Horn AD

[John Danish]

Hello all, after reading the AD over and over, it comes to mind that the explanation by Zach on point 2 is the key.
Having checked mine recently, and done SB1189, you have until 100hrs from OCT 22nd- that gives most of us a year! I fly about 45-50 in my PA24 and another 45 or so in a Saratoga [ + some dual given] So that would mean you would have to be in your airplane at least 10 hrs EVERY month, until it needs to be done?
If you are sure yours is fine, and most are, [no one has given me a number of broken] then why by parts from Piper which do not fit or are made wrong? Let the dust settle, and see what happens. Also, I have the thick tube, which is impossible to crush with the bolts, cracking the horn. [7000hrs+ airframe] why would I want to replace with a thin one, which Piper says is the only one made?
Remember the spar AD men, never found a bad one, how many years did we inspect that? What was the number to prompt the AD 1,5,50,750? no one will say. Being a long time repair person, my fix would be to put a steel strap around it with longer bolts!
any thoughts out there?
jdanish

[Tom Veatch]

[AlanBreen]

Can anyone tell me if the publication of the AD is going to result in changes to the SB.

The AD does not include the twins whereas the SB does. Over here in New Zealand we must carry out SB's and AD's. So while the AD doesn't affect the twins we still have to complete the SB since it includes the twins.

Are Piper likely to remove the requirement to inspect the twin from the SB?

[Fred Zervos]

It looks to me that the tail section of most small planes shake, rattle, and roll when doing a runup. The turbulence coming from the prop buffets the tail section causing tail section vibration. I would think any plane would eventually sucumb to some kind of stress related damage over time. Exactly what would be the results of a failure of the horn? Loss of stabilator or tail seperation? Remember the V-tail Bonanza had problems with the tail section coming off in flight. Steel straps around the horn might introduce corrosion due to dissimiliar metals touching. I think the horn is made out of cast aluminum alloy. My fix would be to install a small plexi glass window in the tail cone to visually inspect the horn on the walk around. How about a sensor that would measure tail flexing limits and give a warning? Some failures happen gradually and then suddenly let go. Assuming that the tail section vibration starts to increase with a failing horn over a period of time, one could be warned before catastrophic failure. If corrosion is found on the horn I would suspect corrosion to be found on other areas of the plane. Seems like an expensive AD to have to disassemble the tail section and magnaflux the horn every 5 years. Would a lazer type examination or eddy current inspection work without disassembly? Every time something is taken apart the possiblity is open that reassembly could introduce an unintended consequence leading to more problems. Leave well enough alone can be useful sometimes. And the money saved on a rescended AD could go towards a new radio or something else to improve safety. It is possible to brush your teeth too much and wear them out.

Fred Zervos 250 owner

[Kristin Winter]

I think that not having the tail go into catastrophic flutter improves safety. The cracking is not due to fatigue. We don't know if the cracks develop slowly or quickly. I am pretty sure that you would get little or now warning before the yoke no longer was connected to the stabilators. Your best hope in that scenario is that the shaking of the aircraft was so violent that it kills the occupants quickly so that they don't have to witness the aircraft disintegrating around them.

[LeWayne Garrison]

The inspection is not really difficult. It was surprisingly straightforward. If nothing is cracked, $1000 should easily cover the inspection. We have now done two, and after the learning curve, not difficult.

[9089P]

Hi Kris,

I agree with your comment as far as it goes, but it is sheer speculation as there has not ever been an in flight failure of the horn with an aircraft break up. So what happens, IF the failure happens, is a guess. The fact that there have been numerous cracked horns and yet no in flight failure suggests there is something else at play here. What, I don't know.

Clearly all the idiots who have been sitting back to "see what happens", and thereby risking the lives of their loved ones, need to get off their a++ and get the inspection done. It just isn't that hard or expensive. This of course is not directed to any ics member as I'm sure they have the requisite intelligence to have done the inspection long ago.

Don

[Kristin Winter]

Hi Don,

I wouldn't call it sheer speculation, but I admit that we haven't tested the theory. However, we do know that a Comanche tail will flutter, courtesy of NASA. From the aerodynamics texts I have read, it is generally considered that out of balance tail surfaces are highly prone to flutter. IIRC, it is a function of having the center of lift and the center of gravity of the surface on opposite sides of the pivot point thus creating a teeter-totter effect. I will have to look that up, but that is my recollection. I would call my speculation more of an educated guess.

[Jim Golden]

Hi everyone. I might actually be able to contribute something here:

Flutter is where the structure hits its natural frequency. Like the node points on a guitar string. That is typically a bad thing in airplanes, and so we try to make the structure stiff enough that it's natural frequency cannot be hit in operation. I have three books on flutter and used to work right beside the flutter guys at Boeing. Marty Holman was probably the best person at flutter calculation in the world (he built the Carbon Stallion kitplane). Unfortunately, Marty died a couple years back.

