The "F" word

[KevinC]

Yup, I said it:  Flutter.

Saturday when my mechanic/test pilot was flying the plane he had a serious flutter problem just after take-off.  The stick vibrated for about 20 seconds at take-off.  He said at first he thought it was an out of round tire.  Vibration stopped when leveled and slowed, and he returned to the field.

The trim tab departed the airplane.  The piano hinge was ripped apart and the piano wire was broken between the spring and the tab connection.  The rear two support cables have at least and inch of slack in them now.

This is a serious problem and to tell you all the truth, I don't know where to go from here.  I was actualy hoping that the tab had come loos from the paino wire and initiated the flutter - but clearly not the case.  I'll be calling the factory tomorrow morn.  I am lucky someone wasn't killed.

k

[Dave Krall CFII SEL SES]

Luck helps sometimes.
Are your tailfeathers balanced Kevin? My A&P has been suggesting it, but my surfaces are already covered. I think Gary or Steve have balanced their elevators...

[scubarider2]

So sorry to hear the "F" word.  My first thought from your description was that the cables were not properly tight and possible out of balance/level.  If you have an inch slack then something definitely wrong.  I would start fresh with the advice from the factory and relook and rebuild the tail.  If we can help....let us know.  Keep us updated.
Dennis

[KevinC]

Just to be clear:  the slack in the flying wires was a result of the incident.  They were tight before the flight (part of preflight checklist and I checked them myself just out of habit).

k

[scubarider2]

So what you are saying is that the wires "stretched" that much due to the fluttering?  Did not think that the wires could be stretched an inch due to that amount of stress.  Just something to consider when you look into it.
Dennis

[gme9261236]

Cables do not stretch. What he is likely saying is that his horizontal stablizers may have actually "warped" causing the slack.

[scubarider2]

From what I know....{and you know how much you can rely on that}......cables do "give" and that is why you need to check them and tighten them as I have had to do for the first 100 hours.  I would find it hard to believe that "warping" took place since the cables are aligned to have stress placed on them at equal locations so when one is under load the other is pulled to meet that resistance.  Of course when you have fluttering that violent to rip the trim tab off you really do have to look at structural integrity.
Dennis

[KevinC]

I don't think the cables stretched and there is no evidence of the horizontal warping.  The cable attach tabs and the cable 'eyes' straightened out a bit, giving same effect of cable lengthening.

No, my control surfaces are not balanced.

k

[Wes]

Kevin,
First let me say I'm sorry to hear about your problem.
Nobody likes to experience structural failures, especially on your brand new airplane.
A couple of questions:
- does anybody know of a previous failure of this type?
- can you post a picture of the tab installation before the event, especially the cable routing and attachment.
- did the pilot experience any difficulty with trim function prior to the failure? (i.e. unexplained pitch trim changes)
- prior to the failure, did you notice any tendency for the trim tab to move relative to the elevator when the stick was moved thru full down to full up travel?
- did the 'anti-slack' spring stay attached as shown in the picture? or did you stick it back in place?
- why does the remaining piece of cable sheath and end ferrule appear to be on backwards in the pic of the trim tab? (see attached pic of a Highlander from Sebring)

From the pictures, you obviously exceeded the range of motion allowed for that poor little trim tab. I think the only way that can happen is for the tab to be unrestrained, that is, no push-pull wire. The picture shows a serious bend (kink?) at the wire separation point. The question is; did this happen before or after the tab separated from the elevator? Good thing you were able to retrieve the 'departed part'.
I have some experience in aerospace hardware failure analysis and this one hits kinda 'close to home', so please excuse all the questions.
I think, 'on the bright side', it shows again how tuff this little airplane is. It can survive a rather severe shaking and still keep on flying.

Thanks for posting and hope I can help resolve this for you.

