KF IV Support Tube Break

[P Morel]

Had a great day flying on Saturday. Made it to a couple lunch fly-ins and then back home. Taxi back to my hanger on turf and just before pulling in, I heard a loud "POP". I stopped but didn't see or feel anything unusual. I taxied another 30' and parked on my pad in front of my hanger. As I opened the door, it seemed to scrape on the bottom like it was mis-aligned. I looked at the opening and found my fabric on the frame under the door wrinkled. I removed my seat and found the angle support tube broken. This was a clean break and not on the weld.

My Kitfox IV Speedster is a taildragger with just over 700 hours. Metal fatigue? Has anyone with a IV had any type of tube failure? Especially in this area?

The repair will begin this week but the big question is what caused this failure and what kind of stress is placed on this area that would cause separation, especially in this area of the fuselage? If taxi on turf might add to the stress of an airframe, I would think any type of structural failure would be somewhere else on the airframe.

Another question, are the angle support tubes the same gauge metal as the rest of the airframe?

Any suggestions?

[avidflyer]

Looks like a lot of wrinkle for just the one tube getting a bit longer. Are you sure nothing else is bent? Is the cracking paint recent? Jim Chuk

[P Morel]

Looks like a lot of wrinkle for just the one tube getting a bit longer. Are you sure nothing else is bent? Is the cracking paint recent? Jim Chuk

The crack in the paint is due to the wrinkle in the fabric when it broke. The picture of the painted side was taken prior to jacking the fuselage and getting the plane off the gear. The gap in the break was probably 1/2" prior the placing the plane on a jack stand. Once on the jack stand, the gap closed to what is now about a 1/8" gap which is depicted by the other two pictures.

So to answer your question, I'm not quite sure at this time if anything else was bent but a full inspection will be done.

[jrevens]

Hi Paul,

First of all, I'm sure sorry that happened! Just based on the pictures alone, the break looks so clean & straight that I would almost wonder if it had been scored in that area during construction, like perhaps from a tubing cutter which was used in the wrong place and then relocated to make a cut (or the tubing had been scored previously & wasn't noticed when that piece was chosen). I don't know how they cut that tubing, but if it's relatively thin wall a cutter may have been used before shaping the cut for final fit. It's probably unlikely, but you never know. I wish I was close so I could examine it carefully.

[P Morel]

the break looks so clean & straight that I would almost wonder if it had been scored in that area during construction, like perhaps from a tubing cutter which was used in the wrong place and then relocated to make a cut (or the tubing had been scored previously & wasn't noticed when that piece was chosen).

I feel the same way but I won't know how thin the was is until I remove it. My welder friend is an A&P/IA that is also a master welder at Delta that can work miracles with metal. We'll evaluate later this week when he can schedule time. This brings up safety concerns with me at this time and currently don't have any answers.

[kitfoxnick]

I'm at a loss to explain it, other than what Jrevens said. I can't imagine a tube breaking in tension like that if it wasn't all ready scored. I'm no engineer but I can't see that tube being in that much tension to fail like that, without some sort of other weakness in the tube. So far I have helped repair three seat truss failures all from operator error.
I have never seen that tube deform, and it looks like there is no other tubes deformed nearby it. I would expect to see a jagged tear and damage to tubing surrounding it if it failed from being over stressed.
I'm interested to hear what your welder friend has to say.

[HighWing]

Sorry here too. My guess is that the tubing is .049" wall thickness.

I have see similar damage after hard landings. As I understand it, one problem with typical welding techniques is the concentrated heat that will often result in hardening of the metal near the weld.

From "Bearhawken"
"Since the transition temperature required for hardening carbon steels is always less than the melting temperature, there is always a region near the weld where the "hardening" temperature was reached. A quick and precise weld, like TIG or MIG, will heat a narrow zone quickly to the molten point. Since the heated region is very small, it also cools quickly, by radiation, by conduction to the air, and by conduction into steel a short ways away from the weld that was NOT heated. This rapid cooling near the weld acts line a quench and causes a narrow hardened region to form alongside the actual weld. This region is quite brittle and will easily crack unless the weld itself is "normalized."

