Broken Longerons

Several weeks ago, I flew my PA-12 to Whiteplains Airpark (SC99), Gilbert, SC to check on the PA-18 Ed Fisher is rebuilding for me. Just as I'm getting out of the plane, Ed's sharp eye caught some wrinkles in the fabric on the right side of the tail. Removing inspection panels and the tail clean out pan revealed that the right upper longeron was completely broken just forward of the Stabilizer Support Assy. It had been broken long enough to cause some serious fretting. After tearing fabric away, Ed also found that a crack had started on the left side in the same place. I don't know how this would have ended up if Ed had not found this. Hopefully it would have failed on a landing but could have been much worse!

On the PA-18 that Ed is working on, the left longeron had been repaired in the same place and Ed found a small crack on the right side, same place.

I did a search and found a thread started in Jan/2003

What Breaks?...Common faults, not commonly known.

The third posting on this thread by SuperCub MD lists this as a common problem area.

I would urge everyone to check their Cub longerons at this location for cracks. I had no idea anything was wrong back there.

The horiz stab does a lot of shaking and twisting a lot of it is transferred to the longerons by the support assy. Once a crack starts, it likely progresses rapidly.

Incidentally, while there was some surface corrosion inside the tubing, it was structurally sound and not a factor in the breakage.

In addition to the repairing of the broken areas( smaller tube inside, rosette welding, etc) Ed devised a fix to reinforce this area. He installed 3/8" Chromoly stiff legs that go underneath each longeron from the support assy to the intersection of bracing below. See Picture.

Perhaps someone else has encountered this problem and come up with a fix. If so, I would be interested to see what has been done. Ed is doing the same reinforcement to the PA-18 fuselage.

stewartb said:

http://www.fadodge.com/tail-brace-beef-up/

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That looks pretty wide like it might go in well forward of where the stab assy welds on to the longerons.

Gary

the root cause of that crack was because the rear stabilizer liner tube was corroded/stiff into the bracket, and each time you moved the trim instead of it rotating in bracket it flexed longerons.....

normally you weld a nut and put a grease zero over the oil hole.... you can also weld a nut to a MS clamp with a hole and put it over oil hole... I also like to cut a channel so great will flow

mike mcs repair said:

the root cause of that crack was because the rear stabilizer liner tube was corroded/stiff into the bracket, and each time you moved the trim instead of it rotating in bracket it flexed longerons.....

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I do not believe that was the case here. This 12 was completely rebuilt in 2010, upgraded to O-320 and a new 18 horiz stab/elevator was added. Flown 250 hours since and always hangared. At the time this broken longeron was found, everything was well lubed and free.

Gary

Gary Ward said:

I do not believe that was the case here. This 12 was completely rebuilt in 2010,

Gary

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and the other 60+ years of its life???

It always makes me wonder where other hidden weaknesses lurk in these old airframes.

Thank God it was discovered while you were on the ground.

Is this the 12 you bought last fall?

mike mcs repair said:

and the other 60+ years of its life???

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I was only trying to make the point that since a new PA-18 horiz assy had been installed that I did not think that freezing up of the liner tube going thru the stab assy caused the problem. The rebuild in 2010 was done by Ron Jones in Eaton, NY. It was extensive with most of the preferred back country mods. Although I just bought the airplane last fall, I have full confidence in the rebuild. I have met and been to Ron Jones shop. He does outstanding work and I am completely happy with the airplane. There was no evidence that any prior repairs had been done to the this area. I suspect the combination of prior fatigue and the larger engine exposed some weaknesses.

Paul Heinrich said:

It always makes me wonder where other hidden weaknesses lurk in these old airframes.

Thank God it was discovered while you were on the ground.

Is this the 12 you bought last fall?

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Paul,

Great to hear from you.

Yes, I am grateful to Ed Fisher and his sharp eye for noticing the wrinkles in the fabric. I doubt I would ever have noticed it.

This is the 12 I bought last fall. Previous owner was Stan Edwards from Norwich, NY. He flew it as a stock 12 and he and Ron Jones, Eaton, NY, did an extensive rebuild with all the best mods in 2010 and he flew it for 250 hours. I fault no one for the problem with the longeron. The quality of the rebuild is outstanding.

It was painful to watch Ed Fisher cut away the fabric on the tail but he did a beautiful job with the repairs. I'm satisfied that structurally, it is better than new and it would take an expert to find where he repaired the fabric and repainted it.

Take care,

Gary

mike mcs repair said:

and the other 60+ years of its life???

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Yes. There is a reason for the price differential between old airplanes and a new build or new airplane.

Mr. Ward. Thank you for the post.

Gary Ward said:

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Thanks for showing these pictures. They are proof that placing a load in the center of a leg of the load triangle is not always a good structural idea. Your assumption that the flaps and big engine combined to change the stresses in this area is a good one. Piper got away with it when the engines were small and without the added turbulence of the flaps. Though in retrospect it wasn't a good idea.

That is a good fix. The low carbon steel of the fuselage may have contributed to the failure where now we only use Chromoly (4130) which is stronger.

skywagon8a said:

Thanks for showing these pictures. They are proof that placing a load in the center of a leg of the load triangle is not always a good structural idea. Your assumption that the flaps and big engine combined to change the stresses in this area is a good one. Piper got away with it when the engines were small and without the added turbulence of the flaps. Though in retrospect it wasn't a good idea.

That is a good fix. The low carbon steel of the fuselage may have contributed to the failure where now we only use Chromoly (4130) which is stronger.

