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Wing spar failure?


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As mentioned previously, the Mooney wing is one piece, tip-to-tip.  Yes, all the rivets can be removed just like any other wing can be de-rivetted.  As is shown in the picture from Don Maxwell of the Mooney "Predator", it is easier to leave the wing attached (with either 6 or 8 small bolts ... I'm not sure how many bolts are in the rear spar attach.  There are 4 for the main spar).  It is easier this way than to drop the wing straight down ... there are too many systems that need to be removed first, but it is possible.

Airplanes like the Bonanza and Pipers are attached at side of body.  Those airplanes have a really stout carry-through structure.  In those airplanes the wing will break at side of body or the carry-through will break.  Beech lost several wings due to improper welding processes on the carry-through.  The design was changed early (maybe in the first year of production).

If one gets a chance to visit an aviation salvage yard, the Mooney wings will all be maintained as a single piece (maybe with the fuselage section attached as pictured above and maybe not).  All the other airplanes will have the right and left wings detached from the fuselage.

RVs (Van's Aircraft) have multiple designs on the different models.  Some have side of body attachments (easiest but highest stresses at attach points), a carry through where the 2 wings butt together in the middle (carry-through takes all the wing bending loads, and some have spars that extend the width of the fuselage and attach to each other ... like a sailplane.  Van is a sailplane pilot, too.

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On 8/10/2021 at 8:50 PM, carusoam said:

If I read that correctly…

All of the parts of the M20J were found at the accident site…. Except the counterbalances for the elevator…

A subtle sign of control flutter prior to arriving at the final resting site…. Where the lead weights don’t want to move as fast as the associated control surface wants to move…

 

PP thoughts only, not an accident investigator…

Best regards,

-a-

I noticed that in the video footage of the Victoria accident. The entirety of the left stabilizer from where it would have been riveted to the flange on the empennage, and the entirety of the left elevator were resting on the ground where they had fallen short of the impact site, but the counterweight was missing. The notch in the stabilizer to accommodate the counterweight is present but no counterweight on the elevator. As I understand it there is a spar that runs through the stabilizer a few inches after of the leading edge and runs nearly tip to tip through the stabilizer. That spar appears to have broken at the empennage.

I posted the footage on the “Minnesota Crash” thread, for those who have not looked at it, it is here. The left elevator and stabilizer material is at 2:15. Early on, a day or two before the crash, there was also footage of the left stabilizer and elevator from a different vantage point. The footage showed an NTSB representative carrying the left elevator in two pieces, or at least folded in the middle, but I can’t find that footage any longer, the TV station appears to have taken it down.

https://www.fox9.com/news/neighbors-ran-to-help-after-plane-crash-in-victoria-minn

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The Mooney wing spar is not “one piece”. It is in fact made up of many pieces all fastened together with an assortment of different fasteners. The fuselage is then mounted on top of the wing.

The Piper Comanche series is built similar, two separate wing panels with the main spar butt ends bolted together under the back seat, and the fuselage bolted on top of the wing.

Many other airframes as pointed out earlier have some form of wing spar box in the cabin, to which the wings attach.

Clarence

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23 hours ago, M20Doc said:

The Mooney wing spar is not “one piece”. It is in fact made up of many pieces all fastened together with an assortment of different fasteners. The fuselage is then mounted on top of the wing.

The Piper Comanche series is built similar, two separate wing panels with the main spar butt ends bolted together under the back seat, and the fuselage bolted on top of the wing.

Many other airframes as pointed out earlier have some form of wing spar box in the cabin, to which the wings attach.

Clarence

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In the posted pictures you can clearly see a seam in the very middle of the web. I’ve not seen one up close but it appears they are put together with Huck bolts, and if so while the wing can be separated into two pieces its apparent that it would  require a lot of work, and I’d adventure to say that the average mechanic doesn’t have the tools to pull Huckbolts.

Add that to the fact that Mooney wings aren’t really all that big in one piece. why not leave them in one piece and avoid a LOT of unnecessary work?

But as Blueontop said the forces on such a wing are usually carried right in the middle. I know because several Thrush and Air Tractor wings have broken there due to fatigue.

