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RV-14/14A Service Bulletin SB 18-09-17 published: possible cracking of F-01478 skin

greghughespdx

Well Known Member
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Van?s Aircraft has released a service bulletin for the RV-14/14A (SB 18-09-17) addressing possible cracking of the F-01478 ?Aft Fuse Forward Bottom Skin? that may occur in flying aircraft. This service bulletin is applicable to all RV-14/14A aircraft empennage/tailcone kits shipped prior to September 26, 2018. It calls for inspection of the F-01478 skin in specific locations, and a modification in the event cracks are found. This inspection needs to be conducted at or before the next annual inspection, and then at each annual inspection until the modifications described in the service bulletin document have been completed.

The Van?s part number to order for the kit used to complete this change is Part no. SB 18-09-17. The cost for this kit is $10 plus shipping.

I?ve inspected my aircraft and did not find any cracks. Should I install this modification proactively?

Installation on an airplane that has not yet developed the described cracks is optional and at the discretion of the builder/owner. Note that for the affected aircraft kits, operation of the aircraft without this modification the aft fuselage bottom skin is likely to result in eventual formation of the described cracks, at which time the skin will need to be stopped drilled and the modification kit will need to be installed. Until the modification kit is installed, you need to conduct regular inspections as described in the service bulletin document. The cracking that may occur is limited in area and is primarily cosmetic in nature, rather than a structural issue. Due to the wide variety of flight conditions and operational characteristics between different aircraft, we cannot predict when the skin cracks may form for a particular airplane. Proactive installation of the modification kit is an acceptable method to prevent these cracks from forming in the first place.

My airplane has not flown yet. Does this service bulletin apply to me?

Yes. The service bulletin and modification parts apply to all RV-14 empennage/tailcone kits shipped prior to Sept. 26, 2018. If you have not yet assembled or flown your airplane and you received a kit that was shipped prior to that date, the service bulletin?s instructions include a section that describes steps to follow when applying the update to affected aircraft kits which are under construction. For kits that were shipped after September 26, 2018 the updates are already incorporated into the RV-14 kit and related assembly instructions/plans; as such the service bulletin is not applicable to those kits.​
 
I reviewed the SB and ordered parts today. The only hard part will be getting back that far to drill and buck rivets, etc.
 
Anybody affected have pics of a crack so we know what to look for?
 
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crawling back is not going to be that difficult, the rivets will be drilled out from the bottom and we only need to buck from inside, beside a couple of rivets which can be squeezed.
Put some plywood/moving blanket that you can put your weight on, lay on one side of the push/pull tube and buck. The backside of the bulkhead might be reachable from the empennage area.
 
crawling back is not going to be that difficult, the rivets will be drilled out from the bottom and we only need to buck from inside, beside a couple of rivets which can be squeezed.
Put some plywood/moving blanket that you can put your weight on, lay on one side of the push/pull tube and buck. The backside of the bulkhead might be reachable from the empennage area.

The drilling I was referring to was the match drilling of clips to bulkhead: Difficulty is proportional to age and mobility/size and I'm not exactly a spring chicken. I need a young, skinny person to help!
 
Here?s the thread I think referenced below.

http://www.vansairforce.com/community/showthread.php?t=161648

It?s interesting to note that the SB addresses the cracks on the lower (bottom) fuselage as shown in the thread, but doesn?t address the location of the oil canning on the sides of the fuselage.

I wonder if the sides will also crack one day?

The question to me is: does the oil canning need to be fixed? I'm currently flying a 38 year old Archer with 3K hours or so, and when I fly I can see the top wing skins distorting between rivet lines - presumably due to the effects of lift on the top of the wing. Not a crack in sight.

Is the flexibility of skin/structure that allows oil-canning to take place a normal/natural/desirable part of the design? Not a rhetorical question - I just don't know, thus the question I posed above.
 
I also ordered the SB parts yesterday. With my fuselage kit currently in crating, tailcone access will never be better. Good timing, for once... :)
 
The question to me is: does the oil canning need to be fixed?

Is the flexibility of skin/structure that allows oil-canning to take place a normal/natural/desirable part of the design? Not a rhetorical question - I just don't know, thus the question I posed above.

