SB-00036 issue

[GlennBfromOz]

Hi all,

Yesterday a friend and I performed the repair as above, on each of our RV7s. Our idea was to do it pre-emptively, as that would avoid the need to open up the tailplane to install the repair doubler. We drilled out 6 rivets top and bottom each side to install the spar doublers, and riveted them in place. So far, so good...

We prepared the Cherrymax rivets for the new hinge brackets as indicated in the SB, by knocking the mandrels part way out and lubricating. Despite this, in about half of the cases the mandrel broke early. There are a couple of ways to know this happened, including the head of the mandrel not being flush with the rivet, but the real evidence is shown with a borescope on the inside of the tailplane spar. You can see the difference between the two rivets in the photo.

So, a few questions arise:

1. If the mandrel doesn't seat fully, how much are the grip and shear strengths compromised? If unacceptably so:
2. Is this an immediate cause to drill out and replace the offending rivets? I note that the rivets appears properly bulbed on the inside, for what that's worth. And I'm acutely aware that removing the rivets invites the possibility of damage to the existing holes. And, if we do replace them:
3. How do we ensure that the same thing doesn't happen again? I've never had this occur before when installing Cherrymax.

Thanks for any input,

Glenn

 

[PaulvS]

Hi all,

Yesterday a friend and I performed the repair as above, on each of our RV7s. Our idea was to do it pre-emptively, as that would avoid the need to open up the tailplane to install the repair doubler. We drilled out 6 rivets top and bottom each side to install the spar doublers, and riveted them in place. So far, so good...

We prepared the Cherrymax rivets for the new hinge brackets as indicated in the SB, by knocking the mandrels part way out and lubricating. Despite this, in about half of the cases the mandrel broke early. There are a couple of ways to know this happened, including the head of the mandrel not being flush with the rivet, but the real evidence is shown with a borescope on the inside of the tailplane spar. You can see the difference between the two rivets in the photo.

View attachment 63717

So, a few questions arise:

1. If the mandrel doesn't seat fully, how much are the grip and shear strengths compromised? If unacceptably so:
2. Is this an immediate cause to drill out and replace the offending rivets? I note that the rivets appears properly bulbed on the inside, for what that's worth. And I'm acutely aware that removing the rivets invites the possibility of damage to the existing holes. And, if we do replace them:
3. How do we ensure that the same thing doesn't happen again? I've never had this occur before when installing Cherrymax.

Thanks for any input,

Glenn

See in this thread, e.g. post #5 and others, there is lots of good info on implementing this SB:

SB 00036 (cracked horiz stab spar) Installation Notes

As the service bulletin kits are arriving in the field and installations occur there may be information that is useful for those contemplating or planning the repair. Please post any details you encounter with the installation that need to be shared.

vansairforce.net

 

[GlennBfromOz]

See in this thread, e.g. post #5 and others, there is lots of good info on implementing this SB:

SB 00036 (cracked horiz stab spar) Installation Notes

As the service bulletin kits are arriving in the field and installations occur there may be information that is useful for those contemplating or planning the repair. Please post any details you encounter with the installation that need to be shared.

vansairforce.net

Thanks Paul; hadn't seen that thread. For the benefit of other readers, we were careful to use the correct -3 rivets without the internal doubler on the spar.

There seems to be a recurring way of avoiding Cherrymax problems, which is: don't use them!

 

[Ron RV8]

Thanks Paul; hadn't seen that thread. For the benefit of other readers, we were careful to use the correct -3 rivets without the internal doubler on the spar.

There seems to be a recurring way of avoiding Cherrymax problems, which is: don't use them!

Had same problem, even lubed and set @ lower pressure. Start with double the number you need and get good at punching / drilling them out. Part of problem was nose of riveter was hitting doubler so started using wedge which helped. Did not damage holes...

 

[JonJay]

There is a reason Cherry Max has very specific, and expensive, pullers and mandrels. I question Vans judgement saying you can pull them with standard pullers even though I have done it and gotten away with it in other applications.

 

[lr172]

looks

Hi all,

Yesterday a friend and I performed the repair as above, on each of our RV7s. Our idea was to do it pre-emptively, as that would avoid the need to open up the tailplane to install the repair doubler. We drilled out 6 rivets top and bottom each side to install the spar doublers, and riveted them in place. So far, so good...

We prepared the Cherrymax rivets for the new hinge brackets as indicated in the SB, by knocking the mandrels part way out and lubricating. Despite this, in about half of the cases the mandrel broke early. There are a couple of ways to know this happened, including the head of the mandrel not being flush with the rivet, but the real evidence is shown with a borescope on the inside of the tailplane spar. You can see the difference between the two rivets in the photo.

View attachment 63717

So, a few questions arise:

1. If the mandrel doesn't seat fully, how much are the grip and shear strengths compromised? If unacceptably so:
2. Is this an immediate cause to drill out and replace the offending rivets? I note that the rivets appears properly bulbed on the inside, for what that's worth. And I'm acutely aware that removing the rivets invites the possibility of damage to the existing holes. And, if we do replace them:
3. How do we ensure that the same thing doesn't happen again? I've never had this occur before when installing Cherrymax.

Thanks for any input,

Glenn

Looks to me likje you are not holding the rivet square as you set it. You can see the mandrel being pulled into yhe side of the casing. It is catching on the sheet edge, creating resistance, then it breaks free. You can see the bulge on the side of the casing where the mandrel is catching and bending it over. Cherry max rivets are nothing like pop rivets and WAYYYYY less tolerant of user error.

Larry

 

[GlennBfromOz]

Closing this story out...

Yesterday we went fearlessly back in, to right the wrongs of the previous attempt. We started by practising on some scrap, to get an idea of how the pulled end should look when properly set.

That done, we punched and drilled out all the rivets; even those that looked halfway right. It's a little disturbing that, when not properly set, the mandrels punch out very easily. Presumably this could have happened eventually if they had remained in service, at the cost of their shear strength.

The key (sorry for insulting those who already know) seems to be getting the tool absolutely square on the rivet anvil. As Ir172 points out, not doing so can cause the mandrel to snag and break prematurely. Similarly, a slow gentle pull reduces the risk of an early break. How the latter can be achieved with a pneumatic puller, I can't comment.

When done properly, there are two distinct clicks. The first is the locking section engaging; the second is the mandrel finally letting go. If you don't hear them both, the rivet is not fully set. Despite our careful approach yesterday, there were still a couple across our two aircraft which didn't sound right, and needed to be redone.

This raises a concern in the installation of Cherrymax rivets. By their very application, they're usually going somewhere that the pulled end can't be seen and inspected. We were borescoping after each rivet, which helped confirm both those which set correctly, and the few that didn't. That luxury isn't available in every location.

In summary, we finally got the job done, and learned a lot. Thanks to all who contributed to our education.

Glenn