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Bellcrank locknut missing during condition inspection

Webb

Webb

Well Known Member
Sponsor
The things that make you say oh ****.

While going about my condition inspection, I almost threw up tonight. The bolt (AN3-7A) that connects the right aileron push tub end bearing to the bellcrank was proudly standing naked without a locknut..... I found the washer and the nut in the wing bay. All others were solidly in place.

How long it's been that way, I don't know but it's been less than 4 months. Fortunately, I had the forethought to put the bolts in so if they ever lost a nut, gravity wouldn't take over and let the bolt drop out. It took me about 2 seconds to fix it with a new nut but but I'm still a bit shakey by what could have happened.

For those building a wing, make darn sure you put those bolts in so they cannot fall out. For those flying, check 'em.
 
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Scary experience for sure! :eek:

How do you know it's been within the past 4 months (new plane, maybe)?

Glad you found this before it showed itself in flight.
 
Webb said:
The things that make you say oh ****.
<snip>
For those building a wing, make darn sure you put those bolts in so they cannot fall out. For those flying, check 'em.
Click to expand...
I went the belt and suspenders route on mine. Used a drilled bolt, castle nut that also has the elastic stop nut material. Torqued, stop nut and cotter pin.
 
That is scary - do you think the nut vibrated loose or might have never been torqued? (easy to miss) I wouldn't have thought there'd be much vibration out there.
 
Try this

I guess flying helicopters for a living, I'm always checking anything that moves, spins, pivots, rotates etc....

I have gotten the habit of pulling that inspection plate during every oil change. You can do it in the course of waiting for a quart of oil to drain or do it when you drain the old oil. For me it is every 25 hours.

I also pull the tail inspection plates as well.

Glad everything worked out. A great reminder for everyone.
 
One of the very first things I was taught as a 13 year old kid beginning to help work on airplanes was "Top to Bottom, Front to Back!" - that's how bolts get installed. Why? "Because nuts fall off, and people die if the bolt falls out...."

Some of those old, gruff, and grizzled guys knew what they were talking about...

Glad to hear you found it when you did Webb !

Paul
 
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Webb Something is wrong with the installation

The way you installed the bolt is correct of course (heads up and forward) but something is wrong with your installation if normal use caused the lock nut to back off of the bolt - vibration is not the cause. Look deeper into the root cause or you may experience it again under worse conditions.

Bob Axsom
 
Bob Axsom said:
The way you installed the bolt is correct of course (heads up and forward) but something is wrong with your installation if normal use caused the lock nut to back off of the bolt - vibration is not the cause. Look deeper into the root cause or you may experience it again under worse conditions.

Bob Axsom
Click to expand...


2 possibilities. An aileron adjustment was made several months ago. That bolt was removed and re-installed. Could have been an over-torque since it is such a critical connection, or an under-torque. I can tell you it has been replaced with great care. And Paul is right about the orientation.
 
An over-torque...

Webb said:
2 possibilities. An aileron adjustment was made several months ago. That bolt was removed and re-installed. Could have been an over-torque since it is such a critical connection, or an under-torque. I can tell you it has been replaced with great care. And Paul is right about the orientation.
Click to expand...

...should be just about impossible to destroy the locking feature - the bolt should break or the nut/bolt threads strip first.

I would vote for an under-torque, but even that should not really make the nut fall off.

Could it have just been installed finger tight for aileron travel testing and never tightened down into the plastic area of the nut?
 
az_gila said:
...should be just about impossible to destroy the locking feature - the bolt should break or the nut/bolt threads strip first.

I would vote for an under-torque, but even that should not really make the nut fall off.

Could it have just been installed finger tight for aileron travel testing and never tightened down into the plastic area of the nut?
Click to expand...

You are correct.....there is a third....heaven forbid but finger tight.

I really don't know but since there is a human factor here, anything is possible. What I really want to convey is exactly what Paul had to say about bolt orientation. It's also a very good reason why we use a checklist and don't "cheat".

Ironically, through this is the ONLY squawk I've found during the inspection but it was a doozie.
 