I would not say it's a normal or desirable attribute of the design...the 14 fuselage is shaped subtlety different from the other side-by-side RVs and has just enough curvature in the lower tailcone side skins to allow oil canning (a bit more or less curvature and it wouldn't happen...I'm sure Vans didn't anticipate this when designing it). Personally I doubt it will ever cause cracking in the aluminum structure...depending on how flexible the paint is, it may degrade the finish over time. I bought some J-channel and bonded it to the side skins with Proseal. No more oil canning, but I added half a pound to my airplane. Which I'm not happy about...but in the end I decided I was more unhappy about the oil canning than the added weight of the stiffeners.

If it bothers you, do something about it. If not, leave it alone. It'll most likely be just fine as is, but nobody knows for sure what the long term effect will be.

Just in case anyone is confused, the oil canning in the side skin panels has absolutely nothing to do with the Service Bulletin to address cracks in the bottom skin (which is the original subject of this thread). I already fabricated my own solution to that problem, but if I had not, I would certainly take advantage of the parts offered in the SB...
 
Mark
If you have not tied the prosealed J stiffener to the bulkheads I would encourage you to do so. The vibration that cased the oil canning will have changed frequency and it will focus on the ends of your stiffeners.
 
The service bulletin addresses cracks in the described aft fuse forward bottom skin, and not tail cone oil canning, first of all, just to be clear.

The question to me is: does the oil canning need to be fixed? I'm currently flying a 38 year old Archer with 3K hours or so, and when I fly I can see the top wing skins distorting between rivet lines - presumably due to the effects of lift on the top of the wing. Not a crack in sight.

Is the flexibility of skin/structure that allows oil-canning to take place a normal/natural/desirable part of the design? Not a rhetorical question - I just don't know, thus the question I posed above.

Norma and natural? Essentially, yes. Desirable? We'd all prefer for it to never happen, of course, but we also need to think about the nature of metal airplane design.

Oil-canning of skins, as described and discussed on these forums in the past, is not completely uncommon in a metal airplane where inside/outside in-flight pressure deltas and skin temperature changes result in some minor movement/expansion/contraction of aluminum skins. While some builders have in the past installed things like stiffeners or adhesive sound-deadening foam to help minimize the effects of oil-canning, it is important to recognize a couple things:

1) The very nature of differential pressures and temperature fluctuations on the skins is such that some oil canning can happen, and this is considered acceptable and normal within reasonable, obvious limits. Oil canning on it's own is not generally a structural concern (unless, of course, some new and unusual permanent skin deformation is seen, in which case we'd look for a cause other than simple oil-canning). Aircraft skin crack formation will tend to occur where skin flexing occurs rapidly and repeatedly. So, an oil-can area that doesn't continuously/repeatedly flex, over and over again at a a relatively rapid rate, is unlikely to develop fatigue-related issues such as cracking. Hence, simple oil canning is not generally a fatigue concern.

2) The addition of custom stiffening structures not specified in the plans can possibly lead to unintended secondary consequences, depending on the details of how they are built and attached. For example, where builders choose to rivet or use adhesive to attach stiffening angles of their own design inside the skins without proper attachment to the aircraft structure, one could unintentionally create a whole new stress concentration area in the skin at each end of the stiffener, which could then cause cracks to develop in the stress concentration area. This type of behavior has been observed in the past on metal airplanes.

Point being, some oil-canning is considered typical, acceptable and normal, and Van's advises using caution if and when determining what custom changes you may want to design and apply. When we analyze our designs for fatigue-prevention, our FEA computer systems and engineers apply a wide variety of cause-effect analysis to help ensure a design is resilient.

Also - as a related sidebar item - when Scott and I were discussing this topic he told me, "If you want to read about some other interesting oil-canning aircraft stories for perspective, try Googling 'oil canning B-52' and taking a look at some of the articles and photos." Interesting stuff!
 
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Thanks for your insight Greg!

Aircraft skin crack formation will tend to occur where skin flexing occurs rapidly and repeatedly. So, an oil-can area that doesn't continuously/repeatedly flex, over and over again at a a relatively rapid rate, is unlikely to develop fatigue-related issues such as cracking. Hence, simple oil canning is not generally a fatigue concern.

Has it been determined beyond doubt that the particular case of skin cracking addressed by this SB is vibration fatigue as opposed to a hard landing? A single cycle is certainly sufficient to cause cracking if the amplitude is high enough. This seems like the place where the crumpling forces would concentrate between the landing gear mount and the tail surfaces.
 