So it will interesting to see where the accident airplanes wings broke, because it seems like it’s not the center?

Edited by A64Pilot
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On 8/24/2021 at 8:40 AM, M20Doc said:

The Mooney wing spar is not “one piece”. It is in fact made up of many pieces all fastened together with an assortment of different fasteners. The fuselage is then mounted on top of the wing.

The Piper Comanche series is built similar, two separate wing panels with the main spar butt ends bolted together under the back seat, and the fuselage bolted on top of the wing.

Many other airframes as pointed out earlier have some form of wing spar box in the cabin, to which the wings attach.

Clarence

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In many ways we are talking semantics.  What you say is indeed true but functionally speaking it is as close to a single spar as is practical. Extruding a truly single piece spar would be both impractical and likely less robust.  Would you consider the Comanche to have a four piece spar? As you know, each wing has an inner and an outer spar that make up the left and right assemblies.  The assemblies are joined in the center by several attachments (plates, straps and channel).  From a practical standpoint, it’s a two piece spar that is made up of four pieces. 

Edited by Shadrach
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Many aircraft especially high wing aircraft have carry through spars that is part of the fuselage structure and its common for the wings to attach with two bolts in shear, they are very easily removed.

‘Some aircraft, very often twins will have a wing center section and the outer portion of the wings bolt on, often in shear, but Beech on some of their aircraft the bolts are in tension.

The attached picture is of the Meyers 200 being built in Albany Ga, as you can see the wings pinned on outside of the main landing gear and the entire fuselage and wing center section structure is 4130 tubing, the Meyers is hell for strong,  but very labor intensive to build.

I know of no aircraft where the wing spars are actually in one piece, except maybe for a small aircraft I can’t see the advantage, the Air Tractor 802 for example the spar caps are four piece and of course there are three splice blocks.
 

 

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Edited by A64Pilot
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On 8/25/2021 at 8:08 AM, A64Pilot said:

So it will interesting to see where the accident airplanes wings broke, because it seems like it’s not the center?

We shall see but my WAG is that the intersection of load and strength will be between WS 59.25 and WS 74. 

 

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Edited by Shadrach
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All this talk and concern about the strength of the wing reminds me of the Bonanza controversy in the mid 1980s.  Somewhere around 300 mph, the leading edge of the V tail would fold over and immediately pull other tail components off, followed by the wings.  The model 33 and 36 Bonanzas did not have this happen, yet the in-flight, fatal loss of control accident rates for both the V tail and straight tail were nearly the same.  Lose control and the result is the same: destroyed airplane and loss of life.  The only difference was the V tail hit the ground in pieces, the straight tails hit the ground in one piece.

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I do not (edited to add the not I missed it apparently) think there is any concern. Anything can be broken, hit a concrete wall in a Volvo at 80 MPH head on and your toast, even though a Volvo is built to be safe.

The Mooney that broke a wing I think pulled massive G’s to do so.

An aircraft built massively strong will not have much useful load, one issue of the Meyers for example. It’s really a two place airplane, that also has a very narrow CG range at close to gross weight. If it weren’t built like a tank, it would have weighed less.

 We are only Certified to pull 150% of 3.8 G’s for 3 sec I believe and at that level things are allowed to bend, but not fail.

Edited by A64Pilot
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39 minutes ago, A64Pilot said:

We are only Certified to pull 150% of 3.8 G’s for 3 sec I believe and at that level things are allowed to bend, but not fail.

There can be permanent deformation above 3.8G ... and complete failure at 5.7G ... but it's typically NOT the wing or tail that may fail at those loads.

Va will protect the wing (and tail) at gross weight, but it's really there to protect the remainder of the airplane (engine, baggage, seats/occupants, etc.) below gross weight.  For example (and using round numbers for easy math ... for me), a 4,000 lbs. gross weight airplane with a 4G limit can fail the wing at 24,000 lbs.(4,000 X 4G X 1.5 safety factor).  If the same airplane were flown at 3,000 lbs., The WING could take 8G, but ...  The engine, seats/occupants, baggage, etc. (which have not changed weight) are still only designed to handle 4G (plus safety factor).

