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Loose Main Gear Legs

Picture of structure

For those unfamiliar with the 12's structure:
570014977_GX2vJ-M.jpg


570014868_3GtZc-M.jpg

The channel is at left in this shot. The gear bolts to the underside of the channel. Rectangular cut for one of the gear bolts is visible, the cracks occur on the front side of the channel in a mirror image of the cut out. The wing spars roll in on the black nylon roller and the pin bushings are obvious.

Tony
 
Something we focused on Tony was that some cutouts for bolts are square, as yours appear to be, some are more narrow at the top. None of the square holes broke out, only the pear shaped ones it seems (no idea what that might mean or why they might have changed the cutout).

Tony_T said:
For those unfamiliar with the 12's structure:
570014977_GX2vJ-M.jpg


570014868_3GtZc-M.jpg

The channel is at left in this shot. The gear bolts to the underside of the channel. Rectangular cut for one of the gear bolts is visible, the cracks occur on the front side of the channel in a mirror image of the cut out. The wing spars roll in on the black nylon roller and the pin bushings are obvious.

Tony
Click to expand...
 
No Cracks

Inspected 912HM today. Have 40+ hours some not so soft landings but not much braking. No cracks or deformed skins.
 
HMMM

I was thinking about this today while I was driving and I think there are two separate issues being discussed on this thread. The original is the wrinkled skin and crimped brake lines. The second is the bolt and channel cracking issue. I think they are probably unrelated.
The channel is one of the beefiest pieces in the fuselage, basically a 1/4" C channel. It will probably be the last piece to actually bend, but the surrounding structure is not nearly as strong, especially the part of the fuselage attached to the rear side of the channel. I noticed that all of the wrinkled fuselage pictures show a vertical wrinkle, as opposed to a horizontal one. This supports the hypothesis that the rear part of the fuselage and tailcone are actually moving in a downward direction. This would push the side skin below the channel forward where it contacts the brake line. The baggage floor attach point to the channel acts like a hinge to allow this. If you think about it, everything above the floor of the baggage compart has very little lateral support, especially above the longeron, since that is where the rear window is. These skins are thin! (.020 or .025 if I remember correctly) Even in a monocoque configuration, even though they are very strong, they can still fail. Like an aluminum can, if you bend it a little too much. Maybe it's aerodynamic force, too many heavy landings, rough fields and heavy braking or a combination of all of these, but I personally think that the rear fuselages are just bending down causing the wrinkling. Maybe a diagonal support from the channel to the rear and up to the longeron would help, I don't know. The fix is for VAN's to design.
As for the channel and cracking bolt holes, maybe it's just a metallurgical issue with some of the parts, or maybe some of the bolts are getting over torqued, I don't know, but I see it as a separate issue from the wrinkling.
Just my observations for you to think about. I hope someone figures it out.
 
Harder to imagine, but you may be correct. That would also explain the mystery of the spar pin not doing anything, which would happen if the channel stood still and the tail came forward enough to pinch the brake line. Will have to think about that some more.
 
DonFromTX said:
Something we focused on Tony was that some cutouts for bolts are square, as yours appear to be, some are more narrow at the top. None of the square holes broke out, only the pear shaped ones it seems (no idea what that might mean or why they might have changed the cutout).
Click to expand...

Not meaning to single Don out (because there has been a lot of different speculation based on incorrect info), but the cutout has never changed. They are all the same. What you think you are seeing as different shapes is likely cause by looking at it at different angles. Try it on your own airplane.
 
sf3543 said:
I noticed that all of the wrinkled fuselage pictures show a vertical wrinkle, as opposed to a horizontal one. This supports the hypothesis that the rear part of the fuselage and tailcone are actually moving in a downward direction. This would push the side skin below the channel forward where it contacts the brake line. The baggage floor attach point to the channel acts like a hinge to allow this.
Click to expand...

I think that is likely part of the picture. Relatively speaking, the rear part of the fuselage would effectively tend to move down as the undercarriage tries to twist the channel back and up. The resulting damage to the skin could make the structure more flexible and possibly allow the rotation about the baggage floor hinge line that you describe.

