What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Horiz Stab cracks/repairs: Don't do this...

Walt

Well Known Member
If you find cracks on the horizontal web (ref SB 14-01-31) please do the repair recommended by Van's.
This aircraft is going to have to have a new horizonal stab made now because of the botched repair job.
The repair parts and install instructions (video) for this "Bracket mod" do not comply with Van's SB intent nor do they have any approval from Van's engineering. (it's also getting new vertical attach fitting).
It would have been much easier/cost effective to do the SB mod than it will be to build this guy a new stab.
I guess the guy who did this repair also preferred the looks of the all thread hex head fasteners more than AN hardware.
 

Attachments

  • IMG_1407.jpg
    IMG_1407.jpg
    105.2 KB · Views: 1,480
  • IMG_1408.jpg
    IMG_1408.jpg
    138.9 KB · Views: 1,473
Last edited:
Be afraid, be very afraid

These are some scary pictures :eek:
Reminds me of the puzzle pictures, how many mistakes can you find?
Random hardware, edge distances, bolt lengths, spacers, etc.
I guess they were worried about cracks. With what you can see in these pictures, I would be worried too and wondering about the rest of the aircraft.
At least the torque seal looks good :D
 
With what you can see in these pictures, I would be worried too and wondering about the rest of the aircraft.
I'm not so sure about that. The visible riveting job looks fine, although hard to see in the small photos. These two questionable mods are clearly not part of the original build.

Aftermarket mods that don't affect structure are one thing. These were scary when they were first announced, and are no less scary now.
 
I'm not so sure about that. The visible riveting job looks fine, although hard to see in the small photos. These two questionable mods are clearly not part of the original build.

Aftermarket mods that don't affect structure are one thing. These were scary when they were first announced, and are no less scary now.

Not so sure about that. I agree, it's hard to see with these small pictures, but even so, I see a few over driven rivets, spar spacers that were not fitted properly, rough edge finishes, an extra hole between two bolt heads that has nothing in it (not sure if it's supposed to or not), a bolt head that appears to have been drilled on the top or it might be another hole behind the bolt head (might have happened during the mod install), nuts installed with maybe a half a thread visible, broken down nyloc nuts. There's likely more, that's all I'm saying.

Totally agree though that most of the "damage" was caused with the mod install.
 
If you find cracks on the horizontal web (ref SB 14-01-31) please do the repair recommended by Van's.
This aircraft is going to have to have a new horizonal stab made now because of the botched repair job.
The repair parts and install instructions (video) for this "Bracket mod" do not comply with Van's SB intent nor do they have any approval from Van's engineering. (it's also getting new vertical attach fitting).
It would have been much easier/cost effective to do the SB mod than it will be to build this guy a new stab.
I guess the guy who did this repair also preferred the looks of the all thread hex head fasteners more than AN hardware.
Walt, are you saying that the aftermarket "horizontal stab bracket mod" is fundamentally and conceptually bad, or that the one pictured was installed badly? I'm not a builder, and the SB 14-01-31 doesn't affect me so I'm not clear on the issues, but I'm interested in your opinion regarding the perceived problems with this aftermarket mod.
 
Walt, are you saying that the aftermarket "horizontal stab bracket mod" is fundamentally and conceptually bad, or that the one pictured was installed badly? I'm not a builder, and the SB 14-01-31 doesn't affect me so I'm not clear on the issues, but I'm interested in your opinion regarding the perceived problems with this aftermarket mod.

In my opinion the 'mod' is worse than nothing as it is based solely on eyeball engineering and makes people think that it has some real value to improve the strength of the Horiz, there is absolutely no engineering data for that claim. The fact that this particular example was installed so poorly it ruined the front spar and cannot be repaired (or simpler to build a new one).

I've been involved in some major structure repairs for a very long time, and I've learned to respect the things engineers do that I cannot see. I do know merely attaching a stronger piece of material to a section of structure to "reinforce" it, unless properly designed just moves the stress area to another perhaps weaker location which may make things more likely to fail.

I can eyeball a lot of repairs using methods in 43-13 but when it comes to primary structure like spars that one goes to eng for approval.
 
Installed badly.

1. Rivets were replaced with 10-32 stainless with threads to the head, this means a thread is carrying the shear loads of the HS.

We can not see the additional fasteners behind the rib. Likely the same. as #1.

As a builder - stay with the plans or know exactly, precisely what you are doing.

FYI, the ASplat 'mod' has no fasteners OB of the rib (unlike the Van's SB repair which carries the reinforcement further out, hence the difficulty of installing the Van's repair compared to this mod).
 
I "re-sparred" a stab after a someone botched the sb. It wasn't as bad you might think. I think I ended up using about 4 cherry rivets and was able to finish the rest back to stock. The parts were a lot cheaper than an entire stab kit. I wouldn't hesitate to do it again.
 
Anti-splat

I'm surprised Allen hasn't come to the defense of his product. He puts a lot of thought into his products.
 
