How much corrosion is too much corrosion?

Question

This great 180hp 1968 Cessna 177 has been sitting for 30 months. During this time reddish, brown rust has streaked the white exterior of the top of the wings. How serious is the problem?

Answer 1

First of all, aluminum doesn't rust, it corrodes. The rust is probably coming from the screws for the access panels. These will be tough to remove and replace (you might have to replace some anchor nuts too) but doesn't indicate a serious problem (other than the aircraft has been in a corrosive environment). Corrosion looks like bubbles under the paint, if you peel it with your finger there will be a white power and pitting in the metal. Inside the wings and fuselage where there is bare metal its not uncommon to see patches of powder on the surface and these are best left alone, if it hasn't gotten worse in 40 years its not going to.

Bad corrosion is when the metal starts flaking off and significant pitting occurs. Typically you are allowed to remove 10% of the material thickness but most manufacturers will limit how much you remove and where you can remove it without replacing parts.

Even light surface corrosion, if it is over the entire aircraft, would require so much work to remove that it might not be worth it. On the other hand, one area of very bad corrosion would be well worth the repair if the rest of the aircraft is in good shape.

If the aircraft has been sitting for a long time, this biggest concern for me would be the engine. Has is been inhibited? Has it been run? Has fuel stabilizer been added? What are the leak down numbers? What kind of junk is in the oil screen/filter?

Hope this helps.

Answer 2

Call me crazy, but the wings seem like a real important part of whether or not this great 180hp Cessna 177 is airworthy. Up to you, but I say where there's smoke there's fire. How serious is the problem? How seriously do you want to find out?

Answer 3

REFER TO THE AIRCRAFT'S SPECIFICATIONS ON TOLERANCES OF THE MINIMUM THICKNESS OF THE MATERIAL COVERING THE WINGS, IF THE CORROSION IS PURELY SURFACE CORROSION IT CAN BE REMOVED WITH LIGHT SANDING BUT THE MATERIAL MUST HAVE A COAT OF ALODINE PLACED OVER IT TO PREVENT FURTHER CORROSION, THIS REPLACES THE 100% PURE COATING OF ALUMINIUM WHICH IF SANDED WILL BE REMOVED. IF THE CORROSION IS PITTED REPLACE THE MATERIAL. NOTE: ALLOYS CORRODE WHILE METALS RUST.