Corrosion. Where and how to inspect?

[Mcstealth]

While I deal with the sites search function, could someone please give me the Cliff notes on the what, where, when, why and how to inspect a 56 year old Mooney for corrosion?

Greatly appreciated in advance. 

David

[KSMooniac]

Biggest areas are the spar (look under the back seat cushion and inspection panels inside the cabin, wheel wells, perhaps some wing inspection panels), and the steel cage behind some interior panels, especially under the side windows.  While looking back there, make sure the original fiberglass insulation was replaced with modern foam too.  You can also pull a belly skin or two and assess overall condition there... ideally you see original sheet metal not bent/beat up, good, solid looking steel tubes, and lots of evidence of regular lubrication, which might be drips, or dirt + lube on the inside of the panels.  Take off the tailcone access panel behind the baggage area and look back in there with a good light for corrosion, dirt, rodent/bird evidence, etc.  I'd also look at sheet metal seams all over to see if you can see anything disturbing the paint... that might indicate paint stripper that did not get cleaned out very well, and has been eating away under the paint.

[A64Pilot]

I’m still figuring out how to get the Da** back seat out, there must be a trick that I’m not aware of.

Is the carpet screwed down?

I admit I haven’t tried very hard, goal is to inspect and spray with Corrosion-X

[Andy95W]

I’ll break my recommendation down into how much time it takes to perform.  In general, you need to look at the main spar, the stub spar, the tubular steel frame (covered in SB M20-208), and then general surface corrosion on the skins and corrosion on the angle aluminum pieces.

Phase 1 (about 30 minutes:  You can see a good bit of the main spar and stub spar in the wheel wells.  The main spar is the heavy frame work where the forward landing gear pivot point is.  The stub spar is the rear pivot point.  Look at as much of those as you can in the wheel wells.  You can google search for “aluminum corrosion” and see some good pictures.  Surface corrosion isn’t as important as intergranular corrosion, which leads to exfoliation (picture below).  Next, look inside the tail cone (see below, marked in yellow with arrow).  That panel comes out in less than a minute with 1/4 turn fasteners.  Look for exfoliation on the angle aluminum pieces and surface corrosion on the inside skins.  While you’re there, look at the pushrods- they are steel and often rust.  That isn’t terrible, but does need to be addressed.

Phase 2 (about 1 hour):  If that initial look is acceptable, then you should dig deeper into the wings.  Open up some of the inspection panels and you’ll see more and more of the main spar (red highlight).  Look inside inspection panels #11 (see picture below) and you’ll be able to see a good bit more of the stub spar (green highlight).  The main spar actually runs wingtip to wingtip, but I’ve noted the location of the extruded angle aluminum pieces that can corrode badly.

Phase 3 (about 2 hours):  If that seems acceptable, you’ll want to take a look inside the cabin.  If possible, remove the back seat and pilot side wall.  There are square panels under the back seat- remove one and you can see the inside view of the main spar (red arrow on picture below).  Inside the pilot side wall, you’ll see the tubular steel structure that is the subject of SB M20-208.  You’re looking for regular rust, particularly below the window frames.  On the picture below, the steel tubes are painted green with zinc chromate paint- but you can see at least one tube with a lot of rust on it.  That tube will have to be replaced.

I’m sure others will chime in with additional suggestions.  Good luck!  I’m keeping my fingers crossed for you!

[RoundTwo]

5 minutes ago, Andy95W said:

I’ll break my recommendation down into how much time it takes to perform.  In general, you need to look at the main spar, the stub spar, the tubular steel frame (covered in SB M20-208), and then general surface corrosion on the skins and corrosion on the angle aluminum pieces.

Phase 1 (about 30 minutes:  You can see a good bit of the main spar and stub spar in the wheel wells.  The main spar is the heavy frame work where the forward landing gear pivot point is.  The stub spar is the rear pivot point.  Look at as much of those as you can in the wheel wells.  You can google search for “aluminum corrosion” and see some good pictures.  Surface corrosion isn’t as important as intergranular corrosion, which leads to exfoliation (picture below).  Next, look inside the tail cone (see below, marked in yellow with arrow).  That panel comes out in less than a minute with 1/4 turn fasteners.  Look for exfoliation on the angle aluminum pieces and surface corrosion on the inside skins.  While you’re there, look at the pushrods- they are steel and often rust.  That isn’t terrible, but does need to be addressed.

Phase 2 (about 1 hour):  If that initial look is acceptable, then you should dig deeper into the wings.  Open up some of the inspection panels and you’ll see more and more of the main spar (red highlight).  Look inside inspection panels #11 (see picture below) and you’ll be able to see a good bit more of the stub spar (green highlight).  The main spar actually runs wingtip to wingtip, but I’ve noted the location of the extruded angle aluminum pieces that can corrode badly.

Phase 3 (about 2 hours):  If that seems acceptable, you’ll want to take a look inside the cabin.  If possible, remove the back seat and pilot side wall.  There are square panels under the back seat- remove one and you can see the inside view of the main spar (red arrow on picture below).  Inside the pilot side wall, you’ll see the tubular steel structure that is the subject of SB M20-208.  You’re looking for regular rust, particularly below the window frames.  On the picture below, the steel tubes are painted green with zinc chromate paint- but you can see at least one tube with a lot of rust on it.  That tube will have to be replaced.

I’m sure others will chime in with additional suggestions.  Good luck!  I’m keeping my fingers crossed for you!

That’s a great write up. 

[47U]

1 hour ago, Mcstealth said:

could someone please give me the Cliff notes on the what, where, when, why and how to inspect a 56 year old Mooney for corrosion?

 

28 minutes ago, Andy95W said:

you’ll want to take a look inside the cabin.  If possible, remove the back seat and pilot side wall.

Excellent post by Andy… a couple pictures of corrosion on the spar under the back seat cushion.

[Mcstealth]

26 minutes ago, 47U said:

 

Excellent post by Andy… a couple pictures of corrosion on the spar under the back seat cushion.

This what inter-granual  looks like?

Edited June 28, 2023 by Mcstealth

[201Steve]

This thread should become a sticky. I’m going to probably start doing my own foggings, and this write up is most helpful.  Thanks @Andy95W

[A64Pilot]

On 6/27/2023 at 8:30 PM, Mcstealth said:

This what inter-granual  looks like?

I woukd say it’s exfoliation corrosion, that’s what inter granular turns into when it’s bad