Possible Rudder Airworthiness Directive

I was hoping that when mine was restored in 1985 after it was flipped on it's back they replaced the rudder. No such luck. In the logs it says they repaired it by TIG welding with 4130 wire. I am going to assume then my rudder is the original 1025 but to make sure i could do the acid test. Any suggestions for a source of Nitric acid? The source listed in the Piper Service Bulletin is about $200 before the hazmat fees and shipping. What kind of place would have that locally? You only need a few drops.
 
Hi Steve, thanks for posting this 4130 liner tube experiment with photos, we were thinking of trying the same thing, and were uncertain how much distortion there might be in to the original tube where ribs etc were welded on back in the stone age.
Bearing in mind this is just a concept for AMOC, allow me to play devil's advocate:
Do you think there is a risk the 4130 liner tube could stress the original tube, passing thru the welded areas as it is tapped into place (slight interference fit in your case, but there were many thousands of those assemblies fabricated by a lot of people over a lot of years, so maybe minor variations in technique and quality.)
How much does the 4130 liner tube weigh?
Would it require a W&B update in the logbook?
I can't recall offhand what the two bolt-holes at bottom of rudder tube are for - would you drill thru the 4130 liner tube and use those bolts to secure liner in original rudder tube?


Best,
GG
 
The bolt holes are for the rudder horn that move the tailwheel so not everyone will have those. Going from the bottom would probably require you to measure and make sure the tube went all the way to the top of the rudder post before securing it. 0.035 3/4" 4130 weighs about 4 oz per ft and 0.049 about 6 oz per ft so around 1 to 1.5 added pounds at the tail and it would require a new W&B plus a 337 as a major repair affecting a control surface. How that would would affect any one persons airplane is hard to guess. If they had a O-320 up front or some other added weight it might not matter but if they struggled with an aft CG then they could have a problem.
 
I wonder about using a shorter piece of tube held in place with structural adhesive. Thinking out loud cause I’m still trying to figure out how to apply it up where it needs to be.
 
I think the internal tube would only need to go top down to just past or right at the bottom edge of the upper hinge as that is the location of the maximum bending load.
 
I think a foot or so would do the job. According to my Aircraft Spruce catalogue 3/4" x .035" wall weighs .2673 lbs per foot. I thought of epoxy and think it would work but nothing in the 43.13 about it so I started thinking maybe a couple of Cherry rivets
 
I too was thinking Cherry Max rivets. All you need is to keep it from sliding. I think it’s totally in a bending mode. Great minds think a like.
 
And as far as a couple of drops of HNO3, you might check with a local high school chem department. Failing that, any labs around your area? I used to use Nitric for sterilizing/cleaning genetic lab test tubes in 1968.

Just keep in mind that stuff is dangerous. I still have a small pockmark on one cheek where a very tiny drop splashed on me -- someone had put a rubber-coated steel test tube rack in my supply and the rubber was nicked. Only took about 30 seconds to dissolve --violently-- that whole rack. It was under a hood and I had big rubber gloves and goggles on, but that one tiny drop managed to land on my cheek. A scene I've never forgotten.
 
I think a foot or so would do the job. According to my Aircraft Spruce catalogue 3/4" x .035" wall weighs .2673 lbs per foot. I thought of epoxy and think it would work but nothing in the 43.13 about it so I started thinking maybe a couple of Cherry rivets

Why wouldn't you slide a piece of 4130 tubing into the top of the rudder down a couple of inches past the top hinge and rosette weld it into place. You could also have it flush with the rudder post on the top and weld them together on the top to prevent it from moving. This wouldn't add much weight. Still needs a 337 sign off and weight and balance update.

Juergen
Pacer N3342Z
 
The top of the rudder is formed to meet the 3/8" tube that shapes the rudder. It could be done but a lot more work. I think a 1/4 lb piece of tubing slid up from the bottom and Cherry riveted to the base tube would be the easiest fix on a covered rudder. Could also use rosette welds and dollar patches if you were real careful with the heat when TIG welding the rosettes.
PXL_20230314_130707299.jpg
PXL_20230314_130717420.jpg
 
OK, that makes sense. I covered my rudder 10 years ago but didn't remember how it looks on the top. I will probable just recover the rudder. It is not that much work compared to just cutting pieces of the fabric of and then trying to match and blend in the paint.

Juergen
Pacer N3342Z
 
You can also "Spark test" to see if it is 4130 or 1020/1025. Find a spot where you can hit it on a grinder and observe the spark pattern to see what material you have, no need for the acid test.
 
You can also "Spark test" to see if it is 4130 or 1020/1025. Find a spot where you can hit it on a grinder and observe the spark pattern to see what material you have, no need for the acid test.

That's pretty interesting.
Do you have a link to some supporting info about that please?
To use that as a AMOC would require some substantiating info and objective observation.
 
I think a foot or so would do the job. According to my Aircraft Spruce catalogue 3/4" x .035" wall weighs .2673 lbs per foot. I thought of epoxy and think it would work but nothing in the 43.13 about it so I started thinking maybe a couple of Cherry rivets

I'm enjoying this brainstorming thread.
This was my thought as well, just a ~12" length overlapping the failure point.
Define a clean & prep method for the inside of the rudder tube (steel bottle brush and cleaning solution safe for fabric/glue.)
Maybe an inspection method using a USB borescope on a cellphone?
Calculate the effective length of an installation tool, say a foot longer than needed to push liner into position overlapping the failure point of the rudder; a 2" long roll pin could act as a stop for consistent installation depth, and the excess length makes a good handle to pull it back out.
EDIT: Since approved adhesives could be problematic (though the Clickbond used for the aileron hinge STC might work), the cherry rivet solution sounds reasonable.

