Quote:
Originally Posted by bobnoffs
if you land hard enough you can break any nose gear. 1700 hrs as a trainer is a lota abuse. i would think this sb is not because of a broken gear on a trainer but rather of the results of an in depth analysis of the area that failed. can we get an idea of how much margin is built into the original gear?
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Okay, here we go. This took a while to type up. Note that the risk of providing detailed answers is that the process of answering new questions never ends. 
So, I won't be able to go into this much depth on every question (and I do need to get on to other fun things here!) but this is a good one to try to explain, I think. Especially if it helps our customers understand a bit more about why we do what we do.
You'd be correct in your assumption about the decision being analysis-based and more specifically not based on the one instance of a cracked/broken gear leg. At the risk of boring folks, I'll try to explain in greater depth here, but know that this is a layman's explanation so please bear with me.
The original gear leg has an area of high stress concentration, as described earlier, which under high loads results in a fatigue issue at the point of high stress concentration. When the original gear leg was designed, we did not have the modern Finite Element Analysis (FEA) computer analysis tools that we have and use today. The issue relates to stress loads that reach the point at which the material in the high stress concentration zone will begin to yield and then fatigue. We conducted drop tests on the original RV-12 gear leg when it was new, and it passed those static tests of course. But what we could not test at the time was the fatigue characteristics over time of the part.
This, in fact, has been a "problem" in aviation and aircraft design since the beginning. Static tests are relatively easy to execute - in this case, you drop the airplane on its gear and observe what happens. If the gear ends up out of shape, you know you have a static strength issue. What you cannot see in static testing is "inside" the material and what happens to the material over time after one or more high/overloads and infinite typical-use load cycle scenarios. That's where FEA software tools and experts (both of which we are now quite lucky to have at Van's) come into play. Van's had no indication that the original RV-12 gear leg may have fatigue issue until that event occurred. The event was surprising to us given the design, and therefore prompted us to perform an in-depth analysis using our FEA tools.
The results of that FEA study identified a stress concentration in the gear leg assembly, located approximately at the lower attachment plate/flange. In this location, several gear leg components come together: The tube, the lower attachment bracket/flange, and another flange that is attached vertically to the tube between the two attachment brackets/flanges. In addition, all of these components are welded to each other. The FEA tools allow our engineers to run reliable analyses that take into account all of the specific geometry, materials, and loading conditions. That analysis determined that with a significant load on the original design, the stress in the leg at the stress concentration exceeds the material yield stress and the fatigue life is significantly reduced. The subsequent remaining lifespan for any given gear leg cannot be determined due to the many variables involved: variation in welding, previous load levels and the number of load cycles.
So, what does this mean? If you do a static drop test of the original gear (which we did many times of course), it will pass. But similar impacts may - depending on a combination of factors - reduce the strength of the leg at the point of high stress concentration. Subsequent loads and load cycles from use of the gear are likely to progressively fatigue the material at the point of high stress concentration. That's why we redesigned the leg. The new leg was thoroughly analyzed during design, and we have confirmed the new design alleviates the stress concentration issue that clearly exists in the original leg.
We cannot predict the loads that will be applied to a given gear leg nor the length of time after an initial event it might take for a crack to form due to the variables I have just mentioned. What we do know is that the stress concentration exists and can become a problem in certain unpredictable/unplannable scenarios and that the new gear leg does not have that same issue.
Our resulting decision was to inform owners of the need to replace the leg at or before the next annual inspection, given the fact although only one failure has occurred our analysis shows it can potentially happen in any RV-12 gear leg under certain load and operating conditions, which may occur in the real world. Our decision process cannot rely on the application of pilot technique to prevent the potential safety issue when we have identified in issue such as this one where we can predict a material failure will occur, and similarly we cannot absolutely guarantee that a perfect pilot (which we know does not actually exist) will never run into an unintended/unforeseeable overload scenario. Therefore we have made the part change and issued the service bulletin, based on the facts and those factors that are under our direct control.
I should also mention that when we need to take actions that impact our customers, such as releasing this SB, we work hard to keep the costs down as much as possible. We are making zero (and when you account for engineering analysis and redesign time, we are actually losing) money on these SB parts. Again, our goal is to promote safety, inform owners and make it as easy as possible to get fixes in the field when they are deemed necessary.
Quote:
Originally Posted by pilotyoung
I have been wondering about this since I first read it yesterday. I wonder if the nose gear leg on the 12 is significantly lighter, weaker, than the nose gear on other RV's? Have there been cracks in the nose gear legs of other RV-s? I also wonder how much abuse an RV-12 used by a flight school for 1700 hours had? I am not an airplane designer, and not an engineer. I was a professional pilot and a flight instructor. I have taught many students to fly. When I first looked at the RV-12 and then flew it, I told my son the landing gear was not made for student pilots. Don't get me wrong. The RV-12 is a wonderful airplane, it flies like a dream, and I love the aircraft. But I personally would not train a student to land in an RV-12.
So what I am wondering is if this change is really necessary for an RV-12 with a lot less hours and that has never been used or abused by students?
Please don't start attacking me. I am not advocating flying unsafe airplanes. I am asking what I think is a legitimate question and wanting to see the thoughts of others more experienced with RV-s than me, people who are trained in engineering, etc. I am wanting to learn.
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No one will (or should) attack ya. And hopefully the detailed info above helps answer some of your questions.
Many, many thousands of flight training hours have been performed in RV-12's. We've made incremental improvements over time to the airplane (and of course we released the RV-12iS with some substantial changes), but know that there are a significant number of aircraft that are used to train students daily.
The other RV models' nose gear legs are fundamentally different in their design, and this issue does not apply to them in any way.
As far as whether it's necessary to replace the gear leg assembly for any given RV-12 I have tried to explain above that the events that can cause the issue to develop are highly variable. It's not necessarily
just the amount of (over)load placed on the gear, which is highly-focused in an area of mechanical stress concentration. Similarly, it's not
just the number of duty/load cycles. And, these stresses and overloads cannot confidently be avoided. So, the actual impact and result is highly variable per aircraft. It's not possible for us to tell you it's ok if less than xx number of landings, or based on the type of runway used, or any of several other variables we could think up. What we did allow for is waiting to replace at the next annual condition inspection if you wish to do so.
Summary: The overall strength of the original gear leg is not the key issue, it's the
concentrated area in the gear leg where the load is focused which results in a crack potentially forming. Once a high-load event has occurred that is significant enough to cause the stress-concentrated portion of the original leg material to yield, an occurrence that is possible based on our analysis, it's just a (highly variable) matter of load cycles before the gear is likely to eventually crack at the location we have described. How many cycles and how much load is required to reach that point will vary significantly. That's why we revised the part and released the service bulletin.
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