I'm surprised Allen hasn't come to the defense of his product. He puts a lot of thought into his products.

Thought or engineering and analysis? The problem that I see with this doubler isn’t the material, or the strength of the material, but the fact that it doesn’t have the fork that extends into the bay beyond the root rib. Only the two rivets (or 4 of you drill the extra ones on his install video) carry the bending moment and the two rivets near the centerline can’t take as much moment as the “fork” that Vans designed into that bay beyond the root rib. It seems that in Alan’s effort to make installation less intrusive, he chopped off the area where the strength is developed! The weakest part is the rivets or fasteners on the little stub part. It NEEDS that forked area that was chopped off to spread the load into the spar. Making it short and stubby to ease installation removed this critical feature.
 
Sorry, been pretty busy. I agree with Walt’s opinion here that structural mods not approved by the designer or done with proper engineering could actually be worse than nothing at all. You really have to be careful when adding doublers or other structural mods to make sure you don’t move the stress riser to a different place, or perhaps increase the stress point at it’s current location.

Here’s a simple saying I keep in the back of my head: bandaids don’t always work; sometimes stitches are needed and it’s best to get a good doctor involved.

You’d be surprised at some of the stuff we see on prebuys. 🙁

Vic
 
For what it's worth, I'm a now-retired aerospace structural analyst. You guys call that a "stress analyst," but stress was only part of the job. Stiffness and deflections were at least as much part of the job and often the entire job. Reason I mentioned that part of it is that these mods, being steel, affect the stiffnesses in that area.

Anyway, I would never use one of these AntiSplat tail beef-up mods on an airplane of mine. I would only use Van's recommendations in this area.

Walt's comments are dead on, in my opinion.

Dave
 
Stabilizer front spar

Having bought a RV7A kit that was almost up to the quick build stage and realizing that the stab front spar mod was not done I set about incorporating the Vans service bulletin doubler plates. Following the instructions it is possible to get it done acceptably well but more likely with oversize holes and poorly formed rivets. I decided to drill out the lower skin rivets (that would also give me the opportunity to inspect the quality of the original riveting). Taking time to carefully drill out the lower skin rivets with an undersize drill went surprisingly fast. I made the decision to replace the front spar so in the end did a fairly extensive disassembly and rebuild. If I had to do it again I would certainly drill out the lower skin rivets, it makes the installation, riveting and inspection of the finished job of installing the doubler so much easier. I also ( after discussions with Vans Tech support) used cherrymax blind structural rivets in about 4 places that are really difficult to drive square rather than having to drill out poorly driven solid rivets, making a mess of it and having to start over. Interestingly during my discussions with Vans I learned that Vans apparently does not have FEA models for the fuselage for the RV-7A - only the wing. This may have changed in the last couple of years since I did the rework in 2017. Having the doubler plates designed to spread the load beyond the first rib would appear to be a well considered strategy that minimizes the probability of creating stress concentrations beyond the first rib where the resultant cracks would be more difficult to inspect. More complexity in doubler installation is a small price to pay for having a better engineered more reliable solution.
 
Interestingly during my discussions with Vans I learned that Vans apparently does not have FEA models for the fuselage for the RV-7A - only the wing. This may have changed in the last couple of years since I did the rework in 2017. Having the doubler plates designed to spread the load beyond the first rib would appear to be a well considered strategy that minimizes the probability of creating stress concentrations beyond the first rib where the resultant cracks would be more difficult to inspect. More complexity in doubler installation is a small price to pay for having a better engineered more reliable solution.

It is true that there is not extensive FEA modeling for the entire RV-7. We did not have the capability when it was designed.

The horizontal stab. spar modification was designed using FEA analysis though, as is most all of the structural modifications that get released occasionally when age of the fleet shows that a particular detail should be re-evaluated.
 
Scott, with seven years on the clock since the SB was published, would you care to hazard a guess as to the percentage of the fleet with HS-0000x doublers installed?

I realize doubler sales are not a perfect indicator, given some get installed as a prophylactic measure.
 
Scott, with seven years on the clock since the SB was published, would you care to hazard a guess as to the percentage of the fleet with HS-0000x doublers installed?

I realize doubler sales are not a perfect indicator, given some get installed as a prophylactic measure.

I can't make a guess that I have confidence would be anywhere close to reality so it is probably best not to.
At the time that the SB went public, the doublers had been incorporated into the standard build so all kits that shipped from that point on included the parts.

I do know that there has been a lot of "prophylactic" installations of the doublers done on flying aircraft (against our recommendation as spelled out in the S.B. document), and unfortunately, as we predicted, quite a large # of those installations went badly which resulted in the modification (and subsequent required repairs) being much more extensive.

From what I have heard, the number of actual cracks discovered has been a rather small percentage of the fleet.

After our initial discovery and analysis we felt that it was entirely possible that an aircraft could go its entire life time and not develop a crack. And now with 7 years of experience, our position hasn't changed.
We recommend the doublers only be added to a flying aircraft if cracks are discovered at inspection.
 
Back
Top