GA Alert, Fuselage Spar Assembly (170 News, 3Q06)

[jrenwick]

Hi George,

In the front of the latest 170 News there's a "General Aviation Alert" about fuselage front and rear spar assemblies, which recommends removal of a wing annually to check for corrosion in the wing spar carry-through structures. Could you discuss this here, please? I didn't receive anything from Cessna about it, and I can't tell from the 170 News article if it applies to all 170 models or just the straight 170s. When was this issued? Who issued it? Is there any other supporting documentation?

Thanks for any help you can give!

Best Regards,

John

[tshort]

Wonder how this applies to us ragwing guys? Mine was completely disassembled and recovered within the last 3-4 years ...

Thomas

[GAHorn]

Unfortunately, this article was apparently used for "filler" for the 170 News. The article has dubious origins, in my opinion, having come from another un-official enthusiasts magazine.
The Cessna 170 forward (and rear) spars are attached to carry-through "hat" sections which follow along the upper cabin, above the headliner. The spar has a thin "prong" which fits into the hollow of the hat-section, and is secured there with a large bolt running fore/aft. To prevent the crushing of the hat section by that bolt, and to lend strength to the area, a machined aluminum "block" is inserted into the hat-section, along with the spar-prong, and the bolt is then inserted and tightened.
This block and internal structure of the hat-section has been hidden from view for over 50 years on most airplanes, unless some reason has been found to remove the wings during that time.
Should a previously never-removed wing be taken off, there is a fair chance that some degree of corrosion will have formed in the structure, especially on aircraft which have spent the predominate part of their histories stored outdoors. (Water will migrate to the block and remain there, providing an electrolyte for the corrosion process.)
My own airplane was rebuilt, and completed in 1996-97, and the original blocks resembled exploded wood due to intergranular corrosion. The restorer replaced them and also scrubbed out the hat-section with bristle bottle-brushes on long handles, then used chromate-soaked rags pushed through with sticks to treat the internal area of the hat-sections against future corrosion.
While I endorse any opportunity to inspect this area for corrosion, I feel that unless clues exist to indicate that such corrosion already exists, then the article's recommendation to inspect that area "annually" by complete removal of the wings or drilling holes thru the cabin roof into the carry-through to be overly aggressive and unnecessary.

In discussions with knowlegeable inspectors (and our own Cleo Bickford) I have come to agree with them that a precursory inspection may be called for, before drilling holes and removing wings. (Drilling holes may constitute an unauthorized repair scheme. Check with your inspector before drilling any holes.) I believe the following method will serve the purpose of determining the airworthiness of the airplane in this area.

Remove the lower wing-to-fuselage fairing, and visually inspect the general condition of the area. Open the headliner at it's zipper, and edging, and with a strong light and mirror, inspect the carry through hat section for evidence of water damage and external corrosion. Examine the wing attach-bolt where it passes through the carry-through hat section at it's outboard ends (where the wing attaches.)
If the bolt shows signs of corrosion and/or rust, support the wing and remove the bolt in order to inspect it's shank and the internal area of the block/hat-section for corrosion. If the bolt appears rust/corrosion-free, loosen the nut one complete turn, then attempt to rotate the bolt within the hat-section. Do not rotate the bolt excessively...the purpose is merely to determine if the bolt is seized or not. If the bolt gives any sign of having seized due to corrosion, it should be removed and the condition of the block/hat-section should be further inspected.
LPS #1 is a penetrating oil and may help in the above procedure. LPS #3 is a congealing preservative, and can be used to lubricate/preserve any bolts, blocks, hat-sections, prior to installation and/or re-installation after any aluminum parts have been treated with chromate or epoxy polyamide.
Rear carry throughs may or may not contain a block, depending upon serial number. But be careful when checking that bolt not to alter the position of the concentric bushings that bolt passes through, or you stand the chance of changing how your airplane handles. (Those bushings are used to adjust a "heavy" wing during final flight testing.) Check your Illustrated Parts Catalog and compare to your aircraft's serial number. Otherwise, the rear bolts should be checked in a similar fashion as the forward bolts.

While the wing attach blocks and bolts certainly are a part of the structural integrity of the airplane, they are not difficult for the average owner to inspect visually to determine if further investigation is warranted. Go out to the airplane with a screwdriver, remove the lower wing-to-fuselage fairing and take a look. If the bolts are found to be corroded/rusted, then the inspection can be carried further under the supervision of your mechanic/inspector. Otherwise, simply include the internal inspection in your list of "squawks" for the next annual inspection. (Your inspector should have at least visually inspected these bolts at every annual already. It's doubtful that any airplane that's been properly inspected will have had this area neglected. While it's important, I don't think there is a wide-spread "the sky is falling" situation that this old article reprint suggests.

This is my own opinion, and is worth what you paid for it.

[jrenwick]

Thanks, George! I appreciate your thorough treatment of the subject!

John

[170C]

George, I have never seen this internal inspection preformed, but if it is necessary to remove the wing, even if only moved horizontally a few inches won't this necessitate the disconnection of the control surface/flap cables as well as emptying the fuel tank & electrical wiring? If so it would be a pretty involved project for either or both wings.

[GAHorn]

George, I have never seen this internal inspection preformed, but if it is necessary to remove the wing, even if only moved horizontally a few inches won't this necessitate the disconnection of the control surface/flap cables as well as emptying the fuel tank & electrical wiring? If so it would be a pretty involved project for either or both wings.

That is correct, Frank. It's a big job to remove the wing. The factory shop manual rates it at 4-6 man hours. (And don't forget to support the opposite wing during removal.)

[spiro]

to remove the wing. The factory shop manual rates it at 4-6 man hours.

didn't know Cessna published one of those; where do I get one?

[GAHorn]

I don't know if Cessna still offers their version of the "flat-rate" manual or not, but it once was available and sometimes they show up in old hangar sales, etc. (When the Tyler, TX Cessna dealer dropped their dealership in the mid '90s they had tons of that kinda stuff sitting in boxes out in the hangar to be thrown away and I grabbed all I could.) Current dealers have the rates which Cessna currently quote (current models) as part of their warranty/factory library...perhaps they are still sold to the public., but I'm not certain.