Anybody affected have pics of a crack so we know what to look for?

I'm very interested in seeing a picture of the actual cracks as well. I see the service bulletin adds a single shear splice plate connecting the forward and aft center stiffener across the F-01408 bulkhead. It also has a couple of tension fittings that connect the F-01486-L/R stiffeners directly to the same bulkhead.

Were the cracks located on the skin at these three stiffeners? Greg, perhaps you can post a picture?
 
The service bulletin addresses cracks in the described aft fuse forward bottom skin, and not tail cone oil canning, first of all, just to be clear.



Norma and natural? Essentially, yes. Desirable? We'd all prefer for it to never happen, of course, but we also need to think about the nature of metal airplane design.

Oil-canning of skins, as described and discussed on these forums in the past, is not completely uncommon in a metal airplane where inside/outside in-flight pressure deltas and skin temperature changes result in some minor movement/expansion/contraction of aluminum skins. While some builders have in the past installed things like stiffeners or adhesive sound-deadening foam to help minimize the effects of oil-canning, it is important to recognize a couple things:

1) The very nature of differential pressures and temperature fluctuations on the skins is such that some oil canning can happen, and this is considered acceptable and normal within reasonable, obvious limits. Oil canning on it's own is not generally a structural concern (unless, of course, some new and unusual permanent skin deformation is seen, in which case we'd look for a cause other than simple oil-canning). Aircraft skin crack formation will tend to occur where skin flexing occurs rapidly and repeatedly. So, an oil-can area that doesn't continuously/repeatedly flex, over and over again at a a relatively rapid rate, is unlikely to develop fatigue-related issues such as cracking. Hence, simple oil canning is not generally a fatigue concern.

2) The addition of custom stiffening structures not specified in the plans can possibly lead to unintended secondary consequences, depending on the details of how they are built and attached. For example, where builders choose to rivet or use adhesive to attach stiffening angles of their own design inside the skins without proper attachment to the aircraft structure, one could unintentionally create a whole new stress concentration area in the skin at each end of the stiffener, which could then cause cracks to develop in the stress concentration area. This type of behavior has been observed in the past on metal airplanes.

Point being, some oil-canning is considered typical, acceptable and normal, and Van's advises using caution if and when determining what custom changes you may want to design and apply. When we analyze our designs for fatigue-prevention, our FEA computer systems and engineers apply a wide variety of cause-effect analysis to help ensure a design is resilient.

Also - as a related sidebar item - when Scott and I were discussing this topic he told me, "If you want to read about some other interesting oil-canning aircraft stories for perspective, try Googling 'oil canning B-52' and taking a look at some of the articles and photos." Interesting stuff!

Greg: thanks for this explanation. Exactly what I was hoping to hear. I will of course do the SB as cracks are clearly an issue, but I don't feel the need to add stiffeners etc for oil canning given that it's not a structural concern.
 
Normal and natural? Essentially, yes.
<snip>
Also - as a related sidebar item - when Scott and I were discussing this topic he told me, "If you want to read about some other interesting oil-canning aircraft stories for perspective, try Googling 'oil canning B-52' and taking a look at some of the articles and photos." Interesting stuff!

bf99_airshow_b52_wrinkles_mwhaley_665-805-22.jpg


50+ years of oil-canning on this airplane, no fuselage failures yet. And you definitely hear the fuse flexing when you go over bumps on the taxiway.

Dave
(former BUFF crew)
 
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I'm very interested in seeing a picture of the actual cracks as well. I see the service bulletin adds a single shear splice plate connecting the forward and aft center stiffener across the F-01408 bulkhead. It also has a couple of tension fittings that connect the F-01486-L/R stiffeners directly to the same bulkhead.

Were the cracks located on the skin at these three stiffeners? Greg, perhaps you can post a picture?

Yes, and that is what the SB 18-09-17 addresses specifically. The thread linked by TASEsq above includes an example photo.
 
Also - as a related sidebar item - when Scott and I were discussing this topic he told me, "If you want to read about some other interesting oil-canning aircraft stories for perspective, try Googling 'oil canning B-52' and taking a look at some of the articles and photos." Interesting stuff!

