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.