Metal Spar wings

[Bob Turner]

On that inexpensive Decathlon I posted a while back - the shop spotted corrosion on the leading edges under the fabric, freaked out, and towed the aircraft back to its parking spot.

I found, through a friend, ACA service bulletin 449, which discusses inadequate corrosion prevention coatings on early metal spar wings, and allows quite a bit of leading edge corrosion. They allow 50% of the linear dimension of the leading edge to be speckled with bumps below .030” high. They allow counting only bumps above .010”, so it is fairly liberal.

You can measure bumps quite accurately with a feeler gauge and a short straightedge. Our aircraft came in at 46%, counting all bumpy areas, so we passed. But none of the shops would touch it - service bulletin or no. (one wing 46%; the other at 10%)

Having rebuilt lots of aluminum/fabric wings, leading edge corrosion really doesn’t bother me - except that, at least with Cub wings, it takes decades to have corrosion take over, and usually it happens because of improper storage. So I am doing the inspection myself. What you do not want, obviously, and what ACA addresses, is a "cave-in" or structural failure. Apparently, ACA has investigated, found lots of these problems, and given us guidelines.

Inside the wing there are lots of little areas of extremely light surface corrosion. No pits at all, just little white rings. Nothing to get excited about, except it leads me to believe ACA used the wrong alloy. The spars in these wings are coated with a green substance that looks like epoxy primer, and are pristine.

 

[Bob Turner]

A second area of concern are the machined aluminum strut attach points for the forward lift strut. They apparently are prone to severe corrosion - this bird had them replaced four years ago - they looked like a telephone book left out in the rain. If it were me, I would have stuck to steel in such a critical area.

During my inspection I really paid attention to the lower fuselage longerons - a big problem area in Cubs. Not a lick of corrosion! But there is some along the channels for the rear cockpit windows - if you park your bird outside, you may want to shoot some LPS or something in those channels now and then. Not terribly worried about them - biggest area is only four inches long, and only one side has two such areas. Still, something to look out for, if you live in damp places near the sea.

So, assuming this will probably be a very short thread, if you wind up with early metal spar wings, be prepared ultimately to really clean the ribs, leading and trailing edges, and strut fittings, and get a good coat of something on them before recover. My Cubs always got ribs soaked in a thinned solution of zinc chromate (which may not be possible any more - chromium is deadly) and then sprayed with an epoxy decorative coat. Ribs I did in 1969 look brand new today. Covering is easy if you do it yourself, and costly if you hire somebody. Corrosion prevention is basic, and I am surprised ACA skipped that step on early wings.

 

[Bartman]

Did the sale fall through Bob?

 

[Bob Turner]

No - new owner is happy. I just feel kinda obligated to help him get it home. It is in better shape than I thought.

But these older metal spar wings are strange. I am actually glad I have the wood spar variants on mine. Zero corrosion.

 

[Hiperbiper]

One other place you might look Bob...
The channel that makes up the door opening is spot welded over the round tubing around the circumference of the door. You have to pull the interior kick panel next to the right rear rudder pedal and peek under the channel with a light/mirror or a camera.
I put this area second only to the lower longerons for corrosion possibilities.
Chris

 

[Bob Turner]

Thanks. I will look at both aircraft in that area.