The ripples in the B-52 fuselage skin are diagonal tension buckling. The weight of the fuselage ahead of the front landing gear provides a downward load on the structure and each section of the skin between the longerons and bulkheads is loaded in shear. Take a piece of paper, hold it vertically with your left hand on the left edge, your right hand on the right edge and pull down with your right hand while keeping your left hand still. The ripples in the paper occur at approximately 45 degrees and the paper on the diagonal from top left to bottom right is in tension. Keep applying load. Even though the paper has buckled it is in tension and will accept more load.

The buckling in the bottom fuselage panels of the 2 seat RVs results from applying up elevator which loads the top fuselage panels in direct tension and the bottom fuselage panels in direct compression. Take the piece of paper again, this time move both hands together applying a compressive load to the paper. It will buckle, and at that point the same force will continue to make the buckle larger. In the aircraft structure, this means the skin no longer carries any further load once the buckle starts, and the load is then carried by the longerons or stiffeners riveted to that skin panel.

So there are a couple of different kinds of buckling (oil canning if you like). In my view, compressive skin buckling is to be avoided.

Hopefully some aeronautical engineers on this forum will weigh in with further insight.
 
We've conflated two different items here. This thread was specifically an announcement about the F-01478 skin cracking potential and the service bulletin/kit that has been released to address the issue. There are already other threads on the forums here discussing skin oil canning, so I'd suggest in the interest of not confusing people unnecessarily we separate these topics going forward, as they are not the same/related.
 
The buckling in the bottom fuselage panels of the 2 seat RVs results from applying up elevator which loads the top fuselage panels in direct tension and the bottom fuselage panels in direct compression.

So there are a couple of different kinds of buckling (oil canning if you like). In my view, compressive skin buckling is to be avoided.

Hopefully some aeronautical engineers on this forum will weigh in with further insight.

More than a couple even.... But once again for posterity..... The cracks addressed by this S.B. are not related to oil canning. That was an entirely separate discussion. Hopefully anyone commenting on the S.B. has read it.

Also, not disagreeing with you regarding your analysis that compression loads are applied on the bottom with up elevator input, but I disagree that you can suppose that that is what must have caused the skin cracking addressed by this S.B..

The type of skin cracks we are talking about here are caused by many load cycles. This type of load cycle is induced into the tail cone in many different ways, so suggesting that it is caused by loads induced by elevator forces is just a guess. It could have been simply the result of vibration induced cycles that acted on the end rivet of an unsupported skin stiffener.
 
We've conflated two different items here. This thread was specifically an announcement about the F-01478 skin cracking potential and the service bulletin/kit that has been released to address the issue. There are already other threads on the forums here discussing skin oil canning, so I'd suggest in the interest of not confusing people unnecessarily we separate these topics going forward, as they are not the same/related.


I only referenced that thread, as it discussed the oil canning issue and included the crack photo on the lower fuselage. The thread indicated that Vans is releasing a SB for oil canning as well. Since these are seperate issues, will there be a seperate SB to address the oil canning? (Or do we wait for cracks to appear from that? Will/could they appear from that?)
 
I only referenced that thread, as it discussed the oil canning issue and included the crack photo on the lower fuselage. The thread indicated that Vans is releasing a SB for oil canning as well. Since these are seperate issues, will there be a seperate SB to address the oil canning? (Or do we wait for cracks to appear from that? Will/could they appear from that?)

Not sure how to be more clear than this, but we do need to be clear and ensure we don't confuse people via discussion: The cracks on the lower fuselage skin (including the one shown in the photo) are not caused by skin "oil canning," and have been addressed using the new parts kit issued with the SB.

There has been no indication made by Van's that a SB is planned for "oil canning." That's because there is no issue related to "oil canning" which needs to be addressed.

Two different topics being talked about here, unrelated, and only one represents an issue that needs to be/is being addressed:

  • Skin cracks on F-01478, which were shown in a photo in the linked forum thread and are addressed by adding stiffening structure in a specific location - Addressed via this service bulletin.
  • Skins may "oil can" in certain conditions related to differential pressures, temperatures and certain loading - Not an issue, not being addressed, not a cause of cracking.

Thanks, all. :)
 
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So for those of us where the dimples are already created. It looks like now there will be a void where the original rivet used to be on the belly skin. For corrosion protection does it make sense to fill this dimple with something before installing the F-14148A SKIN DOUBLER? I think Spruce sells some kind of aluminium filler?
 
