Use of NDI on Cub frame

[TR Dunn]

Sharing some results reference my use of NDI, specifically ultrasound, on our PA-18A frame. I recently removed the wings for re-cover, and always desired the L-21 style “glass”, so I decided it’s a good time to pull that trigger while the wings are off. I purchased the Flanagan STC from Cub Crafters and removed the fuselage fabric. External corrosion is easy to locate, my objective of NDI is to locate any internal corrosion in the tubes, again, this is a ‘54 A model that after departing Lockhaven, went to work dusting and spraying and now spends summers on floats. It retains the original top deck hopper frame support tubes with all their original penetrations possibly affording opportunity for liquid intrusion over the decades.

Starting at the rudder post, I took readings at every cluster followed by readings along intermediate tube locations. My objective is to measure enough locations to allow a numerical data driven assessment of the current frame integrity. If a measurement causes suspicion, more invasive testing is warranted, with a final solution of tube R&R. UT is working flawlessly, (see attached photos).

UT is very accurate, however, very time consuming and requires proper technique manipulating the probe to obtain valid results. The ultrasonic thickness tester I am using is a certified and calibrated instrument and I also have oversight by a Level 3 NDI expert who performs/supervises NDI nearly every working day.

I believe the ability to place a number on the actual wall thickness has its merits. UT provides me the confidence of tube integrity very specifically by providing numerical data which meets my objective of verifying the frame structural integrity. When completed, our “A” frame will have a map of tube wall thickness at each tested location, data that I feel is worth my effort.

This process works for me as we already employ UT, as well as other NDI procedures in our shop for corrosion control inspections. Not looking for a debate, just sharing our use of technology which helps us keep these legacy airframes airworthy.

TR

 

[BC12D-4-85]

When restoring my former PA-11 I applied shop air ~ 90 psi plus to the lower rear longeron drain holes. There was an air leak at the front motor mount which was rewelded. Then a piece of vertical tubing that supported the left side windows departed and shot across the hangar. But this NDI looks like a far better way to inspect beyond our crude stress test.

Gary

 

[Steve Pierce]

TR Dunn, have you found any issues in your fuselage with the UT?

 

[AZinAK]

Not that this isn't good or worth the time/effort, but I dream of replacing my 77+ year old airframe for new metal and upgrades. Not sure how I'll be able to do so unless I go experimental since no one produces the -12 airframe anymore.

 

[skywagon8a]

Not that this isn't good or worth the time/effort, but I dream of replacing my 77+ year old airframe for new metal and upgrades. Not sure how I'll be able to do so unless I go experimental since no one produces the -12 airframe anymore.

Just keep the best piece of your old fuselage and splice in the rest new. Call it a major repair. Then replace the last old piece.

 

[cubdriver2]

Just keep the best piece of your old fuselage and splice in the rest new. Call it a major repair. Then replace the last old piece.

A local IA bought a Cub project to rebuild. It was straight but very rusty. He built a jig from the old frame and built a new fuselage.
Log book entry " replaced all rusty tubing "

Glenn

 

[TR Dunn]

TR Dunn, have you found any issues in your fuselage with the UT?

Steve, I have 137 locations (data points), I have yet to transfer this into tabular form. Going into this I was confident UT would immediately identify tubular sections requiring R&R, I have not found that to be the case thus far. I almost prefer I did as others may use this example to validate their hypothesis that tubular corrosion is non-existent internally, a potentially dangerous assumption.

The PA-18 has near non-existent data employing NDI on the tubular frame that I am aware. Therefore, I cite the DHC-2 as an example; we are required to UT the forward tubular frame every 5 years, along with rudder pedal torque tubes and forward fuselage struts among other NDI procedures on components now mandated per PSM 1-2-5 which has recently been incorporated in its entirety into an AD. Internal corrosion has been discovered by UT with no external indicators, hence the mandate. But wait, that’s not a Cub; Any structure comprised of similar materials, fabrication methods and service exposure are subject to possible internal degradation due to corrosion (the A model perhaps more so due to hopper frame design).

My objective is to simply share this procedure to this community as a possible diagnostic method even though not currently mandated…yet (ref DHC-2 history). I understand NDI being a very specific field, may not be accessible to some. I am fortunate to have access to various NDI methods and use it to support the structural integrity of various components from float attach fittings, flight control rods, struts, and tubular structure wall thickness.

In this case, UT provides near indisputable scientific data as opposed to manual methods such as “pick and hammer” which have too many variables (lack of consistency in applied pressures for one) and when degradation is discovered using this non-precise method, the tube member most likely has long failed to meet design strength criterion.

