Mooney spar design

[MyNameIsNobody]

1 hour ago, cliffy said:

Every metal wing Boeing is designed the same way as our Mooneys, wet wing technology. Even 50 year old 727s have wet wings so the design style is well proven as doable. As mentioned, the difference "may" be in inspection technique and interval to find corrosion. 

I have always found the mind set we exhibit toward 50, sometimes 60, year old airplanes (when viewed from the position of maintenance) and how we view automobiles quite interesting. We invariably drive cars that are less than 6-10 years old (FOR RELIABILITY) yet we think  nothing of jumping into a half century old airplane with lots of "deferred" maintenance or with maintenance we as amateur mechanics have done (when a lot of times we don't even work on our own cars, hangar fairies?) and fly across the country over hostile terrain. Its mind boggling. Probably needs another thread here. 

I have always said if I run into someone named “we/us/them” I am going to Punch “them” and walk away...

Edited April 22, 2018 by MyNameIsNobody

[MyNameIsNobody]

22 hours ago, rpcc said:

seems like snowmen are very detail oriented

 

 

Anal?  Pretty sure that light switch in the closet is off about 5 degrees from vertical...

[carusoam]

One oddity of spar design, is...

The selections that need to be made.  Material scientists ( or engineers) get to choose... 

 

Material attributes...

• strength
• weight
• Volume
• cycle limitations
• age limitation
• corrosion susceptibility 
• technology available the year the plane is built
• COSTS

 

 

Material selection.

The aluminum that is selected for the spar caps...  the thick plates that are stacked up reinforcements....

Plus column....

• High strength
• Light weight
• Very flexible

Minus colum...

• Susceptible to intergranular corrosion

 

We have two types of corrosion to be aware of...

• Filiform... a surface corrosion, sand and repair as needed... common occurrence on old sheet metal with failing paint and coatings...  once started, it progresses further along the surface...
• Intergranular... a form that progresses into the aluminum structure, destroying the connections between microgranules of metal... once started, it progresses deeply into the molecular structure of the piece.

 

Mooney sheetmetal (T-clad?) is designed to resist the Intergranular type of corrosion.  The surface layer (cladding) is pretty effective and not very deep... don’t accidently sand this layer away...

Mooney structural aluminum is incredibly strong and more susceptible to the Intergranular type of corrosion.  It is not as much as a ‘design flaw’ as it is a forced design compromise...

 

Either way, if you see corrosion starting... this is your opportunity to get it stopped.... the sooner the better...

Timing...

• It typically takes years for corrosion to form and spread.  
• This is why the annual inspection is so valuable and important.
• wait too long to fix a filiform challenge, repainting a bubbled spot becomes repainting a much larger surface area in a year...
• wait too long to find and fix an Intergranular challenge, the mechanic has to measure the depth of the corrosion before fixing it...

Allowable depth is pretty small.  If you see corrosion starting, you don’t want to wait for the annual to arrive to take care of it...

Reading the MM can be very enlightening for what needs to be done or what can be done when this stuff occurs...

keeping your eyes wide open when doing the preflight, look under the rug in the back seat to see the most exposed piece of structural aluminum we have... a stacked spar cap, with its coating worn thin from little feet, beach sand, and soda spills.....

Ordinary PP knowledge only, not a material scientist or mechanic...

Best regards,

-a-

 

[rpcc]

18 hours ago, M20Doc said:

That’s the way they were aligned when we looked in for inspection.  I’d be more concerned that the washers are installed correctly against the spar.

Clarence

Is there a washer between the spar and the top right bolt in this picture?  Maybe it's just the angle of the picture but does not look to be present.

[Guest]

4 hours ago, rpcc said:

Is there a washer between the spar and the top right bolt in this picture?  Maybe it's just the angle of the picture but does not look to be present.

Yes, a washer under each bolt head and each nut.