Don't forget, this is all happening in a dynamic environment, with a good deal of movement and flexing going on, so it's a pretty complex picture. We are not seeing all the movement that has occurred - only the permanent deformation. You may also be correct about the bolts, as not all those with skin damage have cracks around the bolts. Anyway, we don't really have any answers yet.

Edit: I'd add that since we can't be sure we understand the failure mechanisms, we should also be very cautious about suggesting any fixes. We really need to leave that to Vans, IMHO.
 
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Channel Rotation

Steve
Not really knowing the what happen while landing, the tail rotating down on a very hard landing may be possible.

However the baggage floor would go into compression and the baggage ribs would keep floor very straight , this is a very strong section and would be hard to bend.

If this was a from breaking the center of momentum in the horizontal axis of the tail would be too high to cause this damage. ( watch a break applied landing the nose goes down and the tail goes up )

All you builders should remember when you put the F-1276 bottom skin on how easy it was to move the baggage floor up and down ( rotating at the channel ) to put the cleco in, then add all the F-1268 triangles to tie the ribs to the bulkhead.

The 1/4 Channel in the F-1204 Center section assembly really has no clear path resistance other then the side skins and the spar pin in a bushing that the channel can move away in rotation.
I stand firm in my opinion.




sf3543 said:
I was thinking about this today while I was driving and I think there are two separate issues being discussed on this thread. The original is the wrinkled skin and crimped brake lines. The second is the bolt and channel cracking issue. I think they are probably unrelated.
The channel is one of the beefiest pieces in the fuselage, basically a 1/4" C channel. It will probably be the last piece to actually bend, but the surrounding structure is not nearly as strong, especially the part of the fuselage attached to the rear side of the channel. I noticed that all of the wrinkled fuselage pictures show a vertical wrinkle, as opposed to a horizontal one. This supports the hypothesis that the rear part of the fuselage and tailcone are actually moving in a downward direction. This would push the side skin below the channel forward where it contacts the brake line. The baggage floor attach point to the channel acts like a hinge to allow this. If you think about it, everything above the floor of the baggage compart has very little lateral support, especially above the longeron, since that is where the rear window is. These skins are thin! (.020 or .025 if I remember correctly) Even in a monocoque configuration, even though they are very strong, they can still fail. Like an aluminum can, if you bend it a little too much. Maybe it's aerodynamic force, too many heavy landings, rough fields and heavy braking or a combination of all of these, but I personally think that the rear fuselages are just bending down causing the wrinkling. Maybe a diagonal support from the channel to the rear and up to the longeron would help, I don't know. The fix is for VAN's to design.
As for the channel and cracking bolt holes, maybe it's just a metallurgical issue with some of the parts, or maybe some of the bolts are getting over torqued, I don't know, but I see it as a separate issue from the wrinkling.
Just my observations for you to think about. I hope someone figures it out.
Click to expand...
 
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DonFromTX said:
Harder to imagine, but you may be correct. That would also explain the mystery of the spar pin not doing anything, which would happen if the channel stood still and the tail came forward enough to pinch the brake line. Will have to think about that some more.
Click to expand...

The wrinkled skins are almost identical to the damage I had to a lease back 1-36 glider. The glider seems quite comparable in the aft section to the RV-12 with no real longerons, just stringers.

A power pilot forgot he had no throttle and landed 10 ft too high, 100 ft short and on a road 10 ft too narrow...:rolleyes:

The very strong Schweitzer cockpit area and skid held together reasonably intact, impacted first and saved the pilot, but the aft fuselage continued downwards and produced nearly identical wrinkles to the previous photos. IIRC the replacement skin was pretty thin, 0.020 or 0.025, similar to the RV-12.
 
Bottom of Fuselage

Can someone with damage produce photos of the bottom of the fuselage to see if that skin was in compression or tension during the failure.


