Wing Spar Bolts and Engine Mount Re-Torque

[JonJay]

I am curious to hear from experience RV mechanics like Mel and Walt on the subject of re-torqueing spar bolts, both early laminated spars and current machined spars, and engine mount to firewall bolts.
What are others doing, if anything, and if so, at what intervals.

 

[Walt]

I check them on every condition inspection and I have found both spar and engine mount bolts (mount/airframe) under-torqued/loose on many aircraft.

 

[Low Pass]

Very interesting. Old spar, new spar??

 

[JonJay]

Thanks Walt.
Interesting that most of the condition inspection check lists I have reviewed do not include checking these.

 

[Walt]

Very interesting. Old spar, new spar??

Both

Thanks Walt.
Interesting that most of the condition inspection check lists I have reviewed do not include checking these.

I check all critical bolts with a wrench to ensure they are tight, looking at them don't cut it (pretty hard to tell if a bolt is loose or sheared by looking at it).

This includes but is not limited to all: engine controls, flight contols and primary structure (especially vert/horiz attach and wing spar), fuel/oil line fittings/B nuts etc..

I also check things like sump bolts, mag bolts, exhaust hanger bolts, exhaust/intake cylinder bolts, MLG attach/fairing attach, tail wheel attach nuts/bolts.

I also don't care if "torque seal" is there they can still be loose (and have been).

You get the idea.

That's also the reason I schedule a week for condition inspections, anyone that says they can do a good inspection in 1 or 2 days.... well I better leave that one alone before I get in trouble.

 

[Mel]

Believe it or not; I have found loose bolts/nuts on initial airworthiness inspections that have torque seal on them.

 

[JonJay]

I check all critical bolts with a wrench to ensure they are tight, looking at them don't cut it (pretty hard to tell if a bolt is loose or sheared by looking at it).

.

Ditto - Specific to the Mount and Spar, this is how I discovered a couple bolts needing to be tightened. I chose to re-torque all of them, which revealed that a couple where slightly under torqued, even though they passed the "wrench" test.
I assume you do not put a torque wrench to each bolt at each CI or am I mistaken? I think there are some odd 60 AN3's in a 6's spar alone.

I am not trying to get carried away here, just want to know what the "pro's" do. I only have one RV to take care of.....

 

[Christopher Murphy]

Spar bolts

I check them on every condition inspection and I have found both spar and engine mount bolts (mount/airframe) under-torqued/loose on many aircraft.

I am curious if these under torqued bolts were installed that way and then discovered or if they were properly torqued and found loose at a later inspection.
The spar bolts, when properly torqued using lock nuts should not be coming loose.

Chris m

 

[JonJay]

I am curious if these under torqued bolts were installed that way and then discovered or if they were properly torqued and found loose at a later inspection.
The spar bolts, when properly torqued using lock nuts should not be coming loose.

Chris m

Chris - mine did. They where torqued at one time, 100% positive. They where not "loose" but a few did need to be tightened. Don't ask me how a torqued bolt loses it's torque, but.....
Also, keep in mind most bolts in an RV are shear, not compression or tension. A slightly loose bolt is probably not going to do anything or be a concern in most of our applications. However.....

 

[Walt]

Ditto - Specific to the Mount and Spar, this is how I discovered a couple bolts needing to be tightened. I chose to re-torque all of them, which revealed that a couple where slightly under torqued, even though they passed the "wrench" test.
I assume you do not put a torque wrench to each bolt at each CI or am I mistaken? I think there are some odd 60 AN3's in a 6's spar alone.

I am not trying to get carried away here, just want to know what the "pro's" do. I only have one RV to take care of.....

I have a "calibrated" arm and use a wrench to check most things. Certain things I just get out the torque wrench (like exhaust studs) and torque them.

I am curious if these under torqued bolts were installed that way and then discovered or if they were properly torqued and found loose at a later inspection.
The spar bolts, when properly torqued using lock nuts should not be coming loose.

