Airframe corrosion discussion

Reading this one http://www.sportaviationonline.org/sportaviation/201211?pg=34&pm=2&fs=1#pg26 you might ask to yourself whether AD shouldn't deserve some research before a course of action is taken for maintenance of your aircraft. I don't know a bit about your mentioned AD, but personally totally agree with Busch's approach to maintenance, even when AD are concerned.

I know nothing about this AD.

I would have expected on discovery of a corrosion issue Socata would have put something in-place like better corrosion protection to solve the issue.

Do all the TB series have the same corrosion protection? Are most of the TB10s components build in the same time period?

Hi all,

I used to work for a Socata distributor back in the late 90's and early 2000's and saw plenty of servere main wing spar corrosion on both TB20s and TB10s with the worst on a TB20. The root cause of the corrosion was poor heat treatment at production of the main spar leading to intergrannular corrosion. The nature of this type of corrosion means no amount of paint or corrosion preventitive compounds will help as it starts at the alloying grain boundaries inside the material.

The good news is that Socata always managed to create a repair solution but the drawing alone was in excess of 10,000 euros before work commenced.

As far as I remember the S/N's indicated aircraft manufactured around 1985 seemed to suffer most.

I'm not aware of a DGAC or EASA AD though but it's been a while, mainly surrounded by Globals and Learjets these days.

Regards

Darren.

I would be grateful for any tips on removing and repainting minor corrosion that appears (mostly leading edges) so that it does not continue to spread. There are a very few areas about thumbnail size that I really do not want to take it to a paint shop for. Aircraft is C172 repainted about 10 years ago. Many thanks

I have this comment from an A&P/IA colleague who I maintain my plane with and who does a lot of this kind of work:

Unfortunately there is no easy way to remove that kind of corrosion. The best way is to have an orbital sander, rub down the affected area about three to four times greater than the corroded area so there are no traces of corrosion left, then treat it with alochrome, then prime and paint it.

Personally I use an epoxy (2-pack) primer, not the common zinc chromate which is frankly WW2-era rubbish compared to epoxy. I have bought a big pack so if you want some I can send you a bit

When I do small patches like that myself, I rub them down with emery paper and then prime with epoxy direct. Important to not use a steel brush or steel wool as they leave steel in the aluminium which then corrodes.

One of the biggest challenges here, is to match the paint correctly. With about 5 dozen differing shades of white, it can easily develop into a patch work quilt effect, not particularly easy on the eye.

For small areas like that, I get the local "Chips Away', car repairer to pop in. Give him the colour match, either RAL or BS, and he does a decent job.

Again, getting the correct conditions for primer, and paint to dry, can be a PITA.

I am not good at this stuff, I tend to make a right mess, therefore I tend to leave it to the pros, who also have a habit of making a right mess.

If you are persistent, confidence when sanding is key, getting the depth and spread of the area sanded is vital, also a good idea to check what primer is actually on the wing, again, I have seen a different primer, making the paint applied, look different. Prior research may save you a lot of heartache.

My view is that preventing the corrosion from spreading is the main thing, and visual perfection is second.

A plane is for flying to places, and the ex-showroom beauty is not going to last 5 minutes once the average EASA 145 company starts maintaining it

For the first 3 years I was G-reg and used a JAR 145 company to do everything. They used to strip all the screw heads with power screwdrivers; I had a key to the hangar and sneaked in after hours and changed the screws with fresh ones out of a Socata screw kit

Now, my 11 year old plane has little bits of corrosion in many places, and there is no way to get a perfect colour match if touching up just a small patch. So I sand out the corroded bit and prime it, and the white primer looks OK from a distance

Also it is impossible to do a good visual job with a brush. One has to use a spray gun, or an airbrush if you are really skilled with one. But for spraying one needs facilities. There are only a few days each year when one can do it outdoors, and the whole plane needs to be masked otherwise the overspray goes everywhere. It's a lot of hassle for a small patch. I have a portable spray kit which uses a scuba cylinder and 2 regulators, but the masking takes so long I rarely use it.

A few years ago somebody did some spraying somewhere near or upwind (nobody owned up to it) and I found the plane covered in white overspray. It took me a day to get most of it off the windows but the rest of the plane is still somewhat covered on the upper surface. I paid somebody £200 to try to get it off with a power buff but it didn't do much.

One day, when I really have nothing better to do, I will get a complete respray, and then everything will match, but there is probably 10 years to go before that.

Cessna recently issued SID’s (Special Inspection Document) for many of their models including the 172. Some registers have taken this as mandatory but it is not. I know the American Register say they are voluntary not mandatory. The cost of an SID on a 172 is reckoned to be around £12K plus the cost of repairing what they find.
It might be worth having the SID done then you would be certain of the condition of the aircraft.
The elephant in the room is “who pays”.