioned above that got me concerned.

Thanks for any advice!

william halford · Post by **william halford** » Sun Nov 16, 2003 10:39 am

The FAA just issued a ageing aircraft inspection pamphlet. It goes in details on what to look at. I received mine thru the mail. I do not hve the pamplet ## but will get it and respond back to you..

I would get this and follow it to the letter..170mb

zero.one.victor · Post by **zero.one.victor** » Sun Nov 16, 2003 4:44 pm

william halford wrote:The FAA just issued a ageing aircraft inspection pamphlet. It goes in details on what to look at. I received mine thru the mail. I do not hve the pamplet ## but will get it and respond back to you..

I would get this and follow it to the letter..170mb

I just got mine in the mail,haven't really done more than glance at it yet. It's entitled "Best Practices Guide for Maintaining Aging General Aviation Airplanes". It doesn't really appear to have a publication number,but is referenced in the attached Special Airworthiness Information Bulletin No. CE-03-52,dated Sept 4,2003.

Eric

Post by **GAHorn** » Mon Nov 17, 2003 9:31 pm

The firewall IS structural, and can be either completely replaced or repaired. When performing restroation work, you should decide how far you are willing to go. One can easily become involved in a complete remanufacture of the aircraft which can be very expensive unless you are doing a lot of the work yourself.
I suggest you carefully inspect the tail and it's structural attachment and bulkheads, the wing attach and inner attach blocks, and the wing spar carry through sections.
Consider completely removing and replacing with new all the electrical wiring. This may be a task you can perform under your mechanic's supervision. Use only aircraft wire, preferrably meeting Mil-W-22759/16 (so-called "Tefzel" wire.)
Consider replacing all fluid carrying lines and hoses. Inspect also your pnuematic (air/static system) lines.
Strongly consider replacing all vacuum lines/hoses.

Walker · Post by **Walker** » Sun Jan 04, 2004 10:35 pm

It is a sticky and opinionated subject, the removal and prevention of corrosion. I am almost finished with the complete stripping of the fuselage. I agree with GA's advice. My last area is a few last sections under the floor. My arms need to heal for a while after first session of floor stripping. So here I am, every bit of grease and tar gone throughout the inside. It looks beautiful except for some mouse pee on the upper bulkheads in the rear of the cabin and a chalky upper half of the fuse aft of the cabin. I've removed everything that has prevented corrosion up till now. I've decided on Randolph zinc chromate for the interior and Randolph Strontuim Chromate for all exterior parts. I would like to leave the floors natural, but am concerned about corrosion. The rest of the interior I'll do my best prep and lay the chromate down lightly. Any advice on the long term results of what I'm planning? Would ATF or red brake juice be sufficient to stop corrosion in the floors? Does the tar that Cessna puts inside the fuse do more than sound deadening? Does it also help reduce structural fatigue as well? I would rather put a modern sound material in instead of the tar.

Post by **GAHorn** » Mon Jan 05, 2004 2:15 am

I never liked the "tar" treatment either, but I've never seen it in a 170. Walker, are you working on a 170 that you believe is factory-treated with that stuff? I thought Cessna only started doing that in the late 60's.
The British are reknown for anti-corrosion techniques and the only place I've seen them use strontium chromate is in bags tied to bulkheads in lower fuselage areas. What method do you intend to apply it externally?

Walker · Post by **Walker** » Mon Jan 05, 2004 11:44 am

I just assumed that Cessna had applied the tar. It was sprayed in, not brushed as I have seen on other Cessnas. The Strontium Chromate primer by Randolph is a two part epoxy. They highly recommend it as the base for Ranthane. I think the regular chromate is the best choice for the interior because you can see through it (when applied properly) easily for inspection and it is easier to remove than epoxy. I don't understand the strontium chromate in bags on bulkheads. Could you explain. Another place that is worth inspecting carefully for corrosion is above the doorframe. Mice like that area also. What do you think about leaving the floors natural with some petroleum based preservative? They look so nice natural and the possibility of a good paint based prevention seems impractical as it is almost impossible to even see all of it. The other nice thing about ridding the plane of all of the tar, grease, interior glue, paint, and fifty years worth of lost parts is the weight savings and that it looks so clean that other mechanics might actually take the time to clean out their mess before they put the plates back on.

rudymantel · Post by **rudymantel** » Tue Jan 06, 2004 12:57 pm

Yes George, please explain about the bags tied to the lower bulkheads-
Rudy

johneeb · Post by **johneeb** » Tue Jan 06, 2004 2:14 pm

George,

Just like Walker I have just finished cleaning all of the debris that collects in 50 plus years of airplane life. All around the lower half of my hull was coat with the same black tar that Walker mentions and the way that it was applied beneath the floor panel one could assume it was applied at the factory. I have no log record of the tar being applied after manufacture.

John

Walker the sores on my fore arms from reaching through those little round holes went away in about 3 days.