So for those of us where the dimples are already created. It looks like now there will be a void where the original rivet used to be on the belly skin. For corrosion protection does it make sense to fill this dimple with something before installing the F-14148A SKIN DOUBLER? I think Spruce sells some kind of aluminium filler?

This is a good question, something that I have been wondering about. What material would be best suited to fill this void. I have never bucked a rivet with such void in the sandwich pieces so I am wondering what percautions need to be kept in mind when riveting this piece?
 
So for those of us where the dimples are already created. It looks like now there will be a void where the original rivet used to be on the belly skin. For corrosion protection does it make sense to fill this dimple with something before installing the F-14148A SKIN DOUBLER? I think Spruce sells some kind of aluminium filler?


Maybe a dab of proseal or similar to help keep water out?
 
So for those of us where the dimples are already created. It looks like now there will be a void where the original rivet used to be on the belly skin. For corrosion protection does it make sense to fill this dimple with something before installing the F-14148A SKIN DOUBLER? I think Spruce sells some kind of aluminium filler?

You could make some countersink washers by driving some AN426AD3 rivets into some scrap (countersink the scrap first, of course) and drilling out the heads carefully. If you're careful you can break the head off intact, deburr and use it in the dimple.

Proseal is commonly used on the faying surface of repair doublers on large aircraft repairs. It is used to prevent moisture ingression into the joint. Without an electrolyte, corrosion is hard to form.
 
Yes, you are correct the plans call for countersinking due to the thickness of the doubler. With a thick doubler on the bottom and the smaller new doublers inside I see no reason to worry about the previous dimples as they will be captured between the new parts.
 
My Crack(less) Status

I ordered and very quickly received my SB kit (Thanks Vans!). The upside is that I already now have the parts on hand to fix the issue, or prevent the issue.

The downside is that I'm already built/painted/flying, and it's now cold out, so it's very hard to be interested in drilling into painted rivets and painting a new doubler and doing the install right now. It may be a good spring project.

I wish I knew how likely it would be that the cracks would affect me. I'm right around 295 hours on the plane now, and I can see no sign of cracking or anything wrong from the outside belly of the plane. If there are cracks, they haven't yet caused anything to be visible through the paint. I'll have to get my fat a into the tail and see how it looks from that side.

Considering that I have in no way been babying the airplane, I really am curious as to the frequency of issues that have been seen, also. At present though, I just feel fortunate.
 
SB

I believe Van's found the cracks in both of the prototype aircraft.

Here's a quote from the SB Van's published.

"These cracks have been discovered in the factory RV-14/14A aircraft, as well as some flying aircraft in the field."

As a side note, I've already received my repair kit, (I live in KY and had it in a matter of days after ordering....Thanks Van's!!) I'm in the build stage and will be doing the repairs over the holidays, when my helper is home from school.
While the process seems fairly straight forward, I'm hopeful some of you folks completing the process before then will post some tips and pictures. :)

Cheers
 
I'm very interested in seeing a picture of the actual cracks as well. I see the service bulletin adds a single shear splice plate connecting the forward and aft center stiffener across the F-01408 bulkhead. It also has a couple of tension fittings that connect the F-01486-L/R stiffeners directly to the same bulkhead.

Were the cracks located on the skin at these three stiffeners? Greg, perhaps you can post a picture?

Greetings.

Please see this VAF post for examples of cracks on a flying RV-14 (not RV-14A).

Here are the parts contained in SB 18-09-17. Rounded forward, blunt aft and beveled edges are apparent on F-14148A Skin Doubler. Remember to countersink F-14148A/C to accept the dimpled J-stiffeners.

I will be installing these during my annual in two weeks and will report back with my experience. My airframe has 87.6 hours, is flown by a pilot who prefers boring flights and shows no indication of cracks in the area associated with the SB.

IMG_9543.JPG
 
Machine countersinking:F-14148A

When I prepared F-1414A, I did not double check the depth of the countersink on a scrap in a # 40 hole. Let me just say, you may not want to order that part again, if you forget the basic steps in countersinking🤪
Not deep enough is fixable. To deep . . . Well, plan on a moment of interspection at best! Yikes - after building a ?completed? airplane, than this!!!
 
