FAA Issues SAIB for MLG Tie Struts

[barnstmr]

Re: FAA Issues SAIB for MLG Tie Struts

RobertP
I am not sure what you are getting at. All I am saying is that at the present time, TMc has basically two options to legally obtain FAA approval for his repair. Not magnifying anything. Yes this is a rather simple repair, but requires a "process" for approval. It is our government in its glory!

[TMc]

Re: FAA Issues SAIB for MLG Tie Struts

Barber; I understand the field approval process.
The reason I performed the repair is I never thought I deviated enough to need to field approve it. It is such a simple repair, I was planning on doing a 337 using AC43.13 as my approved data. It wasn't until I brought my finished repair and my 337 to my IA to sign off when he thought I needed to do a field approval.

[Guest]

Re: FAA Issues SAIB for MLG Tie Struts

Hi TMc,

Your splice looks a lot like FIGURE 4-44. except that you substituted a larger tube (almost never a bad thing).


I am not sure what the real beef is with the welded fitting (bolt) end. Seems like that if you duplicated what had been there then you are ok.

Saib's have little to no bearing here, they are not in PART 39 as I recall. Tell me if I am wrong.

The top of page 7 of FAA-G-8082-11A says (as do the prior versions);

Minor deviation from approved data is permissible if the change is one that could be approved as a minor alteration when considered by itself. Be sure to list the deviations on FAA Form 337 and make an entry in the maintenance record when completing the aircraft records. When in doubt, contact the local ASI who may decide the change is not minor and would need specific approval or an amendment of the original approval.

Seems like you have a combination of FIGURE 4-40 oversize tube splice and 4-44 cabane splice with rossette welds.

Talk with the FAA guy, this part should be easy to get approved in the feild on your own. OR determine if these are minor deviation that the IA can simply sign off. Sounds like that has smaller chance.

I've gotten similar hybrid approvals.

Cheer up buck-a-roo I think it is not so bleak as it may be sounding like.

Was that TIG or gas welding? Looks nice.

Dave

[Guest]

Re: why the lower bolt?

When I recovered my gear a couple of years ago I was curious about the design of the tie strut.
Why was it welded at the top and bolted at the bottom?
It can't rotate around the bolt (being welded at the top).

What was the engineering idea behind the design of weld at the top and bolt at the bottom?
Why not weld both ends, or bolt both ends?

Always wanting to learn a little bit more of that engineering stuff...
Thanks in advance for any enlightenment.
Skip
BC12-D-4-85, N34237, SN 7700

Hey Skip,

I had to chuckle when read the blue text, not at you though.

Taylor designed a door on his airplane that's bigger than the doorway.

I am not surprised that there is a tie strut that is welded on one end and bolted on the other.

He was super at aerodynamics for sure!

Dave

[flyguy]

Re: FAA Issues SAIB for MLG Tie Struts

Dave,
Those are my favorite little "make it stronger" tubes. Give the little guys a break. Chuckle.

[RobertP]

Re: FAA Issues SAIB for MLG Tie Struts

Terry,

Not being critical in anyway and I was just making a general observation that we needed to make sure it didn't go the same way as the lift Struts which seemed to cause aggravation for most! You guys obviously have your fingers on the pulse which is the main thing I guess.

[Dano"T"]

Re: FAA Issues SAIB for MLG Tie Struts

My plane had the larger streamlined tubing welded right over the top of the original tie strut tubing and a 337 was done (sometime in the 60's) My right tie strut sheared off on each end at the welds...it appeared the stresses were all concentrated in the area of those welds where the smaller tubing and the new larger tubing meet. The idea, I think was to make it stronger....I still have the piece somewhere...I'll take a picture and show you guys. Once the tie strut broke, of course the gear collapsed....no other damage occurred (lucky) slid to a stop on the grass, wiped the grass stain off the wing tip and put a strap between the gear legs and pushed it to the hangar.

[Guest]

Re: FAA Issues SAIB for MLG Tie Struts

You are thinking that the larger tubing actually weakened it at the weld joint, did I get that correct?

That would be interesting to see the picture. I hope that you can find it.

Dave

[Hank Jarrett]

Re: FAA Issues SAIB for MLG Tie Struts

Making one part of an assembly stronger usually DOES weaken the whole assembly. It creates stress concentrations that push the loads to the weakest area. You will usually see that when you put a stronger or bigger component in it isn't THAT component that fails, it is right next to it.
Hank

[barnstmr]

Re: FAA Issues SAIB for MLG Tie Struts

Based on the following discussion, I suggest you consider a 3rd option.

3. Cut out the repair and re-do the repair with same wall thickness as the original base material.

Under that scenario, you can use the AC 23-27 (see attached) as APPROVED DATA as long as you follow AC 43.13-1B to design the repair.

HERE's Why...

FAA advisory circular 23-27 is the current FAA guidance on material substitutions for Vintage Airplanes. Appendix 2 has some detailed discussion on this very issue. Note that is says... "...any change to material gauge is beyond the scope of this AC." In other words, the FAA is saying is... Go get a blessing from a structures specialist (DER).

I am not a specialist in structures, nor am I authorized by the FAA as a structures DER. But I have worked with structures enough to understand the issues that the FAA is concerned about.

