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Vertical Stab Attach Plate Crack

This is my first post, and I want to advise what I found removing the panels and fairings for my condition inspection.

While doing the Condition Inspection on my RV9A I found this fault on the vertical stabilator attach plate F-981. For the RV9A this part is visible after removing the vertical stabilizer fairing. It is the top right bolt on F-981. You can see the crack is prevalent on the right side of the bolt and extends left of the bolt about a 1/16. In addition, you can see all four-attachment boltholes are elongated. It is obvious at some point the attachment bolts holes had been re-drilled and to move the vertical stabilator boltholes upwards thus lowering the whole assembly. You can also see the bushing for the manual trim cable is missing. The repair is to remove the whole empennage and carefully drill out the rivets, replace F-981. Both parts $6.83 from Van?s. Hours to repair??.. a few. So while the tail is off I will be looking for other obvious anomalies.

The original builder completed the airplane in 2001; I am the third owner after the builder. Total airframe time is 815 hours. I want others to see taking off that fairing and not just for a quick look deserves the attention to really inspect the attach points.

The pictures are at the following link if I goober the picture post here. http://s1254.photobucket.com/albums/hh616/RV9aer/[/IMG]


N994KSF-9812.jpg


N994KSF-9811.jpg
 
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If I remember rightly, these holes are pre-punched elongated. The problem with this is the sheared edge from the punch, with its fine cracks, won't be tidied up by match drilling with the HS spar. It would need to be done separately. I guess if it's left as a raw edge this is the result.
 
Welcome to VAF!!!!

This is my first post,

Rob, welcome aboard the good ship VAF.

Those photos are scary:eek:

That part failing in flight would be a way to ruin your whole day.

Luckily, as you found out the fix is simple and cheap.

Good catch.
 
The elongated holes come that way from Van's.
Here's what mine looked like during the fitting to the fuselage.

1195159674_Y8aWC-M.jpg


I'm not sure why Van's does the elongation of the holes to this part, since you have to drill the forward spar in the VS to set the position and angle of the VS rear spar to be straight up and down. Makes me wonder if there should be washers under the bolt heads on the enlongated holes to provide better clamping pressure on the bracket.
 
Washers under bolts

"Washers under bolts". My thoughts exactly.
Especially that from the photo, it appears the crack
radiates from the corner of the bolt. I will be sure to polish
Up that elongated hole well.
 
Elongated hole

It seems from a design standpoint, the only
Reason for an elongated hole would be to
Compensate for shim variation (under the HS mounting brackets)
from what is specified as 1/8" stock. Shouldn't those holes be undersized, non
Elongated and matched in the assembly? Is Vans allowing
For an additional, if needed shim to trim out the horizontal axis of the
Plane? I have seen photos and read reports of the elevators
Having to be in a down position relative to the angle of the HS in order
To achieve level flight.
 
Metal Bent?

Did I detect the metal having been bent a bit back and forth at the elongated hole?

New part carefully fit and installed you'll be fine!
 
Thanks for the info Rob,
Since it's an easy place to look I'll be inspecting this weekend. As a reminder, The same vertical stab is also used on the 7's so it would probable be a good idea to post something over there or in the general section just in case they don't see it here.
 
Can't tell

from the photos, but it looks like there is a fair amount of surface corrosion in that area. Was is primed?
 
F-981

Thanks to all for your comments. Thanks to BILLC suggestion, I will post in the 7 Forum.

The crack extends left and right of the bolthole. Looking at the drawings, the drawing hole appears to be elongated. All four-bolt holes on the plate have bolt score marks from being fastened in place at the lowest point of the elongated hole. For the cracking occurrence to me it is difficult to pin a cause, i.e. too much torque, not enough torque, non burred hole, or defect on original and subsequent installation and adjustment, too much weight for a few moments on the one hole, washer placement, etc. I am not an engineer; my conclusions are just guessstimates. If anyone else or Van?s has previous experience with this fault would be good to know.

However, the fix is a simple repair; just time consuming.

In response to GAHco attached is another wider view of the F-981crack.


http://s1254.photobucket.com/albums/hh616/RV9aer/
RV9aer
 
7 attachment is different...