Chris m

Who knows but I suspect many were not torqued correctly from the start.

Also, keep in mind most bolts in an RV are shear, not compression or tension. A slightly loose bolt is probably not going to do anything or be a concern in most of our applications. However.....

This may be true, but just like the recent nose gear bolt failure, a loose bolt will allow the pieces to start moving relative to each causing wear to the bolt, bolts holes etc, things start getting loose and so on.

 

[Toobuilder]

...This may be true, but just like the recent nose gear bolt failure, a loose bolt will allow the pieces to start moving relative to each causing wear to the bolt, bolts holes etc, things start getting loose and so on.

This is a good opportunity to bring up the importance of a properly reamed bolt hole. A drill bit will not produce a round hole, and if you can push the bolt in by hand, it's not carying the total load. A proper joint with the bolt loaded in shear (spar bolts, for example) will carry the full load with no nuts whatsoever.

 

[Walt]

Very true, but even with a properly sized hole if the pieces are not clamped together to prevent relative movement wear will take place and eventually that tight fit won't be so tight anymore.

 

[JonJay]

Thanks Walt. That is consistent with what I do.

 

[JonJay]

This is a good opportunity to bring up the importance of a properly reamed bolt hole. A drill bit will not produce a round hole, and if you can push the bolt in by hand, it's not carying the total load. A proper joint with the bolt loaded in shear (spar bolts, for example) will carry the full load with no nuts whatsoever.

Good point, but I am pretty sure Van's didn't design to this level of precision for most of the airframe, or any of it for us older models, or the fleet would have fallen out of the sky by now. They sure didn't produce their instructions to that level anyway.
These are amateur built remember, many by people with no sheet metal, machining, or certainly aircraft manufacturing experience.

We don't need to hit the bulls eye, just the hay bale.

 

[Christopher Murphy]

true

Chris - mine did. They where torqued at one time, 100% positive. They where not "loose" but a few did need to be tightened. Don't ask me how a torqued bolt loses it's torque, but.....
Also, keep in mind most bolts in an RV are shear, not compression or tension. A slightly loose bolt is probably not going to do anything or be a concern in most of our applications. However.....

JonJay,
You are correct about the shear bolts especially in the case of the spar bolts. One loose bolt in this case would not be a real problem. I was a phase inspection tech on the F-4, have an A&P, and I was an airline mechanic too at one point in my life, working in a component shop inspecting, repairing, and overhauling large jet aircraft components. ie.. cowlings, thrust reversers, etc. Inspecting an aircraft and not missing defects takes a disciplined mind, a sharp eye, and lots of practice. A good inspector is a rarity.

Awsome that you fly a Bucker, there is one at a nearby field.

Chris M

 

[Toobuilder]

Good point, but I am pretty sure Van's didn't design to this level of precision for most of the airframe, or any of it for us older models, or the fleet would have fallen out of the sky by now. They sure didn't produce their instructions to that level anyway.
These are amateur built remember, many by people with no sheet metal, machining, or certainly aircraft manufacturing experience.

We don't need to hit the bulls eye, just the hay bale.

Remember, we're talking about bolts in the spar here, not screws that hold the cowl in place. The fact that Van is obviously conservative does not mean new builders (who are also going to read this post) are relieved from their duty to assemble the airplane correctly. If nothing else, people need to understand that a perfectly round hole and an interference fit is the standard.... The further you get away from that, the closer you get to scrap.

 

[Walt]

Remember, we're talking about bolts in the spar here, not screws that hold the cowl in place. The fact that Van is obviously conservative does not mean new builders (who are also going to read this post) are relieved from their duty to assemble the airplane correctly. If nothing else, people need to understand that a perfectly round hole and an interference fit is the standard.... The further you get away from that, the closer you get to scrap.

Interference fits are not the norm and are only used in critical structures, the only place Van's has an interference fit callout (that I can think of off hand) are the spars which they ream and provide close tolerance bolts for. Unless the bolts are specified as close tolerance types, then the designed fit will not be an interference fit.

