C

Clarkie

Well Known Member
Maybe you guys can help me out here. I was doing the preflight the other day on the -6 and found something quite startling. As I inspected the elevator to HS attachments, I found one bolt the didn't appear to have much thread showing, less than usual anyway. When I slipped my finger in there, I could easily move the bolt with little to no effort. I grabbed a few wrenches to tightened it up and it tightened with not much effort. Ended up finding two that were "loose", one had what's always been the same amount of thread showing.

My questions are these as I did not build the airplane:

1. How much thread should be showing?

2. After flying the airplane, it occurred to me that how did the bolt back out and why was I able to tighten it? Shouldn't there be cotter keys or some other locking mechanisms on the bolts to prevent this very thing from happening?

Thanks
 
Last edited:
Clarkie said:
Maybe you guys can help me out here. I was doing the preflight the other day on the -6 and found something quite startling. As I inspected the elevator to HS attachments, I found one bolt the didn't appear to have much thread showing, less than usual anyway. When I slipped my finger in there, I could easily move the bolt with little to no effort. I grabbed a few wrenches to tightened it up and it tightened with not much effort. Ended up finding two that were "loose", one had what's always been the same amount of thread showing.

My questions are these as I did not build the airplane:

1. How much thread should be showing?

2. After flying the airplane, it occurred to me that how did the bolt back out and why was I able to tighten it? Shouldn't there be cotter keys or some other locking mechanisms on the bolts to prevent this very thing from happening?

Thanks
Click to expand...


Plans call for a locknut on these hinge bolts, but no cotter pin. If these nuts had backed off, I would replace them; obviously the locking feature has been compromised. It'll only cost a couple bucks.
 
On my -4 all those bolts (elevators, ailerons, rudder, etc.) have metal lock nuts, always tight. Maybe something to consider..
 
BTW, you do not just "tighten them up". Use a torque wrench. Without one the tendency is to overtighten, depriving the fastener of some of its strength.
 
How do you get a torque wrench in that small slot? I barely had enough room to get two small wrenches in there.
 
When to use a castle nut and cotter pin

FYI
A castle nut and cotter pin should be used any time the bolt will move or rotate in assembly. If the bolt can be torqued down against a bearing surface and the bolt does not rotate, as it is in your example, a fiber or metal lock nut should be used.
Good luck.
 
It would be my guess that the bolts were never fully torqued when it was originally assembled. It is highly unlikely for a properly torqued fastener installed in joints like this to loosen.

I do not use a torque wrench on a difficult to access bolt like these, but I have a pretty good finger feel for what the correct torque value is.
Proper value is 20 - 25 inch pounds. This is much lower than most untrained people would guess. As already mentioned, the danger of you tightening them, if you don't have a good feel for the proper torque, is that you over did it and damaged the bolt (which can be undetectable visually). The yield point of an AN3 bolt is ~60 inch pounds. Almost 3 times the normal torque, but still not very high.

The thread exposure rule of thumb for AN365 nuts (the ones called out in the plans) is at least 1 thread, but no more than 3 threads visible. This rule of thumb does not work with all nuts, so you would need to determine which ones are installed.
 
To torque, use your wrench as a "crow's foot"; put a nut-bolt combination in the free end, and your torque wrench on that. You will need to calculate the torque wrench setting to get 25"-lbs at the nut.
 
It is probably best not to panic. RVBuilder is most likely correct, as his usually is :). Some one just got side tracked and did not tighten them. It happens!!!! Larry
 
BobTurner said:
To torque, use your wrench as a "crow's foot"; put a nut-bolt combination in the free end, and your torque wrench on that. You will need to calculate the torque wrench setting to get 25"-lbs at the nut.
Click to expand...

It is a good idea Bob, and common practice when possible, but most of us don't have a torque wrench that would accurately read low enough for an AN3 bolt with a 4 inch extension.
 
Not sure I agree. With a 4" extension, and a 10" handle, set 18 in-lbs to get 25 in-lbs at the bolt, with the handle and wrench in line.
If you trust the torque at 25 hopefully 18 is reasonably accurate too.

If you can get a 90 deg angle, set to the usual 25.
 
When assembling the elevators to the horizontal stabilizers, you're supposed to shim the brackets so that you can clamp down the hinge bolts on the rod end brackets and on the bearing in the center and still have the elevators pivot with little friction. That way, the bolts are not subject to rotation, and the nuts are primarily retained by assembly preload, with the nylon locking rings serving as backup.

However, I've observed that some builders don't take the time to find the right combinations of washers or shims to get the friction down to a dull roar, and they get too much friction when the bolts are fully tightened. What they do instead is just back off on the nuts a little until the friction goes away. This reduces the friction, but the assembly is then pivoting on the bolts, and not on the center element of the rod ends. Also, this method relies on the nylon locking features to retain the nuts. These are very bad practices, and are definitely contrary to AC43.13. But on this and other forums, we still see this happening on occasion.

My suggestion would be to see if the elevator friction has increased noticeably after tightening the hinge bolts. If so, you should probably investigate the source of the friction, and see what it takes to address it in such a way as that you can still clamp down on the hinge bolts.

Thanks, Bob K.
 
Caution on replacing the hardware

Be sure the check the drawings the get the correct hardware and the correct hardware stack up. As previously mentioned the design is well thought out and it is important to get it right. Even if you decide not to replace the hardware it would be a good idea to inspect the installations against the drawing requirements. Intuition is only good enough to get you to the point that you suspect something is not right.

Bob Axsom
 
BobTurner said:
Not sure I agree. With a 4" extension, and a 10" handle, set 18 in-lbs to get 25 in-lbs at the bolt, with the handle and wrench in line.
If you trust the torque at 25 hopefully 18 is reasonably accurate too.

If you can get a 90 deg angle, set to the usual 25.
Click to expand...

The inch pound torque wrench I have in my shop would require 16.5 inch pounds with a 4 " straight line extension, but the value scale on the torque wrench starts at 20 inch pounds.
 
20-25" is for "tension". Shear is 12-15".
My understanding is that most, if not all, of the airframe assemblies we are bolting together in our RV's, the "critical" forces where designed and calculated for Shear, not Tension (although both come into play).

If you accept this, you now have a range of acceptable torque using an AN365 nut (tension nut) between 12-25". That is quite a lot of tolerance.
 
Don't forget to add the resistive torque from the nut..

rvbuilder2002 said:
The inch pound torque wrench I have in my shop would require 16.5 inch pounds with a 4 " straight line extension, but the value scale on the torque wrench starts at 20 inch pounds.
Click to expand...

I found the AN365 nuts had about 8 inch lbs . So, 25+8 = 33 required. The 10 inch wrench and 4 inch additional length results in setting the wrench at 23 inch lbs. You can do that. Also, setting the wrench at 20 gives you 28 at the nut in my example. Entirely within range. Don't over think this. You would be close enough. And, if you do as Bob Turner suggested and maintain 90 degrees, your wrench, set at 20 can give you an effective torque of 20-8(nut resistance)= 12. So, your wrench is more versatile than you thought, to the extent your wrench is accurate. Just one person's thoughts.
 
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