I seriously doubt the welds on our fuselages are normalized. I wonder whether the fuselage was welded up in winter or summer.

[Av8r3400]

In the second photo there is a round mark on the lower piece that looks like a "rosette" to me. Could this possibly be a repair from a previous owner or something like that?

[P Morel]

In the second photo there is a round mark on the lower piece that looks like a "rosette" to me. Could this possibly be a repair from a previous owner or something like that?

I wish it were that simple but the answer is no. I'm the original builder. I'm searching for answers to this delima. I don't know if I'll ever get an answer good enought to make me feel 100% comfortable. But I'll be closer to some answers when I start the full inspection and repair.

Right now, I have the wings folded back taking weight/pressure off the longeron. With this tube broken, I can see some flexing on the gear attachments where this area of the fuselage is not rigid any longer. The Fuselage is on jack stands placed under the strut attachment point lifting the left gear just off the floor.

I plan to inspect as much of the fuselage as possible at all weld points to see if any welds were compromised.

[P Morel]

in your second picture the rear support for the landing gear looks like it has a squiggly line front to back. is that a crack?

Thanks for your input. No, that's not a crack, just a very poor job of covering and paint. I will be replacing the attach bolts also just for peace of mind.

I'll post my findings on this post hopefully with repair photos.

[Paul Z]

Had a great day flying on Saturday. Made it to a couple lunch fly-ins and then back home. Taxi back to my hanger on turf and just before pulling in, I heard a loud "POP". I stopped but didn't see or feel anything unusual. I taxied another 30' and parked on my pad in front of my hanger. As I opened the door, it seemed to scrape on the bottom like it was mis-aligned. I looked at the opening and found my fabric on the frame under the door wrinkled. I removed my seat and found the angle support tube broken. This was a clean break and not on the weld.

My Kitfox IV Speedster is a taildragger with just over 700 hours. Metal fatigue? Has anyone with a IV had any type of tube failure? Especially in this area?

The repair will begin this week but the big question is what caused this failure and what kind of stress is placed on this area that would cause separation, especially in this area of the fuselage? If taxi on turf might add to the stress of an airframe, I would think any type of structural failure would be somewhere else on the airframe.

Another question, are the angle support tubes the same gauge metal as the rest of the airframe?

Any suggestions?

Better after landing than while landing. That is a oh **** if I have ever seen one. I'm flag you arrived home safely. A good welder, some fabric work anf a little paint good as new. Too bad the repair will take about a month. A little nerve racking to have something like that happens.

[SWeidemann]

I will be following your findings on the rest of the airframe with interest. I assume all Kitfox airframes are similar in the way the fuselage tubing is constructed. Also would like to know too if others have had similar failures.

Skot

[Kurt.A]

Had a great day flying on Saturday. Made it to a couple lunch fly-ins and then back home. Taxi back to my hanger on turf and just before pulling in, I heard a loud "POP". I stopped but didn't see or feel anything unusual. I taxied another 30' and parked on my pad in front of my hanger. As I opened the door, it seemed to scrape on the bottom like it was mis-aligned. I looked at the opening and found my fabric on the frame under the door wrinkled. I removed my seat and found the angle support tube broken. This was a clean break and not on the weld.

My Kitfox IV Speedster is a taildragger with just over 700 hours. Metal fatigue? Has anyone with a IV had any type of tube failure? Especially in this area?

The repair will begin this week but the big question is what caused this failure and what kind of stress is placed on this area that would cause separation, especially in this area of the fuselage? If taxi on turf might add to the stress of an airframe, I would think any type of structural failure would be somewhere else on the airframe.

Another question, are the angle support tubes the same gauge metal as the rest of the airframe?

Any suggestions?

Paul sorry to hear about the break. It is hard to see in the 2 nd picture but there appears to be a small vertical crack going into the weld directly above the break. A close up of the break and the mentioned weld would help. What I am looking at disappears into the shadow. I am thinking the crack developed from the weld and propagated around the tube. Unusual break that's for sure.

