Why we do Condition Inspections

Ironflight · #1 10-08-2006, 06:22 PM

Yesterday I finished up the firewall forward part of the Valkyrie's yearly Condition Inspection, and today was the day to finish the airframe. Even though it's not due for sign off until the end of the month, coming off last week's long trip, and preparing for next weekend's LOE, I decided that going over everything was a good idea anyway, and I might as well use the checklist and sign it off if appropriate.

The results? The Valkyrie was almost perfect! All the pieces are still there, no cracks on any of the control surfaces (a great way to inspect rivets so to get out your bottle of Wash-Wax-All and clean the whole plane. Takes an hour, and you touch all of the exterior surfaces. Everything works, everything got lubed, and she's ready to go.

But I said "almost" perfect, didn't I? Well, there was one fiber lock nut on a 3/16" bolt that had loosened up, if you can believe that. The fiber was completely disengaged, and the nut was spinable by hand. The only REAL problem with this is that it is on the bolt at the bottom of the pilot's stick that holds the elevator pushrod to the stick.....

Now I am SURE that nut was secured during my airworthiness inspection - four people had gone over the control system before the DAR,and my DAR was particularly thorough. Had I removed the stick since then? I can't remember doing that, and even when I do, I usually pull the pivot bolt and lay the stick down, still attached to the elevator pushrod. I distinctly remember working around the stick when I checked the torque on the landing gear bolts. As I found it today, there was no washer under the nut, and all I can think of is that the nut was grabbing on the control column surface, and slowly turning on the bolt. Hard to believe, but I guess I'll never know for sure.

Of course, I have now checked every single fiber locknut in the control system, by eye and with a torque wrench. The bottom line - that is why we do Condition Inspections! You only see this bolt if you dive head-first into the cockpit, and if you have a boot on (I don't), you won't see it even then. But you can reach down and feel it - which is something that I will probably reflexively do for some time to come!

Paul

az_gila · #2 10-08-2006, 06:58 PM

Paul... really good catch.

To help prevent these occurrences, I recommend at all of the projects I visit as TC, to use this cheap safety device

Only $1.75 from SkyGeek on this site....
It does help mark which nuts/fittings you have declared "done" during the assembly process....

gil in Tucson

AlexPeterson · #3 10-08-2006, 08:26 PM

Paul, that one will keep you awake a few nights....

Have fun at LOE, I've decided that I need to stay home this time.

Ironflight · #4 10-08-2006, 10:27 PM

Sorry you'll miss LOE Alex - I'm glad I got to look over your bird the other day - nice to see one with so many hours still looking like new! I did inspect my airbox top yesterday, and no cracks.....yet!

Paul

Mark Burns · #5 10-08-2006, 10:33 PM

When I was in A&P school (30 years ago!) I'm sure I learned that you are not supposed to use a self locking nut to fasten parts that can impart a rotary motion on the nut. I haven't worked as an A&P for over 15 years but I don't recall seeing any self locking nuts installed this (bad?) way on certified aircraft. It's too late tonight for me to look for the answer in 43.13 !

I think a person just needs to really look at what could happen and decide if the application really needs a castle nut and cotter pin.

I haven't tightened the nuts in the control column area on my 7A yet. I put it off a while back to decide if I wanted to do the castle nut and cotter pin thing. Leaning more that way now after Pauls post.

Thoughts anyone?

az_gila · #6 10-08-2006, 11:12 PM

Quote:

Originally Posted by Mark Burns

When I was in A&P school (30 years ago!) I'm sure I learned that you are not supposed to use a self locking nut to fasten parts that can impart a rotary motion on the nut. I haven't worked as an A&P for over 15 years but I don't recall seeing any self locking nuts installed this (bad?) way on certified aircraft. It's too late tonight for me to look for the answer in 43.13 !

I think a person just needs to really look at what could happen and decide if the application really needs a castle nut and cotter pin.

I haven't tightened the nuts in the control column area on my 7A yet. I put it off a while back to decide if I wanted to do the castle nut and cotter pin thing. Leaning more that way now after Pauls post.

Thoughts anyone?

Mark... it is acceptable if the bolt tightens down on the end of a Heim bearing.

The ball in the bearing is meant to rotate, not the bolt itself. This is standard aircraft practice. My Tiger does this in multiple locations.

I believe the joint Paul is talking about is a rod end bearing.... it is in the -6..

A few scenarios could cause Paul's problem
1. Nut not tightened
2. Bolt too short, and fibre locking was not engaged (1 thread showing, min.)
3. Lock on nut was shot... not common.
4. Bearing "ball" is way too tight, and the bolt rotating was the bearing... bad rod end bearing

Can anyone think of other cases?

If you are still concerned...

If you really want a belts and braces fix for these locations, Aircraft $pruce sells a castle nut that has a self-locking insert - expensive....

http://www.aircraftspruce.com/catalo...es/ms17825.php

If you are concerned, use these.

gil in Tucson

Ironflight · #7 10-08-2006, 11:35 PM

Good analysis Gil - exactly the things I was thinking of. 2 is not the case - the bolt is the right length with a new washer and nut. The bearing is free, so 4 is not the problem. I saved the nut on my workbench at the airport, and want to see if it will lock (I suspect that it will). Leaving only improper installation that wasn't caught, or something else....

I was also thinking about going back with a castle nut and cotter pin - I agree that for these installations, a lock nut meets "code" - but I don't see where a cottered castle nut wouldn't work as well. For now, I am going to do it as the plans show, and check it more often.

Paul

az_gila · #8 10-09-2006, 07:42 PM

Quote:

Originally Posted by Ironflight

Good analysis Gil -

...thanks....

I was also thinking about going back with a castle nut and cotter pin - I agree that for these installations, a lock nut meets "code" - but I don't see where a cottered castle nut wouldn't work as well. ............
Paul

Paul ... one minor hazard with the cotter pin/castle nut.

If it's installed so it's full torque is not met, there is a chance that the bolt will become "the bearing". If this happens, then the smaller diameter bolt will probably wear out faster than the larger ball in the rod end bearing....

Probably not likely, but it is possible....

gil in Tucson

DanH · #9 10-10-2006, 08:21 AM

I've been suggesting AN363 or MS21042 nuts for all the control pivots when I make TC calls. The locking friction is a good bit higher than with a fiberlock nut, and you don't need to buy new bolts or drill your kit-supplied bolts. Cheap upgrade.

Dan Horton

N62XS · #10 10-10-2006, 03:28 PM

Nylon locking castle nuts are an option. Expensive, but in critical areas, the best of both worlds.