Jersey
 
Another thought

One thought I had regarding this is the way the brake pedals are drilled and mounted to the rudder pedals. When I got to this step, it occurred to me that I might not want the position of the brake pedal to be quite that generic. I probably have larger than average feet and wanted the brakes to require a deliberate movement of my feet to activate. It took some time to dial in this brake pedal position. Where I am going with this, is it possible that the brake pedal is positioned so far aft that the pilot could be inadvertently activating the brakes while manipulating the rudder? Think about the scenario, Here you are on short final in a crosswind, working the rudder and flying an airplane that you don't have many hours in yet. You are possibly a little high, a bit fast and as you land, you not only have your feet firmly planted on the rudder, but the brakes as well. At this point the brakes are possibly locking up or at the very least, not being used for what they are designed. At these speeds, could a touch down in a crosswind with hard right or left rudder and a locked brake cause this damage? What say the smart guys?
 
Loose Bolts

Removed the wings this afternoon to check for deformation of the fuselage skin at the wing root. Skin looked good and no cracks on the channel. Found the two outside bolts on the right landing gear loose. Estimated 2 ft lbs or less to turn them. The left side was loose also with estimated 15 ft lbs to turn them. Just wondering if loose bolts is resulting in metal fatigue (radial cracking) at the bolt heads or is metal fatigue causing the bolts to be loose and radial cracking forming later .:confused:

Hope we hear something from Vans soon.

Jersey
 
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Check your Bolts

loose bolts can cause the cracking

jersey said:
Removed the wings this afternoon to check for deformation of the fuselage skin at the wing root. Skin looked good and no cracks on the channel. Found the two outside bolts on the right landing gear loose. Estimated 2 ft lbs or less to turn them. The left side was loose also with estimated 15 ft lbs to turn them. Just wondering if loose bolts is resulting in metal fatigue (radial cracking) at the bolt heads or is metal fatigue causing the bolts to be loose and radial cracking forming later .:confused:

Hope we hear something from Vans soon.

Jersey
Click to expand...
 
Worried......

I too hope we hear something from Vans soon. I understand their need to react thoughtfully and cautiously, but this is clearly an issue that warrants their attention.

I will be flying predominantly from a grass strip and am far from encouraged by what I hear. I am currently building the fuselage and will be attaching the gear legs in the not too distant future. If there is to be a modification, this would be a good time to know about it.

:confused: Erik
 
Loose Bolts

Joe you are probably right.

Whats causing the bolts to loosen? Is it a sympton of something else?

Fact: I know I tightened them correctly during construction.

Cause of loose bolt: ?

Jersey
 
Great info and PIREP's rolling in fellas. Please keep them coming.

On another note, I called Scott @ Van's a few days ago, and expressed my concerns over these recent reports of damage. Scott reports in on the forums from time to time (RVBuilder2002), and as I am about to begin construction of my fuselage, I obviously had concerns about whether to do so, or wait for further info. Scott was kind enough to spend a great deal of time on the phone with me, and mentioned that "a few people" had sent in the official reports, but not nearly anything like we are seeing online. Scott told me that Van's was aware of the situation, and had done telephone intervews with a few people. He said that as they gather information (again, this MUST come from the official form that needs to be submitted to Vans), that the company will research the data. He said nothing has been noted as of yet, that seems to be a common event amongst the few people they have spoken with. He did say that if a solution was deemed necessary by Vans, that it would be "a fix that can be applied to an aircraft being built, or an aircraft already flying". So with this new info, I am pressing on and about to crack open the fuselage crate and begin on that.

The folks at Vans are obviously THE experts on this sort of thing, and I am glad that the factory is at least aware that these events are occuring. I too am in hopes that a solution is found, and we can all sleep better at night. As I mentioned, and I hate to keep 'beating a dead horse', but Scott said that specific data is highly critical from anyone that has had issues, and if reported to Vans on the specified forms, they are going to be looking into all instances and trying to form a common occurence that may have caused the aforementioned problems. So please, for the sake of all other builders or flying RV-12's, if you've had a problem, please send the form in to Vans.

I can only imagine the huge amount of calls that Vans receives each day for all types of questions, and I am very appreciative that Scott took the time to speak with me about this direct topic. As we were about to hang up, he asked me if I had any other concerns. I told him no, but if I could change anything, I'd "want to be retired so I could work on this plane all day since it is such a blast".