OK, we have solved the problem, let's go flying!

Cheers,
GG
 
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Thanks Steve for your research, my logbooks show mine was replaced in 1999 due to corrosion with a Univair rudder, I think its the U40622-007 as it has the top tube and nav light bezel. I will be sure to comment when the NPRM or whatever comes out.
 
That's pretty interesting.
Do you have a link to some supporting info about that please?
To use that as a AMOC would require some substantiating info and objective observation.

I'll have to look further, but the one on my desk is The Welders Bible, Don Geary (c) 1980 Tab Books Page 103. Another is Aircraft Welding by Col Rolland H. Drake (c) 1947 McMillan & Co. Chapter V Principal Metals Used in Aircraft Structures and their Identification. About 7 pages that cover spark testing various alloy groups. I'm sure I have several other references as well.

For low carbon, the sparks are long shafts with forks. High carbon steel has significantly more sparks with many sprigs and sparklers. 1018 to 1020 steel ha about .18-.20 carbon and is low carbon. 4130 has between .28 - .30 carbon and is a medium carbon alloy. If you have known sample of each, spark test them, then take the unknown (the rudder) and spark test it and compare which it is closest to. Pretty easy when you see the known samples and compare.

Of course there is always this Spark testing - Wikipedia
 
Has anyone identified a viable inspection tool and methodology? I just tried to use a cheap Chinese endoscope and had no luck - too low resolution, and it doesn’t look to the side well enough. It sure would be nice if there was someway to inspect without cutting away the fabric.
 
Has anyone identified a viable inspection tool and methodology? I just tried to use a cheap Chinese endoscope and had no luck - too low resolution, and it doesn’t look to the side well enough. It sure would be nice if there was someway to inspect without cutting away the fabric.

Real simple, push on the top of the rudder, if it folds over, time for a new rudder. If it doesnt, press on!


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Cheap effective methodology: grab the top of the rudder, shake it side to side really hard. If it doesn’t flop over from the top hinge upward you are good to go. That’s what I am doing as my alternate means of determining it’s good and part of my preflight.

DGA beat me to the solution while I was typing.
 
I added a rudder top wiggle to my preflight as well. Good call.
 
Counterpoint, I inspect quite a few things on preflight inspection that might not be on the manufacturer's list. But they all relate to things that I have uncovered as issues through the years while flying, so now I check them before every flight. Doesn't take extra time, makes it much less likely that there is a surprise issue in something that has previously been working.

Also counterpoint, I have been wiggling the top of the rudder as part of my preflight inspection for years now in any airplane where I can reach that high. Which is quite a few. Not sure why once a year would be a better plan than knowing the specific condition at the time of each flight. Seems like once a year is sufficient for an annual inspection, but for something this quick and easy, why not do it on preflight?
 
The rudder is a pretty important piece of equipment. I check the tail brace wires, elevator, tailwheel, etc. Why not add the rudder for 5 seconds or less. The chance of it breaking is not worth panicking about but worth avoiding. IF it were to break in flight I'm sure it would be a real pucker moment.
 
How many aircraft of all types have had the beacon or nav light on the rudder top? 10,000 or so? How many aircraft have had rudder issues? It is a pretty small percentage even if you consider the different metals and construction in many of the types. Is it something to be aware of? Of course, but it is not something to get your panties in a wad over. When the rudder is recovered, inspect the suspect area for defects and press on if it is good and give it a good look at each 100 hour or annual. How do you know if putting a reinforcing pipe inside the rudder spar will not move the stress concentration to a more critical spot? If you have an aircraft that you can reach the top of the rudder easily during preflight, give it a wiggle if it makes you feel better. Few of us can reach the top of the rudder on a non-converted Tri-Pacer during preflight. I guess that you could drag the refueling ladder around with you. There are a lot more critical items, like the exhaust system, that need to be carefully inspected during preflight, than the rudder. If it is keeping you up at night, spend $5,000 or so and buy a new rudder, have it covered and press on.
Just saying,
N2709P
 
Just checked my rudder..looks and feels in great shape..one other thing I do is check that the fabric is in good shape at the top of the windshield every time I fuel up.
 
When the rudder replacement becomes mandatory….I’ll have to add the stud to the top of a new rudder…I like my white Redbaron strobe. I don’t like that I’ll have to replace the rudder after restoration. After sandblast, paint, cover…paint….I irrigated the open end of rudder tube with linseed oil…. Then because the rudder horn had a bit of play I use epoxy take out the slop. I can’t imagine my rudder ever failing or getting bent unless I do something stupid.


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If the rudder hinge tube could be sleeved, it might make sense to taper the lower end to transition the stress from the sleeve to the original tube, rather than have a stress concentration where the thickness changes at one point. Kind of like this:Piper Rudder Sleeve.jpg
 
But is there any stress in that area? I understand the reasoning but will make it harder to install the tube. Could make a driver with the same shape.
 
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