Thanks for the info Rob,
Since it's an easy place to look I'll be inspecting this weekend. As a reminder, The same vertical stab is also used on the 7's so it would probable be a good idea to post something over there or in the general section just in case they don't see it here.

The attachment of the vertical stabilizer on the 7 is different (different horizontal stabilizer and spar)
 
Since it's a simple part and the elongated holes are for intial assembly only.

I would make my own replacement part from the same thickness material and just have plain round holes for the bolts.

Is there a possiblity that the bolts were not fully torqued?
 
slots?

Slots are not preferred at loads points for this very reason. A better design would be line drilled. OK, now the decision for me, make a replacement part or just add a washer? Vans may consider a Service Bulletin for this finding. Did someone copy Vans?
 
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Good find and inspection.


Thanks for the info Rob,
Since it's an easy place to look I'll be inspecting this weekend. As a reminder, The same vertical stab is also used on the 7's so it would probable be a good idea to post something over there or in the general section just in case they don't see it here.

That may have been in the older kit (RV7) mine which was built in 2009, the attach point was not pre-drailled and was to be drilled by builder.
 
If you make a new part it would be easy to incorporate a rather obvious load path improvement...straighten the left edge:

ei4n41.jpg


Is the left edge in the photo rounded and smoothed, or square-edged with an "as delivered" finish?

The most likely cause of overload is ground handling. Folks tend to use the leading edge of the VS as a handle.
 
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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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.
 
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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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.
 
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.
 
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?
 
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.
 
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...:)
 
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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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...:)

The first time I saw the slotted hole in this piece I was shocked that it came from the factory that way. I agree with Gil and install washers on these bolts on every airplane I inspect.
 
Rob, when you have the vertical stab off for replacement of the F981, take a hard look at the location of the two drilled holes in the VS main spar:

5ezn9k.jpg


It is a very common builder error to locate and drill those two holes in line with the rivet rows above and below. The result is a serious loss of edge distance on the thick spar web doubler.....with potential for the same ligament failure Kurt outlined above in discussion of your front spar attach plate. The correct placement is well inboard of the rivet lines and the doubler edge.

Returning to the cracked F981, would you be kind enough examine the edges of the slot and the perimeter edge of the part after dis-assembly and report here? Kurt's ligament failure is the fundamental mechanism. The crack merely starts at the weakest point, which is almost always a surface defect or an internal inclusion. A square edge with tool marks or an edge with mechanical damage (from the underside of the bolt head) would provide the surface defect. Neither should exist in a well built airplane, but we see it all the time. I'd put poor edge preparation near the top of the "common sins" list, perhaps even before poor riveting.

For those building now, please review your Vans Construction Manual, Section 5B. A part with properly prepared edges will reach its theoretical capacity without complaint. A part with square edges and/or tool marks will fail at some lower load. This is not the designer's fault.

Speaking of design, there seems to be some debate about the slot. I would suggest that the choice of slot or hole makes no significant difference in terms of stress concentration, in this orientation to load. Here's the simple view of elliptical vs round holes. Anyone have something similar for a racetrack shaped hole?

2whlffn.jpg


It should be noted that the stress concentration for a perfectly sharp notch (the perfectly bad tool mark, or the beginning of a shear) is infinity. Infinity is much larger than, for example, 2, 3 or 5.
 
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A couple of years ago when the F1 was still in production the Czech engineers did a FEA analysis of the tail and found one of the weak points was the fwd spar. On my F1 there is a 1/8" tapered finger plate reinforcement that extends up the spar approximately 8" and the attach plate is 1/8" thick. Very overkill but an area I would gladly take overkill with. Especially considering that this is a stressed area from pushing and pulling and particularly when vibrating at lower RPM's such as runup.
 
I sent in some of the suggestion from this forum to Van's In part this is their response to install a washer, and/or make a new piece with a wider right edge.

"Neither of these suggestions could do any harm and the increased edge
distance could make the plate more crack resistant, but it would mean
making a custom part."

For Dan H's suggestion I will look for the alignment and I have concern for the HS that attaches to F-981. I had to catch up with a few things around the house and hope top remove the tail tonight, or on Sunday.

I will reinstall with the washers, but am unsure about fabrication of a modified right edge F-981. IMHO if a crack should develop again the crack just has more room to propagate to the right. It would seem a thicker piece, or stronger alloy is what would be needed for F-981.