Vans is listening guys and gals, but we've got to keep feeding them the incident forms so they have data to work from.

Have a good evening, everyone.
 
Ausmo said:
I too hope we hear something from Vans soon. I understand their need to react thoughtfully and cautiously, but this is clearly an issue that warrants their attention.

I will be flying predominantly from a grass strip and am far from encouraged by what I hear. I am currently building the fuselage and will be attaching the gear legs in the not too distant future. If there is to be a modification, this would be a good time to know about it.

:confused: Erik
Click to expand...

Ausmo, see my post above ^^^^^^^^^^.

Build on, my friend. The factory says it's all OK to do so.

P.S. I just noticed that this thread is up to 13,340+ views. Think this issue is a hot topic? I would say so!
 
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Correct bolt torques

It is hard to tell from the above posts which bolts were found loose and what they were or should be torqued to.

There are 5 bolts per gear leg. One (1) AN6 on the U1203 Doubler. This would have a handbook torque of 160-190 IN-LB or 13-16 FT-LB.

There are two (2) AN5 bolts on the U1202 Outboard Attach Bracket. These would have a handbook torque of 100-140 IN-LB or 8-12 FT-LB.

There are two (2) AN4 bolts on the U1203 Doubler. These would have a handbook torque of 50-70 IN-LB or 4-6 FT-LB.

Numbers are rounded off to avoid decimals. "Handbook torque" because the plans do not specify what to tighten them to.

You could add the resistance of the friction lock nuts to the above.

There is a lot of "chatter" going on, lets make it as specific and accurate as possible.;)


Tony
 
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loose bolts

I would recommend that once you get the bolts properly torued that you put torque seal on them so you can tell at a glance if the bolts have become loose. Just be sure the bolts are completely seated before applying the torque.
 
Just a guess, but from what has been furnished to me, it APPEARS that the outboard front bolt actually stretches a bit from the stress, making it loose. Definitely a good spot for using torque seal.
 
DonFromTX said:
Just a guess, but from what has been furnished to me, it APPEARS that the outboard front bolt actually stretches a bit from the stress, making it loose. Definitely a good spot for using torque seal.
Click to expand...

Don, as I understand the purpose of torque-seal, it is to indicate if the nut has turned on the bolt. If the bolt has stretched, as in your scenario, couldn't it be loose without the nut turning? Or am I misunderstanding?
 
You're right, of course.

There are a number of things that could cause a bolt to loosen without the bolt and nut unscrewing. Some of them are bolt stretch, washer compressed, small gaps finally closing, structure compressing, all sorts of things.

Worth giving the area a thorough inspection, of course, before retorquing.

Dave
 
Torqueing a bolt `stretches' it and induces a tension in the bolt that clamps the bolted pieces together. The bolt won't actually stretch any further until something tries to pull the bolted pieces apart with a force that exceeds the tension that already exists in the bolt.

Depending on how much the bolt was torqued initially, that additional stretch might be temporary, with the bolt returning to its original length when the force was removed, or permament if the applied force increased the tension in the bolt very close to the maximum load it could take without actually failing.

Not sure if that helps.
 
Chatter

Regarding the Chatter.

My Bad....It was the 2 outboard bolts on both the left and right landing gear...with the right landing gear bolts being the most loose.

I'm wondering if maybe the landing gear surfaces are mating with the chanel surface causing the bolts to loosen. Hopefully re-torquing the bolts will solve the issues.

It may also be that I didn't fully seat the bolts during the original assembling.

Ya got to love being experimental.:p


Jersey
 
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For the bolt to stretch and leave a permanent slack it would have to go beyond the elastic limit. A quick calc assuming a yield stress of 125 ksi shows a force of just shy of 10k lbs to permanently stretch the bolt. To translate that into a G force requires some assumptions on the gear leg geometry but a rough cut at it looks like about 5 Gs which would equate to a vigorous arrival on the runway.