I observe an excitement in this community for technological advances in components such as landing gear, flaps or tires. My sub-objective is to encourage the same for using technology to enhance maintenance and ultimately, safety. I will not dispute my philosophy may be called extreme by some, however, when it comes to the safety of the “butts in the seats” there is no substitute for the best available procedure to meet the inspection objective.

A long answer to your question, however, I feel a simple “not thus far” would be a disservice to you and this community.

TR

 

[Greg Harvey]

Are you talking ultrasonic examination or eddy current inspection
MP for surface cracks I believe

 

[Steve Pierce]

Not that this isn't good or worth the time/effort, but I dream of replacing my 77+ year old airframe for new metal and upgrades. Not sure how I'll be able to do so unless I go experimental since no one produces the -12 airframe anymore.

Is Univair not building PA12 frames any longer?

 

[Steve Pierce]

Steve, I have 137 locations (data points), I have yet to transfer this into tabular form. Going into this I was confident UT would immediately identify tubular sections requiring R&R, I have not found that to be the case thus far. I almost prefer I did as others may use this example to validate their hypothesis that tubular corrosion is non-existent internally, a potentially dangerous assumption.

The PA-18 has near non-existent data employing NDI on the tubular frame that I am aware. Therefore, I cite the DHC-2 as an example; we are required to UT the forward tubular frame every 5 years, along with rudder pedal torque tubes and forward fuselage struts among other NDI procedures on components now mandated per PSM 1-2-5 which has recently been incorporated in its entirety into an AD. Internal corrosion has been discovered by UT with no external indicators, hence the mandate. But wait, that’s not a Cub; Any structure comprised of similar materials, fabrication methods and service exposure are subject to possible internal degradation due to corrosion (the A model perhaps more so due to hopper frame design).

My objective is to simply share this procedure to this community as a possible diagnostic method even though not currently mandated…yet (ref DHC-2 history). I understand NDI being a very specific field, may not be accessible to some. I am fortunate to have access to various NDI methods and use it to support the structural integrity of various components from float attach fittings, flight control rods, struts, and tubular structure wall thickness.

In this case, UT provides near indisputable scientific data as opposed to manual methods such as “pick and hammer” which have too many variables (lack of consistency in applied pressures for one) and when degradation is discovered using this non-precise method, the tube member most likely has long failed to meet design strength criterion.

I observe an excitement in this community for technological advances in components such as landing gear, flaps or tires. My sub-objective is to encourage the same for using technology to enhance maintenance and ultimately, safety. I will not dispute my philosophy may be called extreme by some, however, when it comes to the safety of the “butts in the seats” there is no substitute for the best available procedure to meet the inspection objective.

A long answer to your question, however, I feel a simple “not thus far” would be a disservice to you and this community.

TR

Thanks, I was curious. I have worked on a lot of steel tube frames and rebuilt a lot of Piper fuselages. I have found internal corrosion. Usually obvious when the fabric is removed, sometimes when a crack is found and more so when sand blasted. In every instance I have found a hole someone drilled or an unfinished weld that allowed the moisture in. Many, many years ago while sand blasting I found the intersection of my 1949 PA16 lower longerons were severely pitted externally. I was presently surprised when I cut those sections of tubes out and the inside was still oily. Had a diagonal tube in a Stearman tail cracked. When I got into repair I found internal corrosion and then a small hole drilled just above the tube. A buddy had the main gear carry-thru tube crack from internal corrosion on his PA18 and when it was uncovered we found a series of holes that were drilled in the tube when it was used to carry an aerial sign. We don't know what we can't see and I use every effort to identify an issue so am very interested in what you find.

 

[AZinAK]

Is Univair not building PA12 frames any longer?

My understanding was they weren't but maybe they are. If so, do you know if they will build to suit for optional mods/STCs?

 

[QcHunt]

My understanding was they weren't but maybe they are. If so, do you know if they will build to suit for optional mods/STCs?

Univair still produce pa-12 fuselage.
Check univair website, all STC can be added at order process

 

[Bret Corkill]

Is Univair not building PA12 frames any longer?

Talk to them today and yes they do.