Wes

[KevinC]

A couple of questions:
- does anybody know of a previous failure of this type?
- can you post a picture of the tab installation before the event, especially the cable routing and attachment.
- did the pilot experience any difficulty with trim function prior to the failure? (i.e. unexplained pitch trim changes)
- prior to the failure, did you notice any tendency for the trim tab to move relative to the elevator when the stick was moved thru full down to full up travel?
- did the 'anti-slack' spring stay attached as shown in the picture? or did you stick it back in place?
- why does the remaining piece of cable sheath and end ferrule appear to be on backwards in the pic of the trim tab? (see attached pic of a Highlander from Sebring)

From the pictures, you obviously exceeded the range of motion allowed for that poor little trim tab. I think the only way that can happen is for the tab to be unrestrained, that is, no push-pull wire. The picture shows a serious bend (kink?) at the wire separation point. The question is; did this happen before or after the tab separated from the elevator? Good thing you were able to retrieve the 'departed part'.

-I'm afraid I don't know if this has happened to anyone else.
-I've got thousands of photos - none of the trim setup.
-On the first test flight the trim 'bug nut' slipped and the trim setting in the cockpit was way off from the original setting (cable slipped).  I reset it to 'zero' the day of the problem and re-tightened the bug nut - this time with zeal.
-it did not move relative to the elevator with up/down stick movement, however over time the spring would move it.  I tightened the friction bolt on the cockpit control to keep it from moving from the spring on the morn before problems.
-the anti-slack spring (somehow) remained on the elevator. It appears that when the piano wire broke it went through the spring and retained it.
-the small piece of cable sheath was a spacer to work up against the washer on the aft end of the spring.  Wish I had pix, but from the tab connection forward it went bug nut, small piece of cable sheath, cable sheath end (chrome piece), washer, spring, washer, cable sheath end, cable sheath.

I am pretty sure the kink happened during the event.  As I was adjusting and tightening the morning before, I saw no kinks or problem with function.

One thing that is probably important:  the support for the cable clamp on the elevator is 5.25 inches from the arm on the trim tab - it is mounted on the forward set of tabs on the elevator.  Troy said today that it should be no more than 3 inches.

One thing I noticed from the picture was counter-sunk rivets.  That seems to take a lot out of the hinge.

baffled...

k

[jesander]

I posted a note on Nov 7th concerning a similar incident. We experienced a trim cable failure at the trim tab that resulted in flutter of the control stick. Although we didn't experience any flight control surface damage other than the broken cable. We called J/A and were told .040 dia. wire was the culprit. Trim control wire needs to be .060 dia. J/A sent the correct .060 wire.
    Kevin, Please let us know if you have the wrong (.040 dia.) wire. If your failure was with the .060 wire we may have other trim control issues.
Thanks.... Joe S.

[KevinC]

Joe - I remember your post and looked it up again last night.  My caliper battery was croaking, but it measured .063.

k

[jesander]

Something else to check. While installing the new .060 dia. trim control cable, I found that the cable terminal nut or bug nut or whatever it's called, can be secured with the set screws in a way that locks the cable to the arm. This will cause the cable to bend/flex and fatigue as the arm travels through its arc. This condition is easy to spot at the elevator end but can be overlooked at the trim lever down between the seats.  
Joe S.

[Johnny C!]

Kevin,
 Did you redrill the holes in the elevator
halves to make each side line up?

I remember reading where someone noticed
that the halves were not lining up, and made
some adjustments. I asked Troy & he told me
that they were supposed to be offset a few
degrees, to eliminate the possibility of flutter.

This was discussed in a thread, some time back.

Just checking. Sorry about the trouble...

John

[KevinC]

My mechanic/test pilot stopped by last night to do some head scratching.  A few things we noticed:

-the two outboard rivets on the trim tab were sheared, all the rest pulled through the hinge material.
-there was insufficient distance between the edge of the hinge and the rivet hole, especially on those two outboard rivets
-upon first inspection (we'll be doing more) there appears to be no damage to the horizontal/elevator structures & attach points - only the tab, hinge and cables.

and an answer to John's question on misalignment of the two elevator halves:  The factory sent me an undrilled elevator half (side w/o tab).  Troy told me to align the leading edges of the elevator and the horizontal and then drill/bolt them together.

k[/i]