I'm sure the calc can be done with better assumptions, but this back of the envelope calc seems to make permanent stretching unlikely.
 
txaviator said:
Ausmo, see my post above ^^^^^^^^^^.

Build on, my friend. The factory says it's all OK to do so.

P.S. I just noticed that this thread is up to 13,340+ views. Think this issue is a hot topic? I would say so!
Click to expand...

Thanks for the advice TX, I wasn't planning to stop, just wanting to sleep better! :eek:

Erik
 
For the bolt to stretch and leave a permanent slack it would have to go beyond the elastic limit. A quick calc assuming a yield stress of 125 ksi shows a force of just shy of 10k lbs to permanently stretch the bolt. To translate that into a G force requires some assumptions on the gear leg geometry but a rough cut at it looks like about 5 Gs which would equate to a vigorous arrival on the runway.

I'm sure the calc can be done with better assumptions, but this back of the envelope calc seems to make permanent stretching unlikely.
 
It has pretty well been established that the fore and aft movement of the gear leg is doing the damage, not the vigorous arrival on the runway or downward G loading. A few potholes, some tufts of grass, locked chattering tire, etc. is more likely to be aggravating the gear attachment.
A vigorous vertical arrival in itself puts no stress on the outer bolts, mostly that stresses the inner bolts.

RFSchaller said:
For the bolt to stretch and leave a permanent slack it would have to go beyond the elastic limit. A quick calc assuming a yield stress of 125 ksi shows a force of just shy of 10k lbs to permanently stretch the bolt. To translate that into a G force requires some assumptions on the gear leg geometry but a rough cut at it looks like about 5 Gs which would equate to a vigorous arrival on the runway.

I'm sure the calc can be done with better assumptions, but this back of the envelope calc seems to make permanent stretching unlikely.
Click to expand...
 
Dave, just to reply to your question...

I have also adjusted my brake pedal postion slightly, as I found when experimenting during the installation that it was difficult for me to avoid touching the brake with the rudder pedal pushed forward. Although landing with a locked brake would be pretty exciting, the reports in this thread don't seem to suggest that as being a factor in these incidents.

Dave12 said:
One thought I had regarding this is the way the brake pedals are drilled and mounted to the rudder pedals. When I got to this step, it occurred to me that I might not want the position of the brake pedal to be quite that generic. I probably have larger than average feet and wanted the brakes to require a deliberate movement of my feet to activate. It took some time to dial in this brake pedal position. Where I am going with this, is it possible that the brake pedal is positioned so far aft that the pilot could be inadvertently activating the brakes while manipulating the rudder? Think about the scenario, Here you are on short final in a crosswind, working the rudder and flying an airplane that you don't have many hours in yet. You are possibly a little high, a bit fast and as you land, you not only have your feet firmly planted on the rudder, but the brakes as well. At this point the brakes are possibly locking up or at the very least, not being used for what they are designed. At these speeds, could a touch down in a crosswind with hard right or left rudder and a locked brake cause this damage? What say the smart guys?
Click to expand...
 
Bolt moved UP

Gary

You got it right this is what is happening

Re-torque the bolts after a few landings

I cleand up your answer (adjusted Quote )

jersey said:
Regarding the Chatter.

It was the 2 outboard bolts on both the left and right landing gear...with the right landing gear bolts being the most loose.

the landing gear surfaces are mating with the chanel surface causing the bolts to loosen. Hopefully re-torquing the bolts will solve the issues.

I didn't fully seat the bolts during the original assembling.


Jersey
Click to expand...
 
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What?

DonFromTX said:
It has pretty well been established that the fore and aft movement of the gear leg is doing the damage, not the vigorous arrival on the runway or downward G loading. A few potholes, some tufts of grass, locked chattering tire, etc. is more likely to be aggravating the gear attachment.
A vigorous vertical arrival in itself puts no stress on the outer bolts, mostly that stresses the inner bolts.
Click to expand...

