Lower Spar Corrosion - in fuel tank

[gsxrpilot]

As I mentioned in one of the other threads, my M20K 252 is at Houston Tank Specialists for a strip and reseal of the right tank.  We noticed some fuel leaking in the area of the lower spar inboard, and it was actually leaking into the cabin along the front of the lower spar.  After opening up the tanks, stripping and cleaning, an area of corrosion was discovered. I only have the one picture of it here, but according to Carl, it's definitely not the surface filiform type that Allan @Hyett6420 had in his tanks. But rather it appears to be a type of intergranular granular corrosion. Which is of course, much more serious. 

We're still a bit early in the investigation, but it appears that the fix will be either a spar cap splice or a replacement of the entire lower spar.  The situation is being discussed between Carl Sharon, Don Maxwell, and Bill Craig at the Mooney factory.  So I'm confident the answer/solution that emerges from these discussions will the correct one.  The current plan is to grind down to shiny metal and see what we've got at that point.

I'm sure the fix will be expensive regardless and will take quite a bit of time. I am consoled by the fact that no pre-purchase inspection would have found this and therefore it's just bad luck and not poor decisions on my part.  There is no question that the plane will fly again. I'm of the belief that the 252 is still one of the best models ever to roll off the line in Kerrville and could not be party to scraping one regardless of the cost to repair.  But even worse than the cost will be the time this takes. I've been very luck with a very good dispatch rate, but will definitely be grounded for a while now.

[prflyer]

I'm sorry Paul.  Glad to see you have a good attitude about this and that the airplane will be fixed right.

Good luck.

Fernando

 

[aviatoreb]

Paul, that is a real bummer.

As you saw on the other thread I reported that the same thing happened to me - also in process of a tank reseal - on my 1981 M20K. And there was another thread a month ago where I gave much more details.  It was an expensive and long repair, but N10933 is flying again. Feel free to PM me if you want.

[Piloto]

To me it does not appear that bad. I have seen it much worse. Brush the area well to asses the damage. I would rather clean it well and coat it with zinc chromate. An improper repair such as cap replacement will induce more damage than what it is now.

José

[KSMooniac]

Good luck...it doesn't look fatal to me, and you're definitely in the right area to get it addressed.

Sent from my VS985 4G using Tapatalk

[Yetti]

I don't think that is intergranual.  More just pitting. Intergranual will delaminate the aluminum.  Rivets are still good. I would say most of the struture of the angle is still there.   Not an Engineer.

Hope you get if fixed without too much hassle.

[carusoam]

Tonight's team consists of...

1) A guy with similar shared experience.

2) A guy with the most Mooney fuel tank experience.

3) An aviation engineer.

4) A machinist.

5) An owner that is incredibly calm through the experience.

All saying that looks crummy, but not terrible...

Terrible would have spread further and deeper, popping rivets as it went.  Rivets get sacrificed as the layers they are holding together are spread apart by increased volume caused by oxidation.

I hope Don will be bearing good news soon.

Go MS!

 

Best regards,

-a-

 

Edited February 3, 2017 by carusoam

[DXB]

Yikes sorry! Hope fix is not too exorbitant.  Had you not had the leak, you would have never known.  

Which makes me wonder - did the degraded sealant lead to the corrosion?  Did the corrosion mess up the sealant?

Are my bladders hiding tons of this stuff that no one will ever see??  Not gonna sleep tonight...

 

[Guest]

It may be time to start defueling these airplane and inspecting the tank interiors with a borescope.

I'd be curious to know if the corrosion was under a flap of loose sealant?

Clarence

[eman1200]

Big time bummer to hear and VERY discouraging for a person on the hunt for a first plane.

[DXB]

I just worry that by the time one says "to hell with patching these recurrent leaks- lets slap some bladders in there" you've hidden this problem forever.  I assume the sealant doesn't get stripped to put in bladders? Are the  access panels adequate to examine the spar in this context? Seems GXR would have never found the problem had he not stripped the sealant.

[Piloto]

This is just a pimple not a cancer. It is common on planes left outside. Get a Dremel tool with rotary brushes and brush the area well to bare metal. Clean it with MEK and apply zinc chromate. The whole process is no more than 1/2 hour. This is more prone on planes with bladders. Moisture is trapped between the bladder and the structure causing interface corrosion. If the plane is left outside have the bladder area inspected every five years. On integral tanks fuel keeps moisture out. And sealant is not corrosive. The above pimple is not inside the fuel tank but on the outside perimeter.

José  

Edited February 3, 2017 by Piloto

[mike_elliott]

Paul, IMO, Jose is probably right on this one. While it is no fun to find any corrosion, this appears it can be mitigated fairly easily and hasn't caused a structural concern (yet).

[Yetti]

Can't really tell from the one picture.   It would seem that would not even go deep enough to meet the bad criteria for penetration.

[bonal]

From gixxers description it was found beneath the selant so just draining the tanks might not be revealing. I really hope this is determined to be surface and can be cleaned up as Piloto described 

good luck

[thinwing]

I agree with Piloto..this looks like simple surface corrosion....I think it's over kill to talk about sparcaps or even spar replacement

[gsxrpilot]

I hope to have a picture to share with a caliper showing the depth. But according to Carl, it's definitely not surface corrosion. The pits are evidently pretty deep. Stacy and Bill at Mooney have instructed us to grind it down to shiny metal and then we can evaluate if enough is left. I believe the rule is no more than 10% of the original thickness can be removed and still be serviceable.

