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  #21  
Old 06-06-2012, 05:31 PM
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LifeofReiley LifeofReiley is offline
 
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IMHO... slotted holes in aluminum (or steel for that matter) and aircraft do not go together at all. It sounds like the part is shipped slotted. Toss the part and fab a new part with no slots, nice tight - straight drilled and deburred holes. With proper washers, bolt length and torque you should be fine.
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Last edited by LifeofReiley : 06-06-2012 at 05:48 PM.
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  #22  
Old 06-07-2012, 07:44 AM
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It's worth pointing out that the -7A that crashed in Canada a couple of January's ago lost its vertical stab, and the failure point was right about where this plate is. As someone else pointed out, the exact geometry here is slightly different on the -7, but it stresses how critical this part is.

I second the suggestion of making a new one without slots, unless Van has good reasons for them being there.
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  #23  
Old 06-07-2012, 08:22 AM
krw5927 krw5927 is offline
 
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Interestingly, the crack orientation suggests the part was loaded (and fatigued) in the vertical direction at the plate (fatigue cracks typically start perpendicular to the load). It seems as though either the drag load from the vertical stabilizer induced a tension load upward on the plate, or the vertical stabilizer is mounted in an orientation that induces a bending moment (tension load on the left side of the plate - facing forward - and a corresponding compression load on the right side). Or perhaps as Dan suggested it's not fatigue and the vertical stabilizer was subjected to ground handling forces that exceeded the ultimate tensile strength of the plate.

It's easy to think the slots are a problem, however in the photo the bolt is at the top of the slot. If there had been movement of the plate relative to the bolt, the bolt would be at the bottom of the slot. To me this suggests a tension ligament failure rather than a problem with the slot itself. Click thumbnail for larger graphic.



Poor deburring could be a cause for this, however an equally likely cause is that the edge margin (center of slot to edge of part) is inadequate. If you do re-make the part, ensure that the bend at the bottom has a large enough radius that it doesn't crack, and I'd be very tempted to increase the edge margin on the sides (maintaining clearance to adjacent rivets, of course). Dan H's suggestion is great. Good deburring practice on these holes seems to be critical as well.

It would be great to hear what Vans engineers say.
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Last edited by krw5927 : 06-07-2012 at 10:31 AM.
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  #24  
Old 06-07-2012, 08:31 AM
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I don't know about the -9, but the -7 has an offset built in when this plate is drilled. Be sure to check before you drill.
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  #25  
Old 06-07-2012, 10:28 AM
Rob's RV9A Rob's RV9A is offline
 
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Default Vertical Stab Attach Plate Crack

The response from Van's is; "they haven't seen a failure like this, so it does not seem to be a common issue."

"It's possible that it ws caused either by the bolt pushing against the edge of
the slot (if the HS holes are not all drilled exactly aligned with the F-981
slots). Or if the punched edge of the hole was not smoothed and had a
rough part which acted as a stress riser."

They also said, "fitting a new part should be good..."

The rudder has a about a 8" trim strip attached to the right rear edge of the rudder. There is no record in the logs when this was installed on the rudder.

The response from KRW5927 seems to point to the possible stress source along with the possible reason from Van's and others of not a smooth hole during the initial spar construction leaving a stress riser.

I will look for a structures engineer in our EAA Chapter for additional information (we are in the DC area with 250 members); consult with my A&P/IA, before we manufacture a new piece, or install the replacement part from Van's verifying the hole and vertical stabilator alignments.

I will post the corrective action we choose.
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  #26  
Old 06-07-2012, 12:43 PM
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MS19087 MS19087 is offline
 
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Default Did Van's comment on the addition of an washer?

It was suggested earlier on this thread that the addition of a washer under the head of the bolt would spread the load - I assume this would be an oversized washer? If an oversized washer is used, would it not hide a potential crack during inspection? However a standard sized washer may not act to spread the load. Hmmm . . . thoughts?

I suspect this single occurence won't prompt an advisory (from Van's) to do an inspection - or would it?
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  #27  
Old 06-07-2012, 01:00 PM
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Default Washer reason

The washer prevents rotational (tearing) force by the bolt on the slot, if the bolt was turned under load.
A single data point is not a trend, but it certainly warrents an inspection red flag.
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  #28  
Old 06-07-2012, 05:57 PM
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Default engine and prop vibration

Is there excessive engine and/or prop vibration in your 9A?
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  #29  
Old 06-07-2012, 10:31 PM
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Quote:
Originally Posted by MS19087 View Post
It was suggested earlier on this thread that the addition of a washer under the head of the bolt would spread the load - I assume this would be an oversized washer? If an oversized washer is used, would it not hide a potential crack during inspection? However a standard sized washer may not act to spread the load. Hmmm . . . thoughts?

I suspect this single occurence won't prompt an advisory (from Van's) to do an inspection - or would it?
A regular washer would still help to spread the load.

An AN3 bolt has a circular flat area under it's head of 0.360 in diam.

An AN960 washer has an outer diameter of 0.438 in diam.

Above numbers not precise due to tolerances, but you can get the idea that the washer has a much larger flat area than the bolt head.

Since a washer under the head is standard aviation practise, I would just do it anyway...
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  #30  
Old 06-08-2012, 05:02 AM
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Steve Melton Steve Melton is offline
 
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Default bolt contact area

I took a look at mine yesterday. The bolt contact area is minimal at the slot. Bolt head contact is a small arc on each side of the slot. I could easily see how the aluminum material under the bolt head could be crushed with bolt torque clamp and then developing to be a crack. Van's got it wrong on this one. It should be a line drilled hole. The sides of the slot from the factory were not smooth but rough. I removed each bolt, scotch-brited the sides of the slots in place the best I could, difficult to do properly in place. Used the next size longer bolt, one washer under the head and two under the nut. Put this area on my condition inspection list in RED. I would not be surprised if I see a design change from Van's in the future.
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Last edited by Steve Melton : 06-08-2012 at 05:15 AM.
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