Clarence

[Yetti]

8 hours ago, MyNameIsNobody said:

Anal?  Pretty sure that light switch in the closet is off about 5 degrees from vertical...

We prefer large mythical furry creature.   Screws on the switch plate lined up and 5 degrees would not fly.  And yes I can see it from across the room.

[DonMuncy]

9 minutes ago, Yetti said:

We prefer large mythical furry creature.   Screws on the switch plate lined up and 5 degrees would not fly.  And yes I can see it from across the room.

I thought I was the only one who made sure all the light switch and receptacle plate screws were aligned vertically. 

[MyNameIsNobody]

56 minutes ago, DonMuncy said:

I thought I was the only one who made sure all the light switch and receptacle plate screws were aligned vertically. 

No, “it’s a thing”...

[Hank]

1 hour ago, DonMuncy said:

I thought I was the only one who made sure all the light switch and receptacle plate screws were aligned vertically. 

It's aggravating, and a very simple fix. What really annoys me is three way switches (like in the hallway and kitchen) where with the light off, one switch is up and one is down. That's just a lazy electrician. I've gotten good enough, being in my fifth house (and over 40 abodes) that I can remove the switch from the wall, turn it over and put it back together with only the occasional tingle.

[MBDiagMan]

Sounds as if there are some here who have not heard the true story about the destructive testing of the Mooney main structure.  To mak e a long story short,  some engineers built a fixture and pressured the structure against the fixture to see how much stress the structure would take before it failed.  The structure never failed.  The fixture failed instead.  When in Kerrville a little over a year ago I was told that fixture was kept as a souvineer.  This test is one of the many reasons I own a Mooney.  

I am sure that others here know about this story than I.

[DaV8or]

There's a lot of speculation on this tragic accident.  Most seem to be on poor design, or defect in manufacturing, or corrosion, or poor inspection. 

Now my personal speculation-

It seems to me the flight history of the Piper Cherokee and subsequent Arrow model is well proven over the decades. There is no doubt that there are Arrows out in the wild with more than 7600 hours with both wings still attached. I would hazard to guess there are some with over 10,000 hours by now. I think this rules out poor design.

I don't really buy the manufacturing defect theory either because this airplane did have 7600 hours and I would imagine that a stress riser from poor fabrication, or careless assembly would have caused this failure thousands of hours ago. Corrosion is likely a contributing factor as the plane was operated in Florida and likely stored outside, but this is nothing new for Piper Cherokees used for training in Florida. Again, a much older plane years ago would have failed by now had corrosion been the only culprit.

Poor inspection may also be a contributing factor, due to not spotting the signs, but I have to wonder, how obvious and visible were the signs? The insidious nature of aluminum is that it doesn't give a lot of warning prior to failure like steel does. The warning it does give is subtle and sometimes minute. If the part is painted, how easy is it to see? Same goes for the corrosion. If it is internally corroding, how easy is that to see?

So where else can we lay the blame?

Two areas that I don't see people speculating on much is operational history and metallurgy. The case of operational history isn't popular because we may never know the truth. There are logbooks, but do they always tell the truth and the whole story? This airplane was a training plane and therefore basically a rental plane. Who is to say there wasn't a day when a young and bold pilot by him, or herself after watching a bunch of YouTube videos decided they could pull off a loop in the ol' Arrow? A loop performed badly can put a lot of stress on the plane. They may have gotten away with it, scared themselves silly and kept their mouth tightly zipped.

How about IFR flight into a thunderstorm? Those are a pretty regular features there in Florida, who is to say a student on a solo flight didn't accidentally find themselves in the thick of their worst nightmare, only to survive and quietly put the plane away with no words to anybody out of embarrassment, or fear of retribution? Penetration of thunderstorms has been known to stress an airplane.

This is not a popular line of thinking because there is no way to prove any of it... yet is quite possibly the real cause of this accident. Those that may have overstressed the aircraft in the past are even less likely to step forward now that there are fatalities involved. 