Let's look at DanoT's example. I have not seen the details of it, but Dano T's experience falls under the category of changed "aeroelastic properties".. in other words, the stiffness of the tube was not consistent due to a sudden step-up in wall thickness... this resulted in a concentration of stress at this location. To counter-act this problem in repairs, it is common practice to design seams with a acute "scarf" angle. In blessing AC 43.13, the FAA already determined that a repair of this design will be acceptable as long as the same wall thickness is used. This helps to distribute the stresses so they are no longer concentrated at a single point. DanoT's example shows that this scarf design doesn't necessarily work when you step-up the wall thickness.

I notice that TMc has done this. It seems there would be no problem/no FAA concern if he had used the same wall thickness as the base material. But since it is thicker than the base material, and there is no clear FAA guidance on this, the FAA will ask "How did this repair affect the aeroelastic properties (stiffness) of the tube?"

So back to my previous post, the FAA will basically give you the two options I outlined before. OR... YOU MIGHT SAVE A LOT OF HASSLE by doing the repair over again with the thinner wall tube.

[barnstmr]

Re: FAA Issues SAIB for MLG Tie Struts

Again... I am not trying to magnify this issue out of proportion. I am simply trying to explain to the tribe the logic that our friendly FAA uses on these things.

[Guest]

Re: FAA Issues SAIB for MLG Tie Struts

Hi Terry,

I can't find where TMc had a thicker wall. Where did you see that? I missed it.

Also ac43.13-1B is ripe with statements about using tube at least as thick as the original tube. I don't buy the idea that thicker is worse.

I do buy the idea that discontinuities in crossections are very bad. Which is what I think that you are really getting at. Maybe I am correct about that?

I really want to see DanoT's repair. We don't know that it even had a long tapered joint. I am anticipating a huge discontinuity there and maybe not scarf joint at all. We will see.

After typing the line above I went to look at TMc's joint again.

And I just realized after a closer look at TMc's joint that it may not be a long 30 degree joint. I did not notice that before. Maybe cutting out and redoing (your #3) is best if it is infact not a 30 degree taper.

I have cut some out in the past also.

Dave

[barnstmr]

Re: FAA Issues SAIB for MLG Tie Struts

TMc... I got a response from my DER friend. Here is his response...

Terry,
Not a problem. I will cut him a deal for $250, for a quick write-up and 8110-3. I need dims, thicknesses, and copy of his 337 write-up, aircraft info (model, S/N, N#). Who is his FSDO? Which office?
Rocky

If you want to go this route, I will be glad to facilitate. No charge on my part. You can email me at [email protected].

[Guest]

Re: FAA Issues SAIB for MLG Tie Struts

Based on the following discussion, I suggest you consider a 3rd option.

3. Cut out the repair and re-do the repair with same wall thickness as the original base material.

Under that scenario, you can use the AC 23-27 (see attached) as APPROVED DATA as long as you follow AC 43.13-1B to design the repair.

HERE's Why...

FAA advisory circular 23-27 is the current FAA guidance on material substitutions for Vintage Airplanes. Appendix 2 has some detailed discussion on this very issue. Note that is says... "...any change to material gauge is beyond the scope of this AC." In other words, the FAA is saying is... Go get a blessing from a structures specialist (DER).

I am not a specialist in structures, nor am I authorized by the FAA as a structures DER. But I have worked with structures enough to understand the issues that the FAA is concerned about.

Let's look at DanoT's example. I have not seen the details of it, but Dano T's experience falls under the category of changed "aeroelastic properties".. in other words, the stiffness of the tube was not consistent due to a sudden step-up in wall thickness... this resulted in a concentration of stress at this location. To counter-act this problem in repairs, it is common practice to design seams with a acute "scarf" angle. In blessing AC 43.13, the FAA already determined that a repair of this design will be acceptable as long as the same wall thickness is used. This helps to distribute the stresses so they are no longer concentrated at a single point. DanoT's example shows that this scarf design doesn't necessarily work when you step-up the wall thickness.

I notice that TMc has done this. It seems there would be no problem/no FAA concern if he had used the same wall thickness as the base material. But since it is thicker than the base material, and there is no clear FAA guidance on this, the FAA will ask "How did this repair affect the aeroelastic properties (stiffness) of the tube?"

So back to my previous post, the FAA will basically give you the two options I outlined before. OR... YOU MIGHT SAVE A LOT OF HASSLE by doing the repair over again with the thinner wall tube.

Hi Terry,

I think that taken in context its not about this issue of making tubes thicker for repair but rather of making them thinner walled due to the added strength of 4130.

Dave

[Dano"T"]

Re: FAA Issues SAIB for MLG Tie Struts

I'll explain further. The larger material was welded in almost the full length of the tie strut...there was about 2" of the original material left on each end and it broke at both welds without any other distortion to the strut. It could very well be that the welds were bad...they looked nice, but the metal actual looked brittle of crystalized where it broke. imagine my surprise when I touched down...hit a little hump in the grass strip and down she went skidding to a stop. I do believe Hank is correct in my case..the loads concentrated at those locations and the rest was history.