As mentioned, typically you see control surfaces flutter before you do major structure like the fuselage. "Aileron Buzz" was a common thing with some of the WWII fighters and early jets. Hopefully they could slow down before the control comes off. There are two tried and true ways to help prevent control surface flutter. Well, three actually. (1) the surface itself must be torsionally stiff and strong. (2) There should be no slop in the surface's movement. (3) The surface should be "dynamically" balanced. By dynamically balanced, as opposed to statically balanced, the surface should be balanced along its entire length. Imagine you cut a slice out of an aileron anywhere along its length....that slice should be balanced. So the bob weight ideally would be like a pencil of lead running the length of the control surface. This is as opposed to statically balancing it, where you just have one big bob weight out on the end. Yes, the whole thing is balanced....statically, this way. But each individual "slice" is not. You are reliant upon the stiffness of the surface to not deflect. If the surface is built stiff enough, you can get away with this. And most light plane makers do. But it is not the ideal way to build a control surface. And, as for the balancing, you want it to be at least 100% balanced. Slightly nose heavy doesn't hurt a thing. Why?

Take the example of an aileron. Let's say it is not balanced at all, and there's lots of slop in the rigging. A gust hits your wing and pushes it up sharply. Since the aileron is tail heavy without being balanced, and there is slop in the rigging, as the wing pops up, the aileron wants to go trailing edge down. That gives even more lift, pushing the wing up even more. The wing hits a point where it stops deflecting, and then it snaps downward. Again the aileron, being tailheavy, reacts the opposite way. So it goes trailing edge up, which creates more downforce driving the wing down. This is an unstable and self amplifying situation. Were the surface balanced to 100%, it should just move with the wing and not contribute one way or the other. If it were overbalanced, it would actually have a dampening effect because it would try to act opposite of the gust. In other words, wing deflects up, but aileron is overbalanced to be nose heavy, as the wing goes up, the aileron goes trailing edge up, dampening the effect. Same thing if the wing snaps down, the nose heavy aileron wants to deflect trailing edge down, dampening it. That is how you want it.

A stabilator is a slightly different animal, but the theory is the same. You want stiff, balanced, and no slop.

Hope this helps,
Jim

[Kristin Winter]

Jim,

Thanks! That makes perfect sense, but I never really thought about it before. Have you seen the YouTube video showing a Twin Comanche tail fluttering. NASA was playing games. Search it on YouTube or Google and you will find it immediately, if you haven't seen it.

[Jim Golden]

Hi Kristin,

Glad I could be of some help. I know you guys will answer ten questions of mine for every one I can help you with So glad I could do a little something.

I HAVE seen that video, and it's enough to strike fear into the hearts of strong pilots! They said the NASA test pilot was doing something like 250 knots (I think knots are for sailors...so that's 288mph for me) to get it to do that. By the way, that test pilot was wearing a parachute and had the door rigged to open quickly so he could get out in a hurry. In the video, if you can imagine yourself looking at it from directly behind the airplane, you see the classic sine wave curve shape being formed. I once had a Huey helicopter blade on a shaker and we shook it until it hit its natural frequency, and this is exactly what you see. But, it also appears to be doing a sine wave shaped shaking along the chord direction as well. This bird is rocking and rolling!

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

Personally, I'm not a huge fan of stabilators, just because of this. You have to really beef them up. It's a lot easier to avoid with a conventional horizontal stab and hinged elevator. Not to say you can't avoid it with the stabilator, it's just a little tougher. They need to be stout. I understand Roy LoPresti has some kind of beef up kit for Comanche's that raises the Vne (which is simply the design cruise speed multiplied by a factor....it's not an actual speed where the airplane magically disintegrates...). I'd be curious as to what it was...maybe just a heavier torque tube? I'm looking into it.

As an aero-engineer myself, I will admit that I have not only read most of the posts on here I could find on the horn AD, but also looked into its design as well. I believe a simpler but stronger design would fix this entire problem and do away with the AD altogether. I know when I buy one I don't want to tear it apart every few years for this....and I figure most other folks don't want to either. I realize the Aussie horn helps (and it does look nice), and it buys you 10 years/1000 hours until you revert back to the 5 year/500 hour. But, I have an idea for a replacement assembly that I think would be even better for the Comanche owner and the A&P. How about a drop in assembly that is far stronger than the original, won't fatigue, is completely inspect-able while as-installed in the plane, and would sell for about the price of two inspections on the OEM part? Throw out the old assembly, install the new, and forget about it...other than look at it every 100 hours and say "Yep, it's fine."

I'm working on it

[Kristin Winter]

Jim,

You don't happen to be a structures DER by chance? I will introduce you by email to a Twin Comanche owner who is, and worked on the horn issues.