Thanks for posting Webb - I am careful to put torqur seal on every fastener that I torque, immediately after I torque it. That way I know if (or when!) I find one without torque seal, then I haven't torqued it. Can't be too careful ... thanks for sharing your cautionary tale.
 
Wow

Hey Webb I don't have anything to add that has not been said, but I am really glad you found it on the ground safe and sound.
 
Now I'm wondering how an RV would fly with one free floating aileron; if it didn't somehow jam. Seems like it would work..... I could always test a R/C model to find out.

L.Adamson
 
Be Careful

Make sure that the bearing involved still rotates freely and there are at least 1.5 threads of the AN3 bolt extend through the nut after it is tightened.

Bob Axsom
 
Standard practices!!!

Holy Shinikees!!!! Thats why standard practice is to cotter pin when a bolt is subject to rotation.:)
 
...but this bolt isn't supposed to rotate, is it? The idea is to clamp the "ball" of the bearing tight and let the bearing rotate around it.
 
Torque Seal

Every bolt that gets torqued also gets torqued sealed as soon as I remove the torque wrench.

No seal, no torque. Don't wait, mark it as soon as it is torqued. Don't go to a second bolt, do nothing until it is marked.

Even when I was beta testing Dynon's auto pilot and knew I would remove a servo motor after one test flight, everything got torqued sealed.

Just good "shop practice".
 
derLuigi said:
...but this bolt isn't supposed to rotate, is it? The idea is to clamp the "ball" of the bearing tight and let the bearing rotate around it.
Click to expand...

It rotated didn't it? I'm just saying it wouldn't hurt to use the belt and suspenders method. I know I'd fly a little more relaxed.
 
Maybe the phone rang.....

...and the nut was never more than finger tight??

Don't laugh because the right engine failed on a Cessna 310 I was test flying after an annual, just as the gear came up.

The mechanic had only started the fuel line fitting to the spider and never put a wrench on it. He later quipped, "that d*** phone!"

Regards,
 
N941WR said:
Every bolt that gets torqued also gets torqued sealed as soon as I remove the torque wrench.

No seal, no torque. Don't wait, mark it as soon as it is torqued. Don't go to a second bolt, do nothing until it is marked.

Even when I was beta testing Dynon's auto pilot and knew I would remove a servo motor after one test flight, everything got torqued sealed.

Just good "shop practice".
Click to expand...

This is so valuable, yet it is not a universal practice of builders. I've "inspected" quite a few new planes before their DAR official sign-off, and this is my most common observation - no torque seal. I've found finger tight nuts more than once, and it usually gives the builder a little motivation to follow the advice.:eek:
 
Rudder petal nut at OSH

In the mid 90's I was looking at an RV-6's that had just arrived at OSH. I was talking to the builder/pilot as I looked at his rudder pedal installation which is were I was at in my building process at the time. Next to the right rudder cable I noticed a nut on the floor and realized that it had came off of the rudder cable bolt, which was still in place. I still remember the look on the pilots face when I pointed it out to him as he ran through his mind what could have happened on landing.

In this case it was a castle nut. Changing the aileron bolt and nut to a castle type would not prevent the loss of the nut if not installed correctly, same as the lock nut.

Thanks for sharing your cautionary tale.
 
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When I do a certification inspection, my checklist covers all flight and engine control connections. If is VERY common for me to find at least one or two loose nuts, no threads showing, and/or missing cotter pins.

Another hint; Anytime you perform maintenance of any kind, including a condition inspection, before you close it up, have another set of eyes look it over. You'll be surprised at what you can miss.
 
How many threads should be showing on AN3 bolts?

Mel,
I am having trouble finding that answer. On two of the bolts on my RV6A tie down receiver, I see one thread.
 
WenEng said:
Mel,
I am having trouble finding that answer. On two of the bolts on my RV6A tie down receiver, I see one thread.
Click to expand...

AC 43.13-1B, Chapter 7, section 3: "All bolt installations which involve self-locking or plain nuts should have at least one thread protruding through the nut."
 