How has this been established? As far as I know, VAN's hasn't commented yet, since they haven't received builder forms in any quantity and I haven't seen any thing from the NTSB or FAA, either. Let's not jump to any conclusions.
In my opinion, aside from the cracked channels, I personally attribute the wrinkling and brake line damage to rough use on the airframe that exceeded the design limits. (I don't think it's possible for the gear to move that much fore and aft without breaking the bolts. Again my opinion.)
The best thing RV12 pilots can do is work on landing skills. I like to practice touch and go's so that the nose gear never touches down during the rolling portion of the touch and go. As you practice, use progressively slower landing speeds but keep the plane rolling on to the pavement, rather than dropping it on. If you practice this, you'll get amazingly soft landings and also save the nose gear attachment points. You'll also be able to get shorter landings over time since you are learning better control of the aircraft.
Just my opinions. You can flame them if you like.
 
sf3543 said:
The best thing RV12 pilots can do is work on landing skills. I like to practice touch and go's so that the nose gear never touches down during the rolling portion of the touch and go. As you practice, use progressively slower landing speeds but keep the plane rolling on to the pavement, rather than dropping it on. If you practice this, you'll get amazingly soft landings and also save the nose gear attachment points. You'll also be able to get shorter landings over time since you are learning better control of the aircraft.
Just my opinions. You can flame them if you like.
Click to expand...

Good advice Steve. No flames from here! ;)
 
Time will Tell

Don
I agree and add do not use the breaks aggressively until Van Replies and they will.
They are working on this aggressively to determine if they think they need to add reinforcement ( Remember they have to determine, then conclude there is a problem, design a fix and test the fix ( this will not happen quick )

Until then Check your bolts land slow, soft and rollout steering with Rudder and break easy. ( IF possible ) (Forget trying to make the first turnoff )

Also Note: all information in the background is not posted on this site.

DonFromTX said:
It has pretty well been established that the fore and aft movement of the gear leg is doing the damage, not the vigorous arrival on the runway or downward G loading. A few potholes, some tufts of grass, locked chattering tire, etc. is more likely to be aggravating the gear attachment.
A vigorous vertical arrival in itself puts no stress on the outer bolts, mostly that stresses the inner bolts.
Click to expand...
 
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With regard to the aft fuselage pivoting down around the cargo bulkhead and pinching the brake line and causing compression buckles in the skin, recall a trailering thread a while back. I think the gist was that the aft fuselage needed support during extended ground transport to prevent damaging the aircraft's monocoque structure caused by vertical loads.

This implies a certain fragility in the aft fuselage structure and suggests high vertical loads during a hard landing could exceed design limits, resulting in some of the damage seen.

Bob Bogash
N737G
 
sf3543 said:
The best thing RV12 pilots can do is work on landing skills. I like to practice touch and go's so that the nose gear never touches down during the rolling portion of the touch and go. As you practice, use progressively slower landing speeds but keep the plane rolling on to the pavement, rather than dropping it on. If you practice this, you'll get amazingly soft landings and also save the nose gear attachment points. You'll also be able to get shorter landings over time since you are learning better control of the aircraft.
Just my opinions. You can flame them if you like.
Click to expand...

Sound advice Steve. The -12 will respond to a light touch. Dropping it on the runway like a 150 is not going to end well. ;)
 
I spent about two hours today with a knowlegable friend looking at the gear/channel/skin areas, and reached no conclusions other than loose gear bolts. Even with that I don't know how the leg can bend the aft skin unless they are very loose. Of course that (very) may be the case in the damaged plane reports thus far. Dropping it in would bend or distort the skin above the gear leg and I have not seen comments of that happening. Twisting the c channel is impossible w/o telltale signs of skin distortion in the vertical supports, side skins, and bottom fuse skins. The aft fuse bending down theory would show damage on the side and bottom skins and no reports on that either that I have seen. Of course Van's will confirm the cause after it's study which will nail it down. Until then I believe that dropping it in hard with loose bolts, and hard braking could do it, and maybe hard braking alone if bolts are loose enough. Lets face it loose bolts on gear legs are not easily noticed , as you can't see what the gear is doing while landing. Most pilots seldom look for problems like that in preflight inspections, and it's doubtful one would see anything if they did look. I am doing a Condition Inspection in two weeks and plan to again check gear legs for wobble while it's on wing jacks. Will also check torque and re-seal with Torque Seal. I don't know if that's the answer, but I do know it will help avoid the current problem.
Dick Seiders
 
One word of caution regarding Torque Seal. At work we often need to mark nuts and bolts (and oil filters on Rotax motors) and we have found that paint is better than Torque Seal. Tipp-Ex works very well too.