People ask me a lot about angle of attack and assumed to be "minor" changes due to atmospheric conditions (vertical gusts).  To put this in perspective, I doubt many pilots would exert enough control force to pull 4Gs (in a Cessna, Mooney or Piper), but the atmosphere (especially in a storm) has no problem taking the angle of attack (AOA) of 4-5 degrees (typical cruise) to stall AOA ... with the applicable G-load change.

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Scott Dennstaedt has posted the NTSB Preliminary Report on the Minnesota Crash thread. Both the front and rear spar were broken off. There were apparently multiple fractures of the wing spar near the center, and a piece of it was found a few hundred feet back along the plane’s trajectory. Read it for yourself.

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NTSB preliminary report.  note that both wings were folded up prior to impact. Also “ A 6-inch section of the main wing spar upper cap splice was found about 300 ft southwest of the accident site.”  That seems to clearly show main spar failed at center section well before impact  

pdf

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4 hours ago, ed said:

NTSB preliminary report.  note that both wings were folded up prior to impact. Also “ A 6-inch section of the main wing spar upper cap splice was found about 300 ft southwest of the accident site.”  That seems to clearly show main spar failed at center section well before impact  

pdf

Upper slice cap would be highly unusual, as during a pos G loading, the upper cap is in compression, so it gets even more interesting. Spar caps and splice blocks don’t usually fail in compression, so maybe it did go through an unusually high neg G loading in the accident as well as Pos?

I get only page one which indicates the wings separated outboard of the landing gear.

Edited by A64Pilot
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Mooney's have no established reputation for corrosion. From my experience, I imagine that the choice of alloys and assembly techniques were wise, even though the financial strength of a company can sometimes compromise the process. As far as I was concerned, I chose to control the main spar by ultrasound, since there was no surface treatment that could create discontinuity between the soles of the splices. I found a very healthy spar.

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I worked for a few years at the manufacturer Dassault Aviation, my job was airframe adjuster, I realized the manual forming of the wings of the Falcons, also the assembly of the plane center (where the wings are joined to the fuselage). What saddens me in the construction of Mooney's are the steel fittings, without special protections other than zinc phosphate, like the one visible in the main cargo hold.

Edited by Raymond J
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6 hours ago, Raymond J said:

Mooney's have no established reputation for corrosion. From my experience, I imagine that the choice of alloys and assembly techniques were wise, even though the financial strength of a company can sometimes compromise the process. As far as I was concerned, I chose to control the main spar by ultrasound, since there was no surface treatment that could create discontinuity between the soles of the splices.

I worked for a few years at the manufacturer Dassault Aviation, .... What saddens me in the construction of Mooney's are the steel fittings, without special protections other than zinc phosphate, like the one visible in the main cargo hold.

I believe that Mooney switched from zinc chromate to epoxy on the steel bits/frame starting in 1986.  Raymond J highlights the connections of dissimilar steel to aluminum.  In BeechTalk Norman Earle highlights that he has seen intergranular corrosion on the wing spar caps and doublers when parting out Mooneys.  He said he suspects poor heat treatment of the alloys rendering it a quality control issue that does not necessarily effect the entire fleet.

Intergranular Corrosion in aluminum from Materials Science and Materials Engineering 2020:

"Intergranular corrosion is a special form of microstructurally influenced corrosion, whereby the grain boundary ‘region’ of the alloy is electrochemically different to the bulk or adjacent alloy microstructure.

Intergranular corrosion can occur in a variety of aluminum alloys, but is especially prevalent in the heat treatable 2xxx, 6xxx, and 7xxx series. Alloys of the 5xxx series with more than 3.5 wt% magnesium can be sensitized to intergranular corrosion by thermal exposure. 3xxx alloys containing manganese will occasionally exhibit intergranular sensitivity (Colvin, 2001).