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Pete

Just having a passing acquaintance with metallurgy and the physics of stress/strain--there are structural components of aircraft airframes that must flex or they will result in a catastrophic structural failure ie wings etc. I have had people watching my plane when I (formerly) made very high AOA take offs remark that they were amazed at the flexing (twisting) of the horizontal stabilizer. Your observations r/t the incorporation of higher HP engine, addition of flaps, and other additional air frames changes having a significant affect on an airframe that was not intended for these stressors is a reminder that closer preflights and periodic close inspection of the tail section is warranted. Each time I fly I give the tail brace wires a "twang" to ascertain if their a change in pitch, increase flex, and movement that shouldn't be there---just how much flex in the tail section is too much and is there an issue with insufficient flex. Is there a practical method to measure?
Best practice to do frequent inspections to preclude a failure. Is a failure more likely to me a brake in just one longeron?

Gary, I wonder if there is any connection between this issue and your heavy control stick forces

These 70+ year old mild steel Piper airframes have reached their service limits. I have repaired many rusted hulks and cut away tubing that is perfect and also found paper thin tubing in the same fuselage in the same general area. Many get that warm fuzzy feeling knowing they have the brute 5/8" barreled strut forks when they are attached to a fuselage that may be questionable. Luckily Mr Piper had the heaviest wall and diameters in the floor/strut attach area. That extra tube that Ed Fisher added to the subject aircraft is standard on Pitts Specials, they have a rear fuselage truss arrangement that emulates the Piper design.

I don't think getting 60 yrs out of that fuselage is a design flaw. I believe they are perfectly designed. We tow signs many thousands of hours and sometimes they just break. IMO.

Seaworthy said:

..Just having a passing acquaintance with metallurgy and the physics of stress/strain--there are structural components of aircraft airframes that must flex or they will result in a catastrophic structural failure ie wings etc...... Is a failure more likely to me a brake in just one longeron?

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Just assuming here Tom, The natural frequency in the tubing would likely be different on the left vs the right side of the fuselage due to the clockwise rotation of the prop. Thus even though the tail may appear to be shaking the same on both sides the vibration frequency in the individual tube could be different with as in this case the failure taking place. The frequency of the other longeron was enough different so that it's failure mode was not reached. Once the right longeron failed the frequency of the left tube would have been changed due to the load path changes. It could easily have failed in the next few minutes, or not.

barijr said:

I don't think getting 60 yrs out of that fuselage is a design flaw. I believe they are perfectly designed. We tow signs many thousands of hours and sometimes they just break. IMO.

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I don't believe that this break could be considered a design flaw by Piper since this particular airplane seems to have been heavily modified in the aftermarket. Perhaps the aftermarket persons should have looked a bit deeper into the structure and the effects of their modifications?

Guys I`m just a dumb old welder here but that break is right in what is known as the "heat affected zone" it is the area at the toe of the weld and is usually subject to some hydrogen embrittlement. I suspect that in conjunction with the thin wall thickness, age of the tubing and like Mike said the mechanical flexing all added up to a crack at the toe of the weld and it just propagated around the tube. In short it just got tired and broke

http://crosswindsstol.com/stc.html Notice there tail support at the bottom of the page. Also somewhere in this website theresa some pictures of a nice fix for this problem but i cant find them.

I'll keep flying vintage airplanes but "good" preflight is a must.

The description of right vs. left side is likely and why tails seldom(if ever)? just fall off. We should have a warning so keep a good watch.

(And maybe carry a Ox/Ac rig and a big roll of duct tape to get you home) ;-)

Jack

couple points to correct some things in this thread....

#1. the old fuselages used "some" soft tubes(hard to guess where), yes the chromoly is stronger, but it CRACKS much easier ...

#2 the Atlee beef up brace is not to fix this issue, it's to prevent the twisting side loads from heavy aft loaded planes tail wheel when TURNING in soft or tundra..... want to scare yourself, go grab the stabilizers on a -18 on FLOATS without some form of added box brace like that, and pull one side up and down, they fix ALLOT!!

and few other things i can't remember right now...

MT12 said:

Gary, I wonder if there is any connection between this issue and your heavy control stick forces

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I don't believe there is any connection between the control forces and the problem. I flew another PA-12 with an O-360 and the elevator forces were even heavier than mine. Much of the force is due to the spring which holds the elevator up in the neutral position. Even sitting on the ground without the engine running, it takes a lot of stick force to push full down elevator against the spring.

Gary

Cub junkie said:

These 70+ year old mild steel Piper airframes have reached their service limits. I have repaired many rusted hulks and cut away tubing that is perfect and also found paper thin tubing in the same fuselage in the same general area. Many get that warm fuzzy feeling knowing they have the brute 5/8" barreled strut forks when they are attached to a fuselage that may be questionable. Luckily Mr Piper had the heaviest wall and diameters in the floor/strut attach area.

That extra tube that Ed Fisher added to the subject aircraft is standard on Pitts Specials, they have a rear fuselage truss arrangement that emulates the Piper design.

Click to expand...

That's interesting. I would love to see a pic of the Pitts tail section.

Gary

barijr said:

I don't think getting 60 yrs out of that fuselage is a design flaw. I believe they are perfectly designed. We tow signs many thousands of hours and sometimes they just break. IMO.

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If I though that things just sometimes break, I would quit flying. Rarely does something break without a reason. No structure is perfect. There are always weak points that will show up whether it be Cubs or big jets.