 

[TR Dunn]

To bring this thread back on point, the objective of employing NDI (UT specifically in this case) is to identify areas of degradation that would otherwise go undetected through the typical DVI (detailed visual inspection). In my case, there are no A model replacement frames being made that I am aware. Even if there were, I would continue on my present path. I see no need to replace a frame when I possess the authority and skill to appropriately repair and tailor the structure to my specific requirements and tastes.
Just for grins, I did some "soundings" on a set of reject tailfeathers from my last J-3 restoration. UT identified internal wall thickness loss in the tubular structure otherwise undetected through an external DVI.

TR

 

[s.harstad]

Interesting topic and good research. Can you share what equipment (brand name, type) you used, would be the best suited buying for a cub frame ? Would you use 2 different tools for seeing into a tube or a thicker weld , or just different probes ? Are there any way to «see» any part of the tube still fabric covered and still with paint on (by tilting probes or instrument adjustment) or can you only «see» the area with removed paint ? Is there any chance you could see grain difference near weld (heat affected zone) or metal type difference (tube/weld) ?

Does anyone have experience on tap testing with light object for sound change listening (not making dents) on covered frame ? Maybe sound could be recorded and artificial intelligence could help sound analysis. I guess someone could even make a vibrating calibrated NDT tap tool .

 

[MT12]

My understanding was they weren't but maybe they are. If so, do you know if they will build to suit for optional mods/STCs?

I had Univair build a new 12 fuselage about 7 years ago. They were great to work with. They included all of my logbook documented STCs without question and added all the new ones I desired like float fitting and gross weight increase. Just be sure to make your desires known when you order, once They get started They move right along.

 

[akmarty]

TR,
Interesting method. Did you validate any of your findings with destructive examination? What degrees of loss were you able to see with the UT?

 

[TR Dunn]

The certified tester I used reads to the thousandths of an inch. After testing a set of original forward fuselage struts (doorpost struts) off our Beaver, the wall loss was up to .004. I cut them open for inspection as no matter the reading I was installing new Kenmore struts which are near double the wall thickness of original at .065. The destructive examination was just to see the internals; the tester is calibrated before each use with a base material thickness step wedge.

I expected to have locations on our '54 A model frame that require R&R, however, I have not found that to be the case thanks to UT. My locations varied no more than .001 from factory dimensions. I am in the process of installing the Flanagan STC with full "glass" which is what prompted this in-depth evaluation of our frame. It was a long tedious process to map the frame; however, my time is "free". I am also a history buff, therefore keeping as much original parts as possible is important to me.

TR

 

[daedgerton]

The certified tester I used reads to the thousandths of an inch. After testing a set of original forward fuselage struts (doorpost struts) off our Beaver, the wall loss was up to .004. I cut them open for inspection as no matter the reading I was installing new Kenmore struts which are near double the wall thickness of original at .065. The destructive examination was just to see the internals; the tester is calibrated before each use with a base material thickness step wedge.

I expected to have locations on our '54 A model frame that require R&R, however, I have not found that to be the case thanks to UT. My locations varied no more than .001 from factory dimensions. I am in the process of installing the Flanagan STC with full "glass" which is what prompted this in-depth evaluation of our frame. It was a long tedious process to map the frame; however, my time is "free". I am also a history buff, therefore keeping as much original parts as possible is important to me.

TR

Would love to see pics of your Flannagan STC as it comes to fruition. I have an A model that eventually needs rebuild and would like to incorporate this. Already have the STC from CC so part of the way there!

 

[TR Dunn]

Would love to see pics of your Flannagan STC as it comes to fruition. I have an A model that eventually needs rebuild and would like to incorporate this. Already have the STC from CC so part of the way there!

Will do. I did same, called Vera at CC last year or could even be two years ago to get the STC and drawings. This project is nearing the "back burner of the stove". Presently bogged down in what I call the "prep or teardown" stage, removal, cleaning and testing. I have to finish up a current project before I can dedicate the time it requires to turn the corner to fabrication and buildup of the Flanagan. I am already wrestling with "mission creep" just looking at it. The "while I'm in there" I should add this or that conundrum. While I'm not a weight fanatic, I would like to keep from adding non-essential weight. Some may claim an oxymoron as one could argue the Flanagan adds "non-essential" weight. I disagree as my mission for this aircraft has changed resulting in me residing in the back seat instructing mostly on floats. I believe the added visibility is a safety enhancement both on the water and in the air. Been running this old "A" with the Wip 2K, PennYan O-360 and Airglas belly tank for over 20 years, more glass, and I'll be happy.

TR

 

[KevinJ]

Flanagan A models just look right. I think they are the neatest profile of all -18s and you’re right, it’s hard for someone to sneak up on your tail with that glass.