[t j]

With the way this area is constructed, I can't picture in my mind how that tube could fracture and separate a half inch without compromising one of the adjacent welds.

The fabric repair you see was not the same damage as yours. It was to replace the bungee truss.

[Geowitz]

Obviously just a guess by looking at the pics, but my first thought as described by Lowell is a heat affected zone crack from the welding process... perhaps put into more and more tension throughout your 700 hours of flying, taxiing, and maybe an occasional less than stellar landing? Also, you do kind of have a long and potentially bumpy taxi to your runway.

[SWeidemann]

Regarding broken tube at 700 hours on a Kitfox IV:

One thing I have noticed about the Experimental aircraft fleet is that most (Experimental) airplanes out there are relatively low time as compared to the General Aviation fleet of certified aircraft. In this case, compared to Certified aircraft, 700 hours is not much time on an airframe, although in looking at Barnstormers I seldom see homebuilts with more than a thousand hours on them. For instance, I would say most older Cessnas have thousands of hours on them and just keep going, with maintenance done as needed and according to the long list of AD's that is generated. Since a Kitfox fuselage is tube and fabric, it would be similar to something like the Taylorcraft, Aeronca & Champion, Piper Cub, Carbon Cub and collection of others that are durable and have been flying safely for years. Without checking, I would assume there are similar failures in other tube & fabric aircraft. For our aircraft, they are Experimental and we need to stick together and share information about what has happened to our planes to prevent further problems.

I am hoping and expecting that IF there are other Kitfox owners/builders out there that have a similar problem that they please speak up. If this tube break is happening to more than one of us, shouldn't there be a Service Letter or some other documentation for inspections and an airworthy fix? I for one, want to know about it.

My 2 cents, Skot

[jiott]

The x-brace tube that broke is definitely put in tension during a hard landing on the main gear, or a hard bump during taxi. The fact that there is a gap at the break with no load on the gear indicates there is permanent deformation in the structure. I would bet that if you put a straight edge along the bottom longeron under the seat area you will see that it is bowed and not straight.

[Auster]

I have heard of what sounds like an identical occurrence here in Austria. I will alert the owner and he will probably jump on this thread to explain what happened and how he repaired it. Same Kitfox I.e. Mk 4 speedster but I think with well over 2,000 hours. The owner described what happened while we were standing next to the plane and everything he told me matches with what Paul has described. I only saw it after the repair. I did my kitfox endorsement in that Mk 4 and it has been in one ownership since the build and well flown and cared for.

Paul I hope it's some comfort knowing you aren't the first.

Kerry
S5 Outback
OE-AMJ

[P Morel]

Paul I hope it's some comfort knowing you aren't the first.

Kerry
S5 Outback
OE-AMJ

Thank you Kerry, that's what I was wondering. Am I the only KF4 owner that experienced this problem? I did an inspection on all the welds that I could see and found no indication of weak welds or cracks. Kitfox makes a great plane and safe plane. The factory workmanship is superb.

I'm posting a few preliminary pictures on the beginning of my repair. I'm only in the fitting stage and know that I need to remove more powder coat and fabric.

BTW, I'm also planning to change to the Grove gear that I think might give me that warm and fuzzy feeling ...

[cap01]

don't you just hate cleaning the powder coat off the tubes . the high speed with a scotchbrite wheel seemed to work the best . also have used the propane torch method and melted the powder coat off .
looks like that splice will make a nice repair

[jiott]

Maybe Airfox will chime in; he told me about some kind of gasket remover or something like that which removes powder coat slick as a whistle.

[Av8r3400]

Here is a shot of one of the modifications I did to my "Mangy Fox" project. I did this to strengthen the side truss in expectation of using Grove Gear (a custom longer version).

http://www.avidfoxflyers.com/uploads/monthly_01_2013/post-36-13595152030056.jpg

[Kurt.A]

Nice modification, What thickness of material did you use?

[Av8r3400]

It's 4130 sheet. I don't remember the thickness for sure, but I think .032".

