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FredG

Stabilator bracket crack

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2006 CTsw.  Airframe total time about 1050 hours.  This part of the bracket can be inspected on preflight with no disassembly or special tools.  The "Safety Officer" posted very similar photographs of a failed stabilator bracket about five years ago.  Worth inspecting on any CTsw (as has been noted in the past).  

STABILATOR 1.jpg

STABILATOR 2.jpg

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Fred, nice pictures of ugly crack.  Andy was posting while I was typing.  I have the same question.  Would you know what procedure one must follow to correct this?  I'm wondering if the bracket must be replaced or can it be welded with a follow-up check for any remaining defects - perhaps a magnaflux check afterwards?

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I replaced it with a used part in good condition (at least by visual inspection) that I purchased a few years ago from a reputable vendor.  FD has some guidelines for welding the stabilator bracket but the guidelines are limited to the qualifications of the welder and some comments about the materials to be used (but no guidance for gussets or methods of strengthening the cracked area).  By my observation, the crack forms where a bending force is applied by the stabilator to that somewhat skimpy part of the bracket (I know, no kidding...).  If repaired, I would think that some strengthening of the area would be best.  I will keep the failed part and repair it in the future if necessary.

Disclaimer:  As you know, my plane has experimental registration, so I am permitted to replace the part without inspection, authorization, or any other input by an A&P or LSRM.  

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Welding by nature is stronger than the original material, if done correctly. By itself, it meets the repair requirement of "restore to original or better condition".

Be careful about strengthening without considering any potential issues that could arise though. The experimental homebuilt world is littered with wreckage from people strengthening areas without solving the causes or considering where those stresses will now go.

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Fred, would it be possible for you to provide the FD guidelines for repairing this bracket?  There is a talented and certified welder who is close by.  I examine my bracket frequently and it shows no signs of cracking.  However, the cracking of your bracket is not an isolated incident.  This has occurred to other CT's.  I would like to understand what needs to be done to fix this, if it should happen to me.

Corey, your point is well taken.  Incorrect stiffening can throw stress into areas not designed to accept it.

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10 hours ago, FredG said:

I replaced it with a used part in good condition (at least by visual inspection) that I purchased a few years ago from a reputable vendor.  FD has some guidelines for welding the stabilator bracket but the guidelines are limited to the qualifications of the welder and some comments about the materials to be used (but no guidance for gussets or methods of strengthening the cracked area).  By my observation, the crack forms where a bending force is applied by the stabilator to that somewhat skimpy part of the bracket (I know, no kidding...).  If repaired, I would think that some strengthening of the area would be best.  I will keep the failed part and repair it in the future if necessary.

Disclaimer:  As you know, my plane has experimental registration, so I am permitted to replace the part without inspection, authorization, or any other input by an A&P or LSRM.  

It looks like a classic stress riser crack on an inside curve with insufficient relief radius.  The part definitely needs gussets or a milder bend radius on that turn.  The former should be easy to weld in, the later is pretty much impossible without re-engineering the part.  I'll be inspecting mine carefully on next preflight.  

IIRC that part is steel, I'm slightly surprised to see it crack.  

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Looking at this again.  On the top of the bracket, the vertical flange tapers down washes out and stops right where the crack starts.  On the bottom, the small doubling tab (in the 2nd picture) stops where the crack starts. This demonstrates how reinforcing can lead to bad results if not done correctly.  The crack forms right where the doubling tab ends and the top flange tapers down to nothing; leaving only a couple of small flat plates that are not gusseted or flanged that are taking loads from the stabilator that are trying to bend these plates.  This structure is saying, "bend here".  A solution might be to extend the tab further down the flange to a point where the vertical wall of the bracket is greater depth and able to take the load.  Then, keep good vertical section to the top flange this while running this past the cracked area and go well onto the top surface of the bracket, where it is allowed to wash out to the surface.  I don't think any of these changes would interfere with the stabilator surface.  Of course, I realize that if I designed the CT, it probably would weigh so much that it might never get off the ground.

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Dick, hard to explain a complicated three-dimensional part in words.  But, what the part needs (IMHO) is more metal in an orientation perpendicular to the bending moment of the stabilator (metal that will function like the "I" in an I-beam).  As I think you are pointing out, the flat metal that is parallel to the bending moment is easily bent.  The solution is either thicker metal (that won't flex repeatedly leading to fatigue failure) or a redesign with more metal perpendicular to the bending moment.

 

 

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Fred.  Good summary.  My vote would be to increase the bending stiffness by increasing top flange width plus carry this further towards the bolt hole and to extend the doubling tab to a point where it puts the load into the deeper wall section.  Perhaps just extending the tab would be sufficient and is basically your point of increasing the metal thickness. 

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just noticed these marks on preflight inspection ..... i know it may be difficult to see but does anyone think possible stress marks?  (no visible cracks)   it is slightly rough when i glide my finger back and forth on it 

2525AB5E-1412-445F-BE5B-FC14A1A022E7.jpeg

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On the cracked bracket, it looks like it was impacting regularly with something on the corner. An indentation is worn into the corner, something maybe to investigate.

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