Thanks for the post!

You probably saved more than your own life.

Thanks for owning up to a simple but potentially deadly mistake.

Hopefully WE now are all the wiser! :eek:
 
Note that this...

Mel said:
AC 43.13-1B, Chapter 7, section 3: "All bolt installations which involve self-locking or plain nuts should have at least one thread protruding through the nut."
Click to expand...

...is one FULL THREAD exposed - this can be a bit tricky to see/determine since the ends of the AN bolts have a chamfer on them.

This came from a 3 year old post of mine...

------------- old post (my highlight) ---------------

While the FAA in AC 43.13 allows for 1 thread to be exposed.... I did some hunting on the MIL-Specs, and reading the MS21042 specification (the typical all metal locknut sold by Aircraft Spruce), and found it referenced a NASM specification NASM33588 with the nice long title "Nut, Self-Locking, Aircraft, Reliability and Maintainability Usage Requirements for"...

Anyway, this specification details the use of self-locking nuts in aircraft, and specifies...

2. Bolts, Studs or screws must extend thru the self-locking nut for a minimum length equivalent to two threaded pitches.

This length includes the chamfer.
So it is tighter that the FAA spec. (and also includes the fibre insert nuts) but with the big difference that it includes the chamfered end of the bolt.

So off to the AN 3 specification.... ....and the chamfer is specified at 0.015 to 0.047 inches on the end of the bolt. This is equal to 0.5 to 1.5 of a thread pitch.

So, measuring full threads, the MIL-Spec can be 0.5 to 1.5 full threads exposed, for a 10-32 bolt.

For an AN6 (3/8 bolt) the chamfer is 0.063 to 0.031 inches, which is about 0.75 to 1.5 full threads.... similar results...

I wonder if the FAA just went with a "thread" definition to get around the chamfer bit of the bolt.

It seems like 1 full thread is good as an absolute minimum for the smaller size bolts we work with, but another 1/2 thread or so is better. The Mil-Spec is identical for both fibre and metallic lock nuts. Fibre ones are rated to 250F and metal to 450F.

Interesting hunting around the specifications.....
 
Went with next longest I have.

For the W422 tie down bracket, I took out AN3-12A and used AN3-14A (no AN3-13A) with an extra washer. Now I have 4 threads showing.

Thanks.
Wendell
 
As a DAR, I have inspected at least 2 RV's that were missing ALL of the nuts on the elevator bolts! Bother had been supposedly inspected by others, and a checklist was completed and signed off.
As for bolt head up and forward, that's a good rule 99% of the time. But there are cases where the bolt is actually spec'd to be in a different configuration, and there have been documented crashes because a mechanic defaulted to the "head up/foreward" rule. So do be careful and check for proper orientation and torq.

Vic
 
Limits on both ends...

WenEng said:
For the W422 tie down bracket, I took out AN3-12A and used AN3-14A (no AN3-13A) with an extra washer. Now I have 4 threads showing.

Thanks.
Wendell
Click to expand...

There is peril with too many threads showing also - if the nut runs out of threads, you will torque up the bolt, but the clamping force will not be proper, and may indeed be zero. Use the thin and thick washers to get at least 1 and no more than 3 or maybe 4 threads showing. I just tightened an AN stop nut onto an AN3 bolt, and there are only about 4 to 4.5 threads showing. I did not find an upper limit in AC43-13-1B, but a good rule is 1 to 3 threads showing.

I have found on initial inspections bolts that were indeed torqued, but could be spun by hand (too many threads showing).
 