[beeryboats]

The way I land I may need that mod :rolleyes:
I made a 4G landing in my dad's Baby Lakes. It had a recording G meter. The engine quit on take off, and training took over. No damage was done.

[Dave S]

The modifications Av8r3400 did looks like great improvement to the strength of this vital area.

For comparison - the S7 has considerable beefing up in this area - but it has a higher weight & the grove gear too...see attached.

Dave s

[Esser]

To remove powder coat, use permatex gasket remover. It's in a blue spray can.

[jiott]

Yea that's the stuff I was trying to remember. Way easier than Scotchbrite or sandpaper.

[Dave Holl]

Anyone know if the gasket remover would work on the protective covering on the flaperons?

[896tr]

"Anyone know if the gasket remover would work on the protective covering on the flaperons?"


Dave, if you're talking about the plastic covering on the flaperons ,when I asked Debra McBean how to remove it (mine had been on them for 11 years) she said she had heard that some people had used the gel type paint stripper. I tried that and it worked for me. Put it on thick and let it sit for a while then use a plastic scraper to ease the removal.

[Xengineguy]

Hi Paul,
Might I suggest placing a reinforcing tube on the inside of the original tube.
If it's 049 it might be a little loose but easy to install. Angle cut to fit,drill a pair of small holes thru the original tube at each end. Tach weld the gap,weld thru the end holes flush,then finish weld the gap. Easy and stronger than stock.:o

[P Morel]

Anyone know if the gasket remover would work on the protective covering on the flaperons?

This doesn't have anything to do with the thread but this is how I removed the protective covering on the flaperons. Jasco paint stripper for $11 @ Lowes

https://www.youtube.com/watch?v=pvTJyIaatCQ

[Dave F]

Sorry here too. My guess is that the tubing is .049" wall thickness.

I have see similar damage after hard landings. As I understand it, one problem with typical welding techniques is the concentrated heat that will often result in hardening of the metal near the weld.

From "Bearhawken"
"Since the transition temperature required for hardening carbon steels is always less than the melting temperature, there is always a region near the weld where the "hardening" temperature was reached. A quick and precise weld, like TIG or MIG, will heat a narrow zone quickly to the molten point. Since the heated region is very small, it also cools quickly, by radiation, by conduction to the air, and by conduction into steel a short ways away from the weld that was NOT heated. This rapid cooling near the weld acts line a quench and causes a narrow hardened region to form alongside the actual weld. This region is quite brittle and will easily crack unless the weld itself is "normalized."

I seriously doubt the welds on our fuselages are normalized. I wonder whether the fuselage was welded up in winter or summer.

Lowell is correct - MIG welding works well but without normalization hardness will happen and cracking likely.

This is why I like to oxy.acet welding 4130 especially in joint and cluster areas. he biggest laugh is the guys with 120V MIG home welders that are welding 4130 . Actually quite scary as they go fly these

[Xengineguy]

Lowell is correct - MIG welding works well but without normalization hardness will happen and cracking likely.

This is why I like to oxy.acet welding 4130 especially in joint and cluster areas. he biggest laugh is the guys with 120V MIG home welders that are welding 4130 . Actually quite scary as they go fly these

I guess I don't understand why a 120 volt mig won't do a good job on tubing
We commonly use in aircraft? And as far as worrying about hardness caused
by rapid cooling. Just preheat the area too hot to touch 160/200degrees,
before welding and let it cool slow. Cover if necessary.
I have very good 120 and 220 mig welders and a very nice tig setup,
I think pre and post welding prep has more to do with success than the type
Of welder.?? Just my 2cents:)

[Dave F]

I guess I don't understand why a 120 volt mig won't do a good job on tubing
We commonly use in aircraft? And as far as worrying about hardness caused
by rapid cooling. Just preheat the area too hot to touch 160/200degrees,
before welding and let it cool slow. Cover if necessary.
I have very good 120 and 220 mig welders and a very nice tig setup,
I think pre and post welding prep has more to do with success than the type
Of welder.?? Just my 2cents:)

Exactly -- what I see is the 120V welders go home and the user thinks he is a pro over night and never welder before