When I prepared F-1414A, I did not double check the depth of the countersink on a scrap in a # 40 hole. Let me just say, you may not want to order that part again, if you forget the basic steps in countersinking🤪
Not deep enough is fixable. To deep . . . Well, plan on a moment of interspection at best! Yikes - after building a ?completed? airplane, than this!!!

Other than cosmetics and the loss of .001 knots of speed, any reason not to use universal head rivets?
 
Other than cosmetics and the loss of .001 knots of speed, any reason not to use universal head rivets?

Greetings.

I don't see the advantage of doing so. By the time you get around to installing the parts associated with the SB, you'll have developed the skills to manage it with confidence. And if you do make a mistake, those same skills can be brought to bear to address the error.

Countersinking the F-14148A Skin Doubler (the diamond plate) is straightforward. Use something underneath it with a #40 hole so the pilot on your c/s bit has somewhere to go. Use additional material on the side of the plate so your c/s cage stays normal to the surface

Doing the same as above for the F-14148B/C Stiffener Doublers (the ones that accept the dimples of the J-stiffener), you can follow also the advice on page 05-08:

For a dimpled skin riveted onto a machine countersunk surface the countersink must be slightly deeper as mentioned earlier. Proper depth is .007 deeper than when the rivet head is flush. This depth correction corresponds to seven "clicks" on a microstop countersink tool indexed in .001 inch increments.​
 
Greetings.

I don't see the advantage of doing so. By the time you get around to installing the parts associated with the SB, you'll have developed the skills to manage it with confidence. And if you do make a mistake, those same skills can be brought to bear to address the error.

Countersinking the F-14148A Skin Doubler (the diamond plate) is straightforward. Use something underneath it with a #40 hole so the pilot on your c/s bit has somewhere to go. Use additional material on the side of the plate so your c/s cage stays normal to the surface

Doing the same as above for the F-14148B/C Stiffener Doublers (the ones that accept the dimples of the J-stiffener), you can follow also the advice on page 05-08:

For a dimpled skin riveted onto a machine countersunk surface the countersink must be slightly deeper as mentioned earlier. Proper depth is .007 deeper than when the rivet head is flush. This depth correction corresponds to seven "clicks" on a microstop countersink tool indexed in .001 inch increments.​

No news here: I was asking the question to make a point really - and part of the point is that universal head rivets in a low-visibility area do no harm.
 
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After 25 years of riveting I do not make very many riveting mistakes, but if I do, it is usually a smile on a universal head. Given a choice I will always take flush headed rivets. Best in this location to follow the plans.
 
On the belly, a universal head rivet will be an oil collection point. Even a less-than-flush flush rivet will do so. I believe using a universal would be a regrettable shortcut. YMMV
 
On the belly, a universal head rivet will be an oil collection point. Even a less-than-flush flush rivet will do so. I believe using a universal would be a regrettable shortcut. YMMV

There are already a number of non-flush (Cherry Max) rivets on the belly of an RV14/14A. I’m not advocating this particular approach but I don’t think a handful more rivets with a little oil to wipe off from time to time is going to be a source of regret or angst or even much of a second thought. We’re getting into primer war territory here. Amusing.
 
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Finished the SB install, about 1 hour. My tailcone was completely built but not yet joined the the fuselage. I back-riveted with the tailcone on the ground, very easy.
 
I'm very interested in seeing a picture of the actual cracks as well. I see the service bulletin adds a single shear splice plate connecting the forward and aft center stiffener across the F-01408 bulkhead. It also has a couple of tension fittings that connect the F-01486-L/R stiffeners directly to the same bulkhead.

Were the cracks located on the skin at these three stiffeners? Greg, perhaps you can post a picture?

Sorry for the slow response. These reference images are to hopefully provide some additional location context for those reviewing the 18-09-17 service bulletin. These particular photos are of the Van's factory demo RV-14. Cracks were found on both of our RV-14 demo aircraft (conventional and tricycle models) here at the factory, as was described earlier. No meaningful difference between the two aircraft in terms of cracks/size/location/etc.



 
I completed this project this week and found it to be challenging only because I am 6'1" 200 lbs and 67 years old. it was very difficult to get enough leverage to squeeze the 470-4 rivets, the rest not too bad. Two man job, took 1.5 hours.
 
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