[Piloto]

15 minutes ago, gsxrpilot said:

I hope to have a picture to share with a caliper showing the depth. But according to Carl, it's definitely not surface corrosion. The pits are evidently pretty deep. Stacy and Bill at Mooney have instructed us to grind it down to shiny metal and then we can evaluate if enough is left. I believe the rule is no more than 10% of the original thickness can be removed and still be serviceable.

Use the Dremel tool with the brushes for a meticulous job. To avoid fuel vapor ignition make sure the area is ventilated or you may loose your eyebrows.

Dremel brush with bronze fibers.

 

Edited February 5, 2017 by Piloto

[jetdriven]

15 minutes ago, gsxrpilot said:

I hope to have a picture to share with a caliper showing the depth. But according to Carl, it's definitely not surface corrosion. The pits are evidently pretty deep. Stacy and Bill at Mooney have instructed us to grind it down to shiny metal and then we can evaluate if enough is left. I believe the rule is no more than 10% of the original thickness can be removed and still be serviceable.

You have to get to rhe bottom of all the pitting to eliminate the corrosion. The SRM spells out allowable limits, and I think Mooney is pretty restrictive on spars inside the landing gear attach points. 

if not otherwise specified, then AC43-13 is good data and it says 10% of thickness, generally. Anything on a spar cap is very scary from a financial standpoint. 

[LANCECASPER]

10 hours ago, Piloto said:

Use the Dremel tool with the brushes for a meticulous job. To avoid fuel vapor ignition make sure the area is ventilated or you may loose your eyebrows.

Dremel brush with bronze fibers.

 

 

Since Jose Monroy (Piloto) has spent half of his adult life with his hands inside of a Mooney fuel tank I'd be inclined to trust his judgment.

[glafaille]

Bad news indeed!  But there is a silver lining in those storm clouds.

Sounds like you found this early and may be able to treat it or repair it without too much hassle or expense.  One of those rare cases where a fuel leak may have saved you a truly horrible surprise down the road.

I hope it's an easy and inexpensive fix.

[Piloto]

6 hours ago, Hyett6420 said:

I was told not to use metal brushes as they react with the aluminium leaving further problems behind at the molecular level.  There are some hard plastic type ones that don't react apparently that you should use.  

Apart from that statement I know no more. But the metal interacting with metal does sort of make sense chemically speaking. 

Bronze has minimal reaction and is hard enough. I found the non metal fibers don't last enough to do the job, they are more suitable for soft materials.

José

[ArtVandelay]

Bronze has minimal reaction and is hard enough. I found the non metal fibers don't last enough to do the job, they are more suitable for soft materials.
José

You could do a final treatment with plastic brush, just to clean area of any bronze debris, of course you'll be stripping the tank on top of that.

[aviatoreb]

Hi ALL - so following this current thread and my recalling that I too just went through a complete spar cap repair - at least two different mooneyspacers contacted me for more information - which I will be happy to correspond to them directly by PM, but I decided to repost a very detailed report I made about a month ago, describing my experience, and then a phoenix is rising thread about a better airplane on the other side.

For the reader's digest version - in Anthony-bullet-style:

-intergranular corrosion is different from surface corrosion - once started and even if it seems relatively mild, it will propagate inward and cannot be stopped.  Replacing the part becomes the only option.

-such a corrosion attack can happen in a relatively short time in a relatively small place in an otherwise fine plane.

-spar cap under the fuel sealant is a common scenario.

-there is a specific standard of how deep the corrosion is allowed to be and still be brushed out rather than replace the part

-I had my spar cap replaced at airmods in new jersey.  They KNOW what they are doing and have done this specific job on Mooneys several times before.  They did a fantastic job on my airplane.  It was expensive and most of the expense was labor - I think the part - the spar cap was $500.  They are a no nonsense and very reasonable shop and I think they did me right both in charging hours and in choosing the most efficient repair method possible.  It takes several hundred hours to do.

-they did not remove the wing, but they separate the wing from the plane and displace it just a few inches.  They remove lots and lots of rives and work the whole spar cap out and then jimmy in a new spar cap.  Then apply lots and lots of rivets.  Structural plus skin rivets.

-this method saves the trouble of removing control rods, fuel lines, electronic lines, and in my case, the tks plumbing.

-the alternatives were remove the wing entirely, or maybe even replace the wing.  Replace the wing might be competitive if you find a good deal on a good wing and it fits exactly.  In my case, there were no tks ready m20k wings not from the later series (landing gear and other details changed in what 1984? in the m20k line).  That would have greatly increased the cost.

-before proceeding with an expensive corrosion repair, consider getting the whole airplane re-inspected as if a pre-buy because wisdom says that if there is some corrosion, maybe there is more.  In my case, it turned out to be yes, mostly just a single attack and the rest of the airplane structures, and tubular pieces too, came out fine.

-I think the repair itself took about 4 months in queue waiting my turn on the shop floor with the right mechanics (yea - if I had only planned ahead..... eh?) and about 4 months in process of repair.

-I went through some soul searching before proceeding to decide if I want to keep this plane or get something else.  Every time I looked at something else, I kept coming back to I like this one.

-I am in starbucks in Princeton NJ (Hi Anthony!) today, flew down yesterday for business - at 13,000 ft (low traffic zone) at >200kts the whole way.... and I go home tomorrow.  lifestyles of the rich and famous.  Gotta get back to work now...

 

Edited February 5, 2017 by aviatoreb

[carusoam]

Erik,

Thanks for taking the time to cover so many details.

Best regards,

-a-