Now is it good practice to ream holes and keep things snug, yes.. but thats not the same as saying all bolts should be intalled with an interference fit.

From 43-13:
Generally, it is permissible to use the first-lettered drill size larger than the nominal bolt diameter, except when the AN hexagon bolts are used in light-drive fit (reamed) applications and where NAS closetolerance bolts or AN clevis bolts are used.

PS: as far as spar bolts are concerned, all bolts must be an interference fit as designed, anything other than that should be rejected.

 

[Rick_A]

As long as we're talking about keeping critical bolts torqued and the fit of bolts, let's not forget about landing gear bolts.

During my last annual Walt reminded me to check spar bolts and landing gear bolts. There was a little play in the landing gear bolts, so I replaced my stock AN5-22A bolts with NAS6205-30 bolts in order to get a tighter fit. These are the bolts that go through the weldments at the upper end of the gear legs.

...and yes, my spar bolts also needed to be tightened up a bit.

 

[JonJay]

Well, I don't think everyone is checking some critical items during thier condition inspections. We assume a torqued bolt is going to stay that way.
Check those spar and engine/fuselage mount bolts. You might be surprised at what you find.

 

[plehrke]

Use a good calibrated torque wrench

Remember over torque can be just as bad as under torque. I went in and started to check spar bolt tightness at my last conditional inspection just with a couple of wrenches using my "calibrated" feel. After finding several that seemed to not be tight I went and got the torque wrench to do it right. It turned out that they were torqued correctly and had to go back off on the ones I had over torqued by hand. This was specially true for the AN3 bolts not in the center splice. It has been 8 years since completing the build and my "calibrated" feel is not what it was when I was building.

 

[longranger]

Remember over torque can be just as bad as under torque. I went in and started to check spar bolt tightness at my last conditional inspection just with a couple of wrenches using my "calibrated" feel. After finding several that seemed to not be tight I went and got the torque wrench to do it right. It turned out that they were torqued correctly and had to go back off on the ones I had over torqued by hand. This was specially true for the AN3 bolts not in the center splice. It has been 8 years since completing the build and my "calibrated" feel is not what it was when I was building.

AN3's can be really tricky. It doesn't take much at all to overtorque them. Just a few days ago I final installed the seat ribs to the F-704 bulkhead, and ran the nuts up "just snug" with a small 1/4" drive ratchet before torquing them. None of them took more than half a flat more to hit 25 in-lb.

 

[BobTurner]

Is this possible?

Chris - mine did.
. Don't ask me how a torqued bolt loses it's torque, but.....
...

Is it possible for the close tolerance spar bolts to get "clamped" at the spar-carry thru junction (maybe the wing support shifted) to such an extent that when the nut is torqued, only that part of the bolt between the "clamp" and the nut is stretched? And then after flying, vibration, wing flex, etc., the bolt relaxes and is now uniformly but under stretched?

I recall that when I torqued these nuts it took very little force to hold the wrench on the bolt head. What if it had taken none?

 

[Low Pass]

Is it possible for the close tolerance spar bolts to get "clamped" at the spar-carry thru junction (maybe the wing support shifted) to such an extent that when the nut is torqued, only that part of the bolt between the "clamp" and the nut is stretched? And then after flying, vibration, wing flex, etc., the bolt relaxes and is now uniformly but under stretched?

I recall that when I torqued these nuts it took very little force to hold the wrench on the bolt head. What if it had taken none?

Properly loaded fasteners won't relax (if static and not overloaded). If these are loosening, then it's the pieces in the assembly relaxing/squeezing/becoming closer fitting. Like the -8 landing gear attachment, there are many pieces bound together that, over a period of time and with movement, allow the joint to effectively loosen.

I appreciate this being brought up as I would *not* have expected the (new) spar connection to loosen in this manner.