What I know about the problem is what I read. Eric, "Trojanflyer" on the Delphi site, gathered some data and concluded that the cracking was caused by excessive torque being applied to the two bolts that hold the horn to the torque tube. IIRC, he measured the tube as being as much as .010 out of round when the torque valve for a bolt in tension is applied. This is excessive as the bolts are in shear only. However, back in those days, manufacturers QA was a bit loose. The FAA tested a cracked horn and determined that the cracks were caused by stress corrosion.

Eric tested the theory on the twin torque tubes which have much thicker walls, and determined that there was much less of an out-of-round condition created. That and the fact that a cracked horn has never been found on a twin, the FAA did not extend the AD to the twins. It would seem to me that a new horn and a thicker torque tube would solve the problem. I understand that Hans ran a finite element analysis, but that the results were inconclusive. You will have to ask Hans about that as I probably wouldn't understand his answer, but you probably will. I always wanted to get my AE degree, but I ran out of my money and my parents were not supportive of that goal. They were still gagging on my learning to fly, and getting my A&P put them over the edge.

As for the flutter, Piper did something interesting with the twins starting in 1969. The stabilator got a thicker profile. That meant a longer spare web, and presumably makes it more rigid. Whether they did this because of flutter or because they were trying to increase the effectiveness of the stabilator on landing, I don't know.

On the singles, the AD knocked 40 mph off the Vne. If you put the counterweights on the rudder, you got back 20 mph to the Vne. To get it all back, you had to add counterweights to the stabilator tips which extend forward of the leading edge. Piper added those weights starting with the 1969 C model singles. That AD never affected the twins.

Thanks to you your informative post, I guess my Maggie is less prone to stabilator flutter in that she has the thicker stabilator and she has a counterweight along the leading edge. That counterweight is not meant to reduce flutter. It is mean to break off ice when inflated. However, I guess it does dual duty.

[AlanBreen]

[Jim Golden]

Alan, If I misspoke on this, then please forgive me. I've read so many posts on this that they've started to run together. Blame it on the eggnog.... I thought that it was still up in the air (bad pun...) as to whether it makes the entire AD go away because it's a different part number than any of the ones listed, but FAA could decide to treat it just like a Piper horn. Maybe that's been resolved and the Aussie horn does make it totally go away. If so, then that's great. I'd say buy one and never look back. Oh, and I have sailed a few boats so I guess knots are OK too

Kristin, you make a very good point on the over-torquing of those bolts causing the cracking. That is entirely possible. Like you said, those bolts are only supposed to be acting in shear (and you would think they wouldn't be doing much at all with the hot horn/cold tube press fit), so they really shouldn't need to be any more than snug and be safety wired.

That begs the question: Why not simply install the assembly from the 400/twin into the 180/250/260? I doubt it weighs that much more that it would be a CG buster.

I just read back through every post on here I could find on this subject, and I'm still not sure how it all shakes out with the inspections. It looks like if whichever way you go, you're still tearing it apart every so many years. Maybe I'm making a bigger deal out of that than it is. Just seems like a pain to me. But I also noticed that this whole process on the new horn has been going on since 2010. Seems like a long time to work this out.

I'm not a DER, but would like to be one. (my one buddy was a DER, but he went to work for FAA directly at SACO instead) I was a structural analyst at Boeing, and then moved into Loads and Dynamics with them for awhile. I'm on the east coast now doing civil structures (I've done a few bridges and some neat big buildings) for a living. I'm big with EAA though and have done about every kind of analysis you could think of on a plane. I like to think I'm pretty good with structures. I also got my P.E. rating in Mechanical Engineering and like designing and fabricating all manner of stuff. I'll get with Hans and compare notes.

I know I'm new here, but I'd like to see this problem fixed once and for all and have it go away. More important stuff to worry with like what color the new paint job will be

Take care,
Jim

[Kristin Winter]

Jim,

The Aussie horn is an AMOC for the AD. It does not technically terminate the AD as it substitutes a 100 hour visual inspection for the 500 hour disassembly and dye penetrant inspection.

The reason that you can just substitute a twin torque tube and horn are two fold. They FAA hasn't blessed that as a terminating action, and the stabilator attach bolts on the twin and 400 are 5/16" instead of 1/4". Another wrinkle is that Piper installed some of the thick walled tubes on the singles in some of the early airplanes. Piper has no record of this and denies it, though I have seen two personally and others have been reported. It is likely that the crew just ran out of thin-walled tubing and just the stock from the twin pile to make the torque tubes for the singles. On the two I have seen, the horns were not cracked.

I am guessing that if Hans' FEA had been more conclusive in support of the over-torquing theory, we might have a terminating action. Hans could tell you more than I can on that subject.