The International Cessna^® 170 Association

After looking at the corrosion in the gear box casting thread I was just wondering what is the allowable percentage of skin thickness loss after clean up in .032 Cessna skins?

Ten per-cent is the industry standard.

I thought it might be 15% for non-pressurized areas, 10% for pressurized skins. I've looked at an xp this weekend and I've never seen corrosion on a Cessna like this before. The paint was lifting (blisters) and underneath the metal, had turned to dust, with damage at least .010 deep The area was aft of the firewall on the belly and between the gear legs. There didn't seem to be a reason as in mechanical damage that would have allowed corrosive elements access to the alloy and yet it was spread out in small pockets everywhere in the skin. Its something you would expect beneath a galley or a Lav not aft and below the engine. Did Cessna have aluminium quality problems in the late 70"s? They certainly had good metal for our planes.

was the corrosion downstream of the battery box drain? that can cause it, if they ever had a voltage regulator failure that caused overcharging of the battery

our airplane had some corrosion on the belly but that was from malathion or something from when my father-in-law used it as a sprayer. took some work to get paint to stick when I finally got around to fixing that but it did keep the mice away

Intergranulation and other certain forms of corrosion can start at the manufacture of the alloy it can appear anywhere, this is why we inspect our aircraft no matter how old or new. Surface corrosion is up to us to prevent, control and treat.

c170b53 wrote:I thought it might be 15% for non-pressurized areas, 10% for pressurized skins. I've looked at an xp this weekend and I've never seen corrosion on a Cessna like this before. The paint was lifting (blisters) and underneath the metal, had turned to dust, with damage at least .010 deep The area was aft of the firewall on the belly and between the gear legs. There didn't seem to be a reason as in mechanical damage that would have allowed corrosive elements access to the alloy and yet it was spread out in small pockets everywhere in the skin. Its something you would expect beneath a galley or a Lav not aft and below the engine. Did Cessna have aluminium quality problems in the late 70"s? They certainly had good metal for our planes.

This sounds like filiform corrosion and is common after paint jobs that failed to properly clean and prme areas to be painted. Usually filiform can be nuetralized/treated with acid-etch, prime, and re-paint. Occasionally alodine is required. Only in severe cases is skin replacement or doublers required.

Did these cessnas have some original skins of .032 thickness? Somehow, I never thought of them having any skins that thick.

The 54 I replaced the belly skin on had .032 fwd and aft of the main gear. I bumped it up to .040 Brian