Walker · Post by **Walker** » Tue Jan 06, 2004 8:39 pm

Maybe the same sadistic A+P got after yours too. Seems like the applier held the trigger on a little longer for those long corners in the floors. There was no tar anywhere on things that ran through the floors. Anyhow as you scrape off all of the crud, you can envision weight savings. The area in the roof above the D window showed accelerated corosion in comparison to the rest of the inside. As mentioned before, some little furry friends had been hiding aboard also. Could it also be that there is lack of ventilation in that area because of the headliner?

johneeb · Post by **johneeb** » Wed Jan 07, 2004 1:23 am

Walker,
I did not find any corosion above the rear windows, there is an exterior panel in that area that covers the root of the wing flaps, this panel has some screws that penitrates the cabin skin and could leak and feed corosion groth.
I found lite corosion in several other places all of it treatable with Scotch brite wheels, alumaprep, alodine proir to Zinc chromate.

Joe Moilanen · Post by **Joe Moilanen** » Wed Jan 07, 2004 6:56 pm

My '53 B model Has zinc chromate applied between floorboards and outer fuselage skins and everywhere between the spinner and tailwheel. Am I one of the lucky ones that received the corrosion proofing options or did all '53 models get this treatment? It's applied so thoroughly that I'm sure it was applied during assembly.

Also, I have a spot of damaged skin (3" in dia.) right behind and inboard of the right landing gear leg in the lower fuselage skin that was repaired not so well with Bondo at some time over the last 50 years. I was thinking of installing a standard Cessna inspection hole/cover over this area as it would make a clean repair and also aid in access. What are the ramifications of installing one? Does it require a 337? Any specific rules on this location? I believe Cessna allows up to (5) additional holes in each wing if installed per guidelines but I can't find any specs on lower fuselage skin. Perhaps it was always there and I don't know how it got there? TIA.

Joe Moilanen

Post by **GAHorn** » Wed Jan 07, 2004 8:18 pm

Joe Moilanen wrote:My '53 B model Has zinc chromate applied between floorboards and outer fuselage skins and everywhere between the spinner and tailwheel. Am I one of the lucky ones that received the corrosion proofing options or did all '53 models get this treatment? It's applied so thoroughly that I'm sure it was applied during assembly.

Also, I have a spot of damaged skin (3" in dia.) right behind and inboard of the right landing gear leg in the lower fuselage skin that was repaired not so well with Bondo at some time over the last 50 years. I was thinking of installing a standard Cessna inspection hole/cover over this area as it would make a clean repair and also aid in access. What are the ramifications of installing one? Does it require a 337? Any specific rules on this location? I believe Cessna allows up to (5) additional holes in each wing if installed per guidelines but I can't find any specs on lower fuselage skin. Perhaps it was always there and I don't know how it got there? TIA.

Joe Moilanen

Hello, Joe! I suggest as a reference, the Service Manual, 100 Series, 1962 and Prior, Section 19 "Structural Repair". Especially look at Fig. 19-12 on page 19-23 for illustration on how to make an "insert patch" repair. This is most likely the minimum repair called for by the paragraph 19-43 (whcih defines Negligible Damage that requires no structural repair. Note: "Wrinkles occurring in the lower skin of the main landing gear bulkhead areas should not be considered negligible. The skin panel should be opened sufficiently to permit a thorough examination of the lower portion of hte landing gear bulkhead and its tie-in structure."

In other words, the area you mention is not eligible for "negligible" damage definition and therefore requires structural repair....not an inspection hole. If your inspector does not determine an insert patch adequate repair, then you should consider a complete skin panel replacement per the structural repair section. (Don't forget the edge flange replacement on whole skin panels are required per para. 19-46 Note.)

rudymantel · Post by **rudymantel** » Thu Jan 08, 2004 2:57 am

In the very early '60's and probably in the '50's, Cessna's optional factory-applied corrosion proofing was of a decent quality. The skins were alodined and zinc chromated prior to assembly. However, the rivet holes were then drilled and not to my knowledge protected.
In later years factory "corrosion proofing" was pretty useless; zinc chromate was merely sprayed in the airplane's interior after assembly.

I ordered my airplanes without "corrosion proofing" and when new promptly spayed the interior with a petroleum-based corrosion inhibitor which was re-applied annually. They were not hangared and lived in a highly corrosive environment (Montego Bay) but never suffered from internal corrosion. I still spray the interior of my 170 with Corrosion X at annual time.
Rudy

Walker · Post by **Walker** » Fri Jan 09, 2004 11:12 pm

Thank you for your input, Rudy. It seems I remember talk amongst the saltwater bunch that ATF was a nice choice of petroleum based preventatives. What we see is only an indication of how bad it is where we can't see. You can polish and scrub till the moon turns blue, but you can't do too much to where it really counts: where you can't see. I have drilled apart stuff with a flashy paintjob to find non-existant rivets. If you see action on the areas around a seam, you can be almost guarenteed of a lot more action where you can't see. The basic idea that I have worked around is that you rarely see a rusty oil pan ( granted times have changed). I always like to see what everybody else has been doing to combat the problem before making my own decison. I am leaning towards a petroleum based solution (after extreme "topical" cleaning). The drawbacks that I can see is a messy, dirt gathering treatment that needs to be CONCIENTIOUSLY applied on a regular basis. (The simple big words kill me sometimes as far as spelling) The big question is (if you are truly concientious) is what do you want to have to look at when the time the nightmare you are going through is due again? I always try to do work that will be looked upon the poor SOB behind my attempts has at least something to work with