Correct Length and Thread Protrusion

You need to get both right. On my RV6 I had to monkey with thick and thin washers at both ends of the bolts on the end of large aileron push/pull tubes. If the bolt thread extended too far it came close to the pivot support angles as it swung past them on the bell crank which could have restricted aileron travel.
Jim Sharkey
RV6 - Phase 1
 
jsharkey said:
You need to get both right. On my RV6 I had to monkey with thick and thin washers at both ends of the bolts on the end of large aileron push/pull tubes. If the bolt thread extended too far it came close to the pivot support angles as it swung past them on the bell crank which could have restricted aileron travel.
Jim Sharkey
RV6 - Phase 1
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I had exactly the same thing happen on my RV6 (a 1993 kit). I did not pick this up but the guy who inspected it before the first flight thankfully pointed it out. The threads protruding from the nut were only missing the bell crank support angles by a small amount. He rightfully told me that during aerobatics or if the bearing wore a little, the bolt end could jamb on the support angles:eek:
The only way I could get more clearance was to install the bolt the wrong way (head towards the ground). I also made a notation in the airframe log book about this orientation of the bolt.

Cheers
 
where does one find the torque settings for these bolts?

I haven't looked hard, but wondering where I would find that
 
Okay, I figured it would be bolt specific, I don't see them in the narrative or on the drawings, but like I said I haven't had to torque anything just yet, I'll look in section 5
 
For my 10, the plans section 1 through 5 are in a notebook format, 8 1/2 x 11", and the actual plans/drawings/blueprints are a totally separate set of pages.

On the various places where a torque value is needed, the plans say to look at the torque tables in sec 5, instead of just putting the needed info in the plans -------- kinda stupid way of doing things IMHO.

As I said before, the 7 may be different.....
 
Can't really put all standard information in the field

Mike S said:
For my 10, the plans section 1 through 5 are in a notebook format, 8 1/2 x 11", and the actual plans/drawings/blueprints are a totally separate set of pages.

On the various places where a torque value is needed, the plans say to look at the torque tables in sec 5, instead of just putting the needed info in the plans -------- kinda stupid way of doing things IMHO.

As I said before, the 7 may be different.....
Click to expand...

Can't really put all standard information on the field of the drawing for a couple of reasons. For one the field of the drawing would become so cluttered the illustration value would be lost, hence bill of materials tables and notes (both flagged and non flagged). For another the chance of omission and inconsistency would increase dramatically. Then change control would get caught up in minute recurring standard callouts and sensitivity to important unique design details would be reduced. When standard requirements apply to multiple views and identical hardware there would be some confusion about where the standard callouts should appear on the drawing. It is actually an efficient and smart to specify standard requirements like standard fastener torque requirements in one place separate from the field of the drawing.

Bob Axsom
 
Bob Axsom said:
Can't really put all standard information on the field of the drawing for a couple of reasons. For one the field of the drawing would become so cluttered
Click to expand...

Uh, no.

To say "Torque to 75 ftlb" is a lot less "clutter" than saying "For torque specifications, see the table in section 5"


Bob Axsom said:
It is actually an efficient and smart to specify standard requirements like standard fastener torque requirements in one place separate from the field of the drawing.

Bob Axsom
Click to expand...

Many times this is the best way, and usually in my experience, it has been done with a block at the bottom of each page.

In these plans, it was not even in the same set of paperwork, you had to go find the notebook with the details in it. A specification page included with the blueprints would make a lot of sense.
 
True confessions

I check the torque on all the bolts in the control system every condition inspection. A couple of years ago I was checking the bolt that connects the elevator bellcrank to the push tube that goes back to the elevator. It was loose, or so I thought. I kept tightening and it never tightened up. It was stripped.

I installed it originally, and checked the torque at least 6 times, so it must have been my bad, but I could not believe it. You just can't be too careful.

As far as torque being on the drawings, I think that is a waste of time and energy on the part of Van's. We use AN hardware, and the whole aviation industry knows what the torques are supposed to be. If it is not standard, then it goes on the drawing. It is our responisiblity as manufacturers of the aircraft to know what the requirements of all the fasteners are. If you went down this line of thinking you would also have to call out what the size of the shop head of the rivets should be on every drawing. Where do you stop as a designer? It could go on and on: Cotter key specs, AN tubing fittings torques, cherry max rivets, ... Just make a copy of the standard torque tables and tack them up on the wall so it is always available. And torque EVERY bolt.
 
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