Then there is metallurgy. It is possible that the spar was made with substandard aluminum. Untempered aluminum, or aluminum with the wrong alloy looks identical to good and correct aluminum. There is no way anyone working at Piper would have ever known and it would machine identically. This is a knowable fact and I'm sure the NTSB is doing laboratory testing now.

My speculation based on the odds is, The accident airplane had unknown over stressing of the airframe in it's past history, made worse by the corrosive environment of outdoor storage in Florida. The over stress in the past may well have loosened the bolts just enough to allow a certain amount of moisture into the interior. A contributing factor may have been poor, or sloppy inspections of this area although I don't know how often this area is supposed to be inspected and just how easy this kind of damage would be to see if you did suspect it, so I personally don't lay as much blame here, if any at all.

As usual, just IMHO.

 

Edited April 23, 2018 by DaV8or

[DaV8or]

2 hours ago, MyNameIsNobody said:

No, “it’s a thing”...

Yes, I do it too and my wife is grateful as she would likely make me do it if I didn't already do it.

[Yetti]

Looking at the NTSB fotos don't really think corrosion is that big of a factor....   I do think some flying that was out of spec at some point would be a contributing factor. 

[Igor_U]

On ‎4‎/‎22‎/‎2018 at 8:35 AM, cliffy said:

Every metal wing Boeing is designed the same way as our Mooneys, wet wing technology. Even 50 year old 727s have wet wings so the design style is well proven as doable. As mentioned, the difference "may" be in inspection technique and interval to find corrosion. 

I have always found the mind set we exhibit toward 50, sometimes 60, year old airplanes (when viewed from the position of maintenance) and how we view automobiles quite interesting. We invariably drive cars that are less than 6-10 years old (FOR RELIABILITY) yet we think  nothing of jumping into a half century old airplane with lots of "deferred" maintenance or with maintenance we as amateur mechanics have done (when a lot of times we don't even work on our own cars, hangar fairies?) and fly across the country over hostile terrain. Its mind boggling. Probably needs another thread here. 

Cliffy,

Very good observation!

Every Boeing (or other commercial/business jet manufacturers) have wet wings with somewhat similar construction, however their execution is much better, IMO.

Their design and construction includes fay surface seals between components and then fillet seal with special attention to wet-wing fuel tank areas. All the wing (and fuselage) components are primed which of course helps preventing corrosion and issues we sometimes see on our Mooneyes. Even at Boeing, Sealing and Corrosion protection improved dramatically since days of B727, and even newer programs improving their design and protection by updating materials and processed on each new iteration of the model. Also, progressive maintenance of Transport category airplanes with big checks helps avoiding issues on older airframes over the years flying.

 

I was somewhat surprised that wings on our planes and not fully primed (my ’67 F is certainly not) while fuselage is and this can contribute to corrosion. Enter the sealed areas of tanks that are impossible to inspect and decades passing by…

I was also surprised not sealing the tanks during wing assembly (omitting fay surface sealing) and just filet seal after construction. To my understanding DIY RVs are built with tanks sealed at the time of wing assy and we don’t hear much of leaking tanks on RVs.

 

This is not a really a (big) critique of Mooney, or other GA manufactures as I am sure no one in 60-s or 70-s, at the peak of GA production would think we would fly our planes after 30 years let alone over 50. Also, this reduced the cost of construction and could be rolled down to the customer and keep accounting department happy.

At the time, no one really perceived this as a big issue, considering the life expectancy of the airplanes and of course, cars at the time were not that great and corrosion resistant either.

 

I am guilty of working on my 50+ year old airplane (and cars, except the new one) but this helps me better understand condition, potential issues and lower the cost of ownership. Over past 13 years of ownership my IA and I had done a lot of preventive maintenance that could be deferred since often I was paying just for parts and some reduced rate labor.

So, yes, I still hop into my ’67 Executive to go cross country but I’m aware issues can pop up anytime that can deem repairs not economical with low hull values of our planes.