However, I also think the washer suggestion with a cleaning and deburring of the holes, ensuring proper placement and alignment the VS and HS attach points.

My airplane has accumulated 815 hours and my concern is for other 9's and 9A's. If my airplane is just a one off incident then that is good for the fleet.

I will post our findings and how we proceed with the repair. I hope to have it all done within the next to weeks.
 
A couple of years ago when the F1 was still in production the Czech engineers did a FEA analysis of the tail and found one of the weak points was the fwd spar. On my F1 there is a 1/8" tapered finger plate reinforcement that extends up the spar approximately 8" and the attach plate is 1/8" thick. Very overkill but an area I would gladly take overkill with. Especially considering that this is a stressed area from pushing and pulling and particularly when vibrating at lower RPM's such as runup.

Bob - which model tail did they analyze?
 
As I look more at this picture...

1195159674_Y8aWC-M.jpg


I notice that the location of the upper bend is very critical. If this bend is not formed right (does the builder bend it?) there is a high probablity of a lot of residual stress on the top bolt holes - the cracked one.

A lot of things should line up carefully with no fasteners for a good structural attachment. Torquing the bolts down or riveting the bracket in place "to get things to sit flat" could be bad.
 
As I look more at this picture...

1195159674_Y8aWC-M.jpg


I notice that the location of the upper bend is very critical. If this bend is not formed right (does the builder bend it?) there is a high probablity of a lot of residual stress on the top bolt holes - the cracked one.

A lot of things should line up carefully with no fasteners for a good structural attachment. Torquing the bolts down or riveting the bracket in place "to get things to sit flat" could be bad.

Gil brings up and important point. The part is prebent from vans, however, and this was discussed in this thread, The bracket should sit behind the VS forward spar, not in front as depicted in this picture and the crack photo in question. If the bracket was attached on the front side of the forward spar, and the spar length after trimming was too long... the bracket would be under stress when bolted flat against the HS spar.
 
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Just inspected my VS attach bracket and after 170 tach hours, no sign of cracks. Assembly is installed per plans with attach bracket behind VS spar.
 
Gil brings up and important point. The part is prebent from vans, however, and this was discussed in this thread, The bracket should sit behind the VS forward spar, not in front as depicted in this picture and the crack photo in question. If the bracket was attached on the front side of the forward spar, and the spar length after trimming was too long... the bracket would be under stress when bolted flat against the HS spar.

Nate is correct here. I tried my bracket in front and behind the spar during assembly. When it was in front the upper bolts had to pull the bracket into the horizontal spar to close the gap. With the bracket in the rear per the plans it lays flat and tight against the spar. I could easily see this being the cause of the cracks forming on the top two bolts due to the bending load placed on the bracket through the bolt holes when installed in front of the VS spar.
 
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The bracket should sit [COLOR="Red" said:
behind[/COLOR] the VS forward spar, not in front as depicted in this picture and the crack photo in question. If the bracket was attached on the front side of the forward spar, and the spar length after trimming was too long... the bracket would be under stress when bolted flat against the HS spar.

Gil's instincts told him something was up and a good catch by Nate on what that something was. If I was a betting man, I would double down on this being the likely cause as Don also suggests.
 
Wait a minute....

Nate is correct here. I tried my bracket in front and behind the spar during assembly. When it was in front the upper bolts had to pull the bracket into the horizontal spar to close the gap. With the bracket in the rear per the plans it lays flat and tight against the spar. I could easily see this being the cause of the cracks forming on the top two bolts due to the bending load placed on the bracket through the bolt holes when installed in front of the VS spar.

From the instruction on "Fitting The Vertical Stabilizer" page 8-17.

"If the read spar bends aft at the top when the front spar is clamped, the vertical spar may be mounted on the rear of the F-981 plate. If necessary, you can make a shim to put between the F-981 and the vertical stabilizer spar. If the rear spar bends forward, make a shim to go between the F-981 and the front spar of the vertical stabilizer."
Look like mount on the front or back depends on how your parts look after construction.

