7A elevator spar crack

[vic syracuse]

That those locknuts on the rod end bearings, once tightened and torque- sealed, would never come lose without visibly breaking the torque seal. Yesterday, I found myself shaken to the core, seriously. I always visibly look at the locknuts on the all control surfaces during a preflight for just that, and I routinely touch most, but not all of them. Well, yesterday, after 720 hours on the Hobbs, I checked the tail a little more closely and I found one on the left elevator, and one on the rudder that I turned with my fingers during the preflight!
My rudder also had some wind damage about 2 years ago due to a thunderstorm.
Related? Will never know. But I will certainly keep an eye on the jam nuts a little more closer.
As a further note, I have been a DAR on many rv's in the last 5 years, and I have only found 1, yes one, that didn't have some loose jam nuts on the tail rod end bearings. This seems to be a regularly overlooked area.

Vic

[Caveman]

RV-7 340 hours, IO-360-M1B. Gave it a quick inspection when this thread first opened up and didn't find anything. I had used torque seal and look at it during preflight. Went back today and really gave it a good going over with the flashlight and mirror. Cleaned the area and looked for cracks. Tugged and wiggled, thoroughly. Everything seemed to move smoothly with no discernable flexing or binding. Put a wrench on the lock nuts and they were tight on both the elevator{s} and rudders. Checked the trim servo and hardware. It was fine. Celebrated by putting air in the tires and going flying, Even did a roll and a few wheel landings.

This is a good thread, guys! Thanks for the heads up. This is what keeps me reading VAF even though the plane has been flying 5 years.

[Bob Redman]

G'day folks,

RV-7, built to plans, 93 hours TTIS. 210 landings, 50:50 sealed:unsealed surfaces.

Inspected tail cone & empenage:

both vertical & horizontal stabs felt solid - no movement or noise detected.
free movement of control surfaces (no differential movement of elevators).
no cracks noticed.
slight movement of elevator trim tab, as reported in other posts, and slight noise with movement coming from manual trim cable moving in plastic snap bushing at rear H stab spar.
All rivets OK, nuts tight, BUT, torque had relaxed on the four nuts holding the front Horizontal (and front vertical) stabiliser through shims to the fuselage rear upper deck. All retorqued - my impression is that the two outer nuts had relaxed slightly more than the two inner nuts - not much movement of the wrench before reaching the torque setting on any of the four (torque seal remained intact to my eyes - so wrench needed to confirm - seal only good as a indication that the nut has been tightened - not that it has retained its correct torque).

Thanks for this thread and all the contributors. This site has to be the best value in aviation - so thanks and much appreciation too for Doug Reeves.

[Walt]

Originally Posted by vic syracuse

That those locknuts on the rod end bearings, once tightened and torque- sealed, would never come lose without visibly breaking the torque seal. Yesterday, I found myself shaken to the core, seriously. I always visibly look at the locknuts on the all control surfaces during a preflight for just that, and I routinely touch most, but not all of them. Well, yesterday, after 720 hours on the Hobbs, I checked the tail a little more closely and I found one on the left elevator, and one on the rudder that I turned with my fingers during the preflight!

This is precisely why I have said many times that a dab of torque seal means nothing to me when doing an inspection, its presence does not guarantee proper torque, only a wrench can do that.

[precession]

1993 RV-4, 0320, @1000 hrs., 3rd owner, not builder. Have owned plane for @500 hours and, honestly, never knew to check rod end bearing stop nuts for tightness. Nor, apparently, did my A&P who did annual conditional inspections.

Found 6 of 7 stop nuts loose on tail. Tightened all and plan on adding torque lines (even while recognizing last post that even if torque line discloses no movement only a wrench confirms for sure the nut is still tight).

Unable to find evidence of cracks on either HS/VS side or elevator/rudder sides of attachment points. (But they are both well-painted, so I'm not sure if cracks would necessarily show(?).)

No play of any kind between right and left elevators.

Thanks a lot to all for this thread.

[gtmule]

I'd want to see the actual parts, or see some better pictures, but this crack doesn't look like a typical high cycle fatigue crack to me. Cracks like to start at (not near) holes or edges and grow towards other holes or edges. My thought I'd that looks like low cycle fatigue, with a small number of VERY high stress events causing this weird crescent shaped crack just under a fastener, which could be caused by very high bending loads in that fastener.

From other posts, it sounds like this tail saw some kind of wind or impact event on the ground?

[Zero4Zulu]

It's hard to tell from the earlier photo's but it would be worth verifying that the cracked spar has the grain direction running correctly to the bends of the spar. On my RV6 the grain is running perpendicular to the long bends of the spar. I think if the grain was running parallel to the bends, cracks could develop easier. It's not impossible that a operator could have sheared a sheet the wrong direction.

Looking closer at the photo with the paint stripped it really looks like the grain is running the wrong way.

[ao.frog]

I've been following this tread with great interest, and came to think of one thing regarding thightening the loose locknuts: schouldn't the rod-ends be held in position with a tool during the tightening?
If not, I guess the rod end ball-bearing will be exposed to pretty heavy forces?

When tightening the rod ends during the build, I used a special tool bought from Avery.
This tool will keep the tightening forces to the rod end casing and not the bearing.

In order to use it, the control surface must be removed, so the job gets alittle more complex, but certainly doable.

Here's a link to the tool:
http://www.averytools.com/prodinfo.asp?number=408

[DanH]

Originally Posted by rvbuilder2002

If there was a general issue with the design, it would have become evident many years ago. At the very least, on RV-8's. They use the exact same (prepunched) elevator.

I said it once, but I guess many don't believe... this type of a crack does not occur from one (or even several) overload/bend events localized at the crack. It is many (probably many thousands, possibly even many ten thousands) load cycles. The damage from a single overload in a location like this would be a visible crease or bend. Not an original flat surface with a (fatigue) crack.

Deductive reasoning says that something has been causing a localized load cycle on this (maybe these two) airplane, that does not occur on thousands of others that have much more flight hours on them. The challenge is discovering what. Once we know that, a solution is right behind.

My opinion is that we haven't yet heard all the details.

I've quoted the above merely because it would be good it everyone read it again. Scott makes four fundamental points. I think we have general agreement on 1, 3, and 4. Some debate remains regarding #2, the question of low-cycle or high-cycle loading. Serious addressing #4 would do wonders for settling that question. Gathering all the details is the hard part.

Let's keep it focused. Problem solving in an internet forum tends to be like a Dick Cheney quail hunt; shotgun fashion, in random directions, with poor results. It is fine to propose a theory, but the theorist should be able to offer evidence or a logical path to the conclusion. For example, you could say "It cracked because aliens fired a destructo-ray from orbit". However, without evidence of aliens or destructo-rays or unidentified orbiting objects, the proposal is useless. Please include how you came to the conclusion; evidence, math, logic, a link to an SAE paper on destructo-rays, whatever demonstrates some thoughtful analysis. Heck, even if wrong we'll probably all learn something useful in the process.

Returning to #4, Mark, Bill K, can you confirm the dimension in the drawing was 7/8" or less? I realize you may have dismantled the offending elevator, but the other one should be similar.

[Mark Burns]

Dan,
I measured the right elevator rod ends before dis-assembly.
I used a pocket scale only. They were both identical using a scale.
I'm at work now but I remember what I wrote down. 7/8 minus 1/32.
So it's 27/32.

I won't be able to go to the airport until tomorrow afternoon.
I will check the left elevator alignment like Alex suggested.
Then I will pull it and check the horizontal stabilizer hinge point alignment.

I should finish getting the skin rivited on my new elevator spar this evening.
I finished the internals last night.

Mark