In aluminum-copper alloys, precipitation of Al2Cu particles at the grain boundaries leaves the adjacent solid solution anodic and more prone to corrosion. With aluminum-magnesium alloys the opposite situation occurs, since the precipitated phase Mg2Al3 is less noble than the solid solution. Serious intergranular attack in these alloys may however be avoided, provided that correct manufacturing and heat treatment conditions are observed. "

I can find nothing in the manuals about the alloys used in the many many pieces (as highlighted above by M20Doc) that make up the so called "one piece" wing spar.  Does anyone know?

If it turns out that corrosion did play a role in the wing spar failure, even secondary to an instrument upset/pilot spatial disorientation, this will be bad news for both owners and Mooney.  Could be a Spar AD coming.  If so and caused by years of spilled drinks and water leaks then there will be many Mooneys retired.  If caused by poor material specs or poor QA/QC during manufacture then many Mooneys retired and many lawsuits.  I don't think insurance covers any of this.....

 

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Zinc chromate is the yellow color anticorrosion primer, it is used only on aluminum alloys.

Mooney used this treatment on the internal parts subjected to condensation... Oddly, the inside of the wing is not treated with this while the tail cone is.

For steel, zinc phosphate is used, it is a primer of dark green color, the same color as the primer found in the engine compartment of some Mooney, on the inner part of the aluminum alloy engine cowling. But don't get me wrong because you could order zinc chromate in this color too.

The application of zinc chromate it is very toxic, hence the replacement by allodine which is miscible with water for the application. Our Mooney's are treated with that where there is no zinc chromate.

In its manufacturing process, Mooney does not have operations dedicated to pre-assembly surface treatment (of assemblies or subassemblies). For this manufacturer, both surface treatment and corrosion protection is a final finishing operation. This is not criticable in itself because this design of the manufacturing range finds arguments when one is in a short assembly process with a reduced cycle time.

In aviation, this is not really the case, except in 1943 when some factories released 1 B24 liberator per hour or a Me 109 in 2h30 see an Avro Lancaster in 2h50...

Personally, I think this design of the manufacturing range was justified when Mooney assembled several aircraft in the day as was the case in the 60s... That this design of the range is no longer valid when increasing the cycle time and exposure of bare surfaces to the effects of time.

For a long time, manufacturers of metal aircraft have known about the influence of condensation in factories, the effects of soluble cutting oils on machined parts... At Dassault as at Daher or Airbus, we monitor the basicity / acidity of the cutting oils, the hygrometry of the buildings and we have included in the manufacturing range the finishing operations before the final assembly.

I suspect that on the other side of the lake it is the same for Boeing, Northrop, Cessna, Bombardier Canadair... I don't know if that was the case at Mooney in the ' 80s.

In France, the company Reims aviation manufactured Cessnas under license, you can see that the French Cessnas are not affected by certain AD "corrosion" thanks to this manufacturing process.

Edited by Raymond J
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One thing for sure…

If your spar cap has a corrosion issue… you want to take care of it ASAP…. There is an acceptable, yet tiny limit, to the depth…

The aluminum alloy is known to not hold back intergranular corrosion… so… once started, it moves quickly…

Spar caps in the back seat area, live in a tough environment… having their coatings worn off by the movement of feet and dirt…. And getting coated with soda and drink box liquids…

PP thoughts only… mostly about older Mooneys than the one being discussed here…

Best regards,

-a-

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On 8/29/2021 at 12:16 PM, 1980Mooney said:

I can find nothing in the manuals about the alloys used in the many many pieces (as highlighted above by M20Doc) that make up the so called "one piece" wing spar.  Does anyone know?

If it turns out that corrosion did play a role in the wing spar failure, even secondary to an instrument upset/pilot spatial disorientation, this will be bad news for both owners and Mooney.  Could be a Spar AD coming.  If so and caused by years of spilled drinks and water leaks then there will be many Mooneys retired.  If caused by poor material specs or poor QA/QC during manufacture then many Mooneys retired and many lawsuits.  I don't think insurance covers any of this.....