Torque seal is normally a fairly thick substance and can actually stay attached as a whole to only one item and looks as though everything is as it should be when actually it isn't.
 
sf3543 said:
How has this been established? As far as I know, VAN's hasn't commented yet, since they haven't received builder forms in any quantity and I haven't seen any thing from the NTSB or FAA, either. Let's not jump to any conclusions.
In my opinion, aside from the cracked channels, I personally attribute the wrinkling and brake line damage to rough use on the airframe that exceeded the design limits. (I don't think it's possible for the gear to move that much fore and aft without breaking the bolts. Again my opinion.)
The best thing RV12 pilots can do is work on landing skills. I like to practice touch and go's so that the nose gear never touches down during the rolling portion of the touch and go. As you practice, use progressively slower landing speeds but keep the plane rolling on to the pavement, rather than dropping it on. If you practice this, you'll get amazingly soft landings and also save the nose gear attachment points. You'll also be able to get shorter landings over time since you are learning better control of the aircraft.
Just my opinions. You can flame them if you like.
Click to expand...

Steve, I am with Larry, this is good advice. You read my mind (I would not make a habit of that). btw I found my outboard bolts loose also.
 
Rotating channel

The bottom of the channel is 6 1/8 inches above the bottom of the fuse.
The channel is rotating at the center of the baggage floor , about 7" above the bottom of the fuse.
The channel would have to rotate 3.5 deg. to hit the break line and less then 3 deg if the bolt is loose.
This is what is happening ( the Channel Is rotating )
The cutout for the spar in front of the channel allows the front of the channel to move down about .086" + -. ( skin in tension )
ant the back to flex up about .136" + - ( skin in compression )


look at the rivets
k9b8er.jpg
 
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The bolts would have to be very loose indeed to crush the brake line between the back of the leg and the fuselage skin if that was only factor in the equation, but loose bolts would certainly make matters worse. Dick commented on the lack of visible damage to the vertical supports on either side of the channel and in other areas, but that doesn't mean that these areas weren't distorted during the incident, only that they didn't exceed their elastic limit.

As I said elsewhere, we are not seeing the full picture of what happened, only the permanent distortion that remains. It might be very instructive if someone could mount a camera under the wing to see what happens when landing on different surfaces while braking.
 
Wear Plates

Tightened the two outboard bolts. (right side) Each took about 1 1/2 turns before resistance was felt. Apparently the wear plates are doing their thing. Theyre wearing....

Jersey
 
How?

How are the bolts loosening? Do you think they are loosening or just were not fully tight in the first place? What would be wearing that much in that short of time that would allow 1-3 rotations?

I vaguely recall when I installed the bolts and torqued them the head of the bolt was not fully seated due to friction of the bolt going in. This would mean you could think they were torqued but really never were. I used a rubber mallet to ensure they were seated then torqued. I have not checked mine yet but only have 12 hours on the plane. If the head of the bolt was not fully seated, a few landings would seat the head and the nut would then appear to be loose.

Is that a hypothesis of why these bolts are coming loose?
 
ScottSchmidt said:
How are the bolts loosening? Do you think they are loosening or just were not fully tight in the first place? What would be wearing that much in that short of time that would allow 1-3 rotations?

I vaguely recall when I installed the bolts and torqued them the head of the bolt was not fully seated due to friction of the bolt going in. This would mean you could think they were torqued but really never were. I used a rubber mallet to ensure they were seated then torqued. I have not checked mine yet but only have 12 hours on the plane. If the head of the bolt was not fully seated, a few landings would seat the head and the nut would then appear to be loose.

Is that a hypothesis of why these bolts are coming loose?
Click to expand...