 

Good luck.

[Hank]

Newer Mooney were, and new one are, built with a layer of something [different sealant?] between the spar and ribs in the fuel tank area. This change occurred in the 90s, I think, well after my C left the factory.

[MyNameIsNobody]

14 hours ago, DaV8or said:

Yes, I do it too and my wife is grateful as she would likely make me do it if I didn't already do it.

Sounds like a pretty sweet home situation.  Enjoy!  At least she loves to fly...

[0TreeLemur]

On 4/18/2018 at 5:02 AM, Oscar Avalle said:

Look at this old picture it shows how strong our wings are...

https://goo.gl/images/VYVprx

I clicked on that link, the redirect works, but the image is not there.  I'd love to see that.

[carusoam]

FRED,

The pic references 30 Mooney employees on the wing....  same pic posted above...

That pic was like the shot heard round the world in its day...  How strong the wing actually is in simple marketing terms... and how many women were working in the Mooney factory at the time...  

Best regards,

-a-

[Oscar Avalle]

8 hours ago, Fred_2O said:

I clicked on that link, the redirect works, but the image is not there.  I'd love to see that.

here it goes

 

https://www.google.com/search?rlz=1C1GGRV_enUS752US753&biw=1280&bih=562&tbm=isch&sa=1&ei=nxLfWpv3GY-Q_Qaay77QDw&q=mooney+wing+strength&oq=mooney+wing+strength&gs_l=psy-ab.3...21785.24217.0.25385.7.6.1.0.0.0.52.278.6.6.0....0...1c.1.64.psy-ab..0.0.0....0.qeQ8ITWBEKg#imgrc=K7V9MWjz8xTMYM:

[0TreeLemur]

2 hours ago, Oscar Avalle said:

here it goes

 

https://www.google.com/search?rlz=1C1GGRV_enUS752US753&biw=1280&bih=562&tbm=isch&sa=1&ei=nxLfWpv3GY-Q_Qaay77QDw&q=mooney+wing+strength&oq=mooney+wing+strength&gs_l=psy-ab.3...21785.24217.0.25385.7.6.1.0.0.0.52.278.6.6.0....0...1c.1.64.psy-ab..0.0.0....0.qeQ8ITWBEKg#imgrc=K7V9MWjz8xTMYM:

The photo is awesome.  I'm not surprised that the wing can hold 30 people.  Rated max wing loading is 2575 lb * 3g = 7725 lb, compared to 30*175 = 5350 lb.  They could have put more people up there.  I'm just glad they didn't use my a/c.   The aluminum skin out there is only 0.020!

[0TreeLemur]

While looking into wing attachment this morning, I came across this very sad wing failure on April 18 of this year due to fatigue an spar corrosion on a 2007 Arrow used for flight training with over 7000 h TTAF.  The preliminary NTSB report includes incontrovertible photographic evidence of the failure mode that led to wing separation:

https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20180404X13226&AKey=1&RType=HTML&IType=FA

The report also shows how the Arrow wing is attached.  Quite different from the Mooney design. 

 

[Guest]

9 hours ago, Fred_2O said:

The photo is awesome.  I'm not surprised that the wing can hold 30 people.  Rated max wing loading is 2575 lb * 3g = 7725 lb, compared to 30*175 = 5350 lb.  They could have put more people up there.  I'm just glad they didn't use my a/c.   The aluminum skin out there is only 0.020!

Except that the wing is loaded to the negative G which is much lower than the positive G limit.

Clarence

[0TreeLemur]

Good point @M20Doc.  That would be -1.52g for normal category, in that case they overloaded the wing by  about 1400 lb.   Now I'm really glad they didn't do that with my a/c.

[N601RX]

This isn’t the same negative loading that is saw in flight. The plane is setting on the landing gear. To replicate flight loads the center part of the cabin would be set on blocks and wheels retracted. 

[moodychief]