 

Thanks for the interesting details on metallurgical considerations - I'm also curious how they apply to our spars.  However, the circumstances of this crash are radically different from that Arrow accident that led to the Piper wing AD, and I suspect it will not inspire nearly as much regulatory scrutiny from an airframe perspective.   I also think the center section of spar that failed is relatively easy to examine and no fleet-wide issues have been noted regarding corrosion susceptibility of that area to date?   What worries me more are the portions of the wing structure that are covered with tank sealant - I think someone here stripped a tank for reseal at some point, only to learn that his plane was totaled due to corrosion.   Then there are planes like mine, where someone put in bladders when the leaks became intractable. In that case, not only does the sealant not get stripped (possibly underlying corrosion that was contributing to the leak), but a portion of the wing structure is made permanently inaccessible for visual inspection.  

Edited by DXB
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On 8/28/2021 at 7:07 PM, A64Pilot said:

Upper slice cap would be highly unusual, as during a pos G loading, the upper cap is in compression, so it gets even more interesting. Spar caps and splice blocks don’t usually fail in compression, so maybe it did go through an unusually high neg G loading in the accident as well as Pos?

I get only page one which indicates the wings separated outboard of the landing gear.

That is an interesting thought. Possible the plane flipped, the wings failed in the negative, and then it continued to tumble so the wings were failed in the positive direction when it impacted. That would square with the prior two incidents, the J and the Rocket, where the failure was negative. 

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On 8/28/2021 at 3:42 PM, ed said:

NTSB preliminary report.  note that both wings were folded up prior to impact. Also “ A 6-inch section of the main wing spar upper cap splice was found about 300 ft southwest of the accident site.”  That seems to clearly show main spar failed at center section well before impact  

pdf

Your reading into something that is not there The report is not at all specific about the section of upper cap. Moreover, given the impact energy, it’s no shock that stressed parts might be found 100yds from impact. The report is very specific about where the wing folded. Both sides failed outside of the main gear. This is just outside the station where the stub ends. The wing transitions from Main Spar, Stub Spar and Rear Spar to Main and Rear.

As I read the report, the wing failed between WS 59.25 and WS 74. Illustrated below. Watching the video, the plane hit tail low but level in roll. The center section of the spar was most certainly compromised by the impact. I don’t see anything that indicates it “clearly failed at the center section before impact” as you say above.

 

 

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On 8/25/2021 at 1:24 PM, Shadrach said:

We shall see but my WAG is that the intersection of load and strength will be between WS 59.25 and WS 74. 

 

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I suppose it makes sense that the main spar failed just outside the stub spar, whose structure along with that of the fuselage bolted to the wing may provide some reinforcement to the central portion. It seems kinda remarkable that both sides failed at the same instant - I'd think one side would fail first resulting in the other side being unloaded.

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47 minutes ago, DXB said:

I suppose it makes sense that the main spar failed just outside the stub spar, whose structure along with that of the fuselage bolted to the wing may provide some reinforcement to the central portion. It seems kinda remarkable that both sides failed at the same instant - I'd think one side would fail first resulting in the other side being unloaded.

Yes, looking at the wing’s construction it seem intuitive that it would break where it did. The fact that both sides failed at the same time and in the same place suggests not only consistent [over]loading during the event but also and more importantly to my thinking, very consistent manufacturing tolerances that yielded a structure that failed symmetrically under tremendous load ever after nearly 30 years in service. That may not seem impressive to an engineer but it is to me. After 30 years of flexing, turbulence, lousy landings and anything else to which an aircraft might be subjected over such a period, it yielded in symmetrical fashion at it’s absolute limit.

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4 hours ago, Shadrach said:

As I read the report, the wing failed between WS 59.25 and WS 74. Illustrated below. Watching the video, the plane hit tail low but level in roll. The center section of the spar was most certainly compromised by the impact. I don’t see anything that indicates it “clearly failed at the center section before impact” as you say above.

 

 

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How do you conclude "The center section of the spar was most certainly compromised by the impact"?  If you believe that the wings folded up vertically outboard of the main landing gear before impact then they were no longer placing any load (or minimal at best) upon the center section of the main spar at the moment of impact.  With no loading or levering of the wings transmitted to and upon the center of the spar at the moment of impact, it does not seem reasonable that the impact alone would cause "The main and rear wings spars were highly fragmented in the center of the airplane between the separated left and right wings."

Edited by 1980Mooney
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