Kent
 
RV-6/6A instructions dated 9/10/01:

"Once the VS-603 rear spar has been drilled and bolted, the forward spar of the vertical stab, VS-402, must be aligned with and joined to the front spar of the horizontal stab, HS-602 with the joint plate F-681. At this point the tiny variations between individual aircraft become apparent and the fore/aft match of the spars is seldom perfect. If necessary, F-681 can be attached to the front of VS-402. If this does not solve the problem, use a piece of aluminum sheet, up to 1/8" thickness between the F-681 joint plate and either spar. Of course, longer fasteners must be used as well. If this does [sic] is still not sufficient, a new F-681 may be fabricated with the bend in a different location. Do not leave less than 2" above the bend. The portion below the bend must be sufficient to bolt to both HS-610 and HS-614. The combination of these adjustments should accomodate any reasonable mis-match."

I would have hoped that builders of the RV7/7A wouldn't have faced this type of "mis-match" problem. It is interesting that these instructions seem to indicate that Van does not take the whole "joint plate" very seriously.
 
From the instruction on "Fitting The Vertical Stabilizer" page 8-17.

"If the read spar bends aft at the top when the front spar is clamped, the vertical spar may be mounted on the rear of the F-981 plate. If necessary, you can make a shim to put between the F-981 and the vertical stabilizer spar. If the rear spar bends forward, make a shim to go between the F-981 and the front spar of the vertical stabilizer."
Look like mount on the front or back depends on how your parts look after construction.

Looks as though, either way, care must be taken so that the F-981 does not flex forward or backward when clamped in place.
 
Not quite the same

"Once the VS-603 rear spar has been drilled and bolted, the forward spar of the vertical stab, VS-402, must be aligned with and joined to the front spar of the horizontal stab, HS-602 with the joint plate F-681. At this point the tiny variations between individual aircraft become apparent and the fore/aft match of the spars is seldom perfect. If necessary, F-681 can be attached to the front of VS-402. If this does not solve the problem, use a piece of aluminum sheet, up to 1/8" thickness between the F-681 joint plate and either spar. Of course, longer fasteners must be used as well. If this does [sic] is still not sufficient, a new F-681 may be fabricated with the bend in a different location. Do not leave less than 2" above the bend. The portion below the bend must be sufficient to bolt to both HS-610 and HS-614. The combination of these adjustments should accomodate any reasonable mis-match."

I would have hoped that builders of the RV7/7A wouldn't have faced this type of "mis-match" problem. It is interesting that these instructions seem to indicate that Van does not take the whole "joint plate" very seriously.

The RV-6x assembly is a lot less critical since the part the connects the VS and HS spars does not have a tab on the bottom that is bolted to the deck.

This enables the bent part to be moved up and down somewhat to get alignment. I believe this was never a prepunched part on the -6s.

The RV-9 arrangement is much more critical for alignment due to the lower bolted tab. I guess vans slots were an attempt toease this problem a bit.
 
The RV-6x assembly is a lot less critical since the part the connects the VS and HS spars does not have a tab on the bottom that is bolted to the deck.

This enables the bent part to be moved up and down somewhat to get alignment. I believe this was never a prepunched part on the -6s.
QUOTE]

Thanks Gil, I will be doing the install in a few months and obviously didn't understand the diferences in the models.
 
The elongated holes come that way from Van's.
Here's what mine looked like during the fitting to the fuselage.



I'm not sure why Van's does the elongation of the holes to this part, since you have to drill the forward spar in the VS to set the position and angle of the VS rear spar to be straight up and down. Makes me wonder if there should be washers under the bolt heads on the enlongated holes to provide better clamping pressure on the bracket.

I have never seen cracks in this location before.

The reason for the elongated holes is to allow for an adjustment of stab incidence should anyone ever want to do so.

I thought that washers were specified under the bolt heads (anyone have plans handy?)

Looking closely at the crack photo, it looks to me like there is an imprint where a washer was installed previously (but maybe my imagination)
 
I talked to Van's again today and they say the washer with the bolt should be fine.

Also, I noticed when you look at the drawing you cannot see a washer with the bolt however, in the text of the drawing it specifically says to install a washer.

Today, I received a new F-981 in the mail. After etching priming, painting the part, takeoff the tail this weekend I can hopefully finish it next weekend. New job requires me to work out of town for the week.
 
Great catch. I am sure all will be checking thier stabs for missing washers and cracks. Two other owners and nine or ten cI's and nobody saw this. Way to go.
 
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