Proper torquing requires that you add the amount of resistance or friction caused by tight holes and nylock nuts to the amount of finished torque you are trying to achieve. Under torquing would be a huge factor in this situation.
 
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Joe. the skin damage illustrated looks like the result of a very hard landing as to me it appears the gear legs moved vertically only making severe impact on skin structure above.
Dick Seiders.
 
More loose bolts

Yesterday I checked the torque on all my gear attach bolts. In each case I set the torque wrench to the high side of the torque range. All inboard bolts checked out fine. All four outboard bolts were loose and required a FULL TURN to get them to proper torque.:eek:

I am sending this to Van's as another data point. I have 130 hours on the airframe, with probably about the same number of landings. I don't do many touch and goes - mostly cross-country flights. I operate mostly from good paved runways, with only occasional landings on grass or rough surfaces. I don't think I actually put a torque wrench on these bolts at my first annual condition inspection, so loosening may have occurred gradually over the 130 hours (year and a half).

Even though we don't yet know the cause and effect relationships in the incidents of damage, I think it would be prudent for everyone to check their attach bolts without delay.
 
Check the bolts

This could only be possible if the people reporting are forgetting that they had hard landings.
I Am basing my opinion on that all people are truthful
Also if it was a hard landing you would think there would be damage above the channel and the bottom skin below the channel.
I don't know the radial or the sin of the angle ( t=r f sin ) this would help in knowing if any torque is applied in a hard landing,


Again Check the Torque of the bolts this is adding to the problem.
This is a dynamic connection bolts should be checked after some landings and at every 100 hrs

My View



dick seiders said:
Joe. the skin damage illustrated looks like the result of a very hard landing as to me it appears the gear legs moved vertically only making severe impact on skin structure above.
Dick Seiders.
Click to expand...
 
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Not again ...

I just received my new rear window and have not yet installed it. Obviously, I hate looking at my badly crazed window and yet I hate to take out my tools and become a "builder" again.

At my first annual, among other things, I replaced my gas tank due to financial concerns and warranty concerns of performing the upgrade vs new prefabricated gas tank from Van's. I still have nightmares about SB 11-12-14. I had my FBO perform the annual and change out the gas tank because I didn't want to be a "builder" again.

I currently show 80 hours on the Hobbs and finally read all the posts in this thread last night. I thought "Loose Main Gear Legs" meant "loose main gear legs" ... Boy ... Was I wrong. I can only imagine what the next Service Bulletin
will include! The only thing I'm certain of is "I don't want to become a builder again".

I know the first time I installed the landing gear every part slipped easily into place ... Like a hot knife thru butter ... alone in just a couple minutes I attached the main landing gear legs ... ... ... NOT!

The only thing I'm certain of is "I don't want to become a builder again".

Guess I'll drive out to the airport and check out my -12 and hope for the best ...
 
More loose bolts

At my annual condition inspection, all four outboard bolts were loose. At the time of the inspection I thought I must have forgotten to torque them, which was a big surprise to me. I do recall ensuring the bolts were well seated, by using a hammer to drive them fully home. So now I'm not so sure I had made the mistake.

I too, put torque seal on at the annual inspection. And I note Jerry's caution on its use.

120 hours mostly on hard runways but several departures/landings on grass. I recall one firm arrival on grass during the first 5 hours, when I investigated handling in a strong crosswind. No damage observed then or now.

Cheers...Keith
 
Don, actually the outboard bolts are subject to tensile stress on a hard landing because the gear legs splay out from the fuselage causing a fulcrum point at the point of contact with the bulkhead. In fact with the shorter arm to the fulcrum point they probably see more stress than th inboard bolts.
 
Loose bolts

I am in the process of installing the gear legs and noted the U-1202 and U-1203 are powder coated. I recall a recent discussion on powder coating engine parts and a caution to remove the powder coating from any bolt/nut areas that are torqued. I believe the point was the powder coat material could flow under a high load over time leaving the nut/bolt loose. There are no notes on step 2 of page 35-03 that indicate the power coating should be removed in the area under the bolt head.

Has anyone with a loose bolt removed it to see if the powder coating is intact?
 
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