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Trim System Squawk

RanchPilot

Registered User
Wyoming
Trim cable frayed and bunched up in the fairlead. New cable is on order. Two questions:

1. Anyone have any tips or secrets for replacing the trim cable inside a covered fuselage? (-18 trim system inside a -12 fuse)


2. Should I be worried about flying the plane while I'm waiting for the replacement cable? I won't use the trim (it is currently set at about 70). I also assume that, even if the trim cable came apart altogether, the horizontal stab would stay in its current position on the jack screw? In other words, is there any way the horizontal stab could travel to one trim stop or the other based only on air pressure from flight?
 
2. Should I be worried about flying the plane while I'm waiting for the replacement cable?



would it cause you to crash... probably not

would your insurance be void if you did crash..... YES

I would bet 9/10 IA's would not sign off the airplane as "airworthy" with a control system cable failure like what you are describing. Even though it's a secondary control it's still part of the control system. I would borrow a friends plane if you absolutely had to fly.


Can't help you with info about changing the cable on a covered aircraft. I hope you have a metal interior and don't have to cut fabric to gain access.


Jason
 
If you fly without trim or even suggest it ...I would recommed getting better flight training than you have had. Trim may not be considred a primary control, but in my book it is very important for safe flight.

If you are in a situation were you have to fly to protect people and property, then that may be different. But, the posting implies not.
 
It is a pain if you have a fabric headliner to get to all the pulleys. If you have metal interior not to bad. Draw a picture of where the cable is on the double groove pulleys and where each one goes. Rubbing cables and rewrapping is a real pain. Working through the tail opening on the jackscrew is a pain but doable. You know your airplane better than anyone, there are several Cubs I fly where I never touch the trim and others where I wouldn't be comfortable without it.
 
Thanks, Steve and Jason. I've pulled all the access panels I can and, although it doesn't look fun, I think we can reach everything one way or another. We will have an A&P in the mix to make sure it gets done right.

Thanks also for your measured responses on the second question. It never ceases to amaze me how some on this site can take a straightforward question (i.e., can the horizontal stab be moved up or down the jack screw by air pressure alone) and turn it into an opportunity to impugn someone's intelligence and/or flight training simply for posing the question. Perhaps it was/is a stupid question, but I appreciate that you tried to answer it without suggesting (or stating) that I'm stupid or ill-trained for asking it in the first place. If it weren't for stupid questions, I probably wouldn't have any questions at all...
 
"... Perhaps it was/is a stupid question, but I appreciate that you tried to answer it without suggesting (or stating) that I'm stupid or ill-trained for asking it in the first place. If it weren't for stupid questions, I probably wouldn't have any questions at all..."

Not stupid. Sorry for my terse analogies! Nobody knows everything! Not even me :oops:
 
I haven't installed one on a covered fuselage but if I had to do it. I wouldn't just rip the old one out I would either leave it in place or cut it and pull a string thru. Then just go at it one fairlead/pulley at a time. Remove the old cable from a section put the new one in its place and move on to the next section. If you pull it all out then you forget how it was routed around something and will be even more aggravated.
 
Thanks, 8G. I appreciate it, and also recognize that you were looking after my best interests with your answer.

Great tip,Bill. Just what I was looking for. I'm a little aggravated about having to try and replace this cable with the fabric on--new PA-18 trim system installed less than 400 hours ago, during recover.
 
Pilots need to stick together, even during disagreements. Hope you get the cable intalled soon!
 
Hopefully they put a big square inspection plate on the outside fabric right by the trim handle and pulleys. That saves pulling all of the interior. With a -12, it just depends who put the trim in and did the last rebuild. If you are thinking ahead, and the rebuild is designed right, it is not a big deal to change a trim cable. Fortunately for me, Mike Skup of @Mikes MCS Repair did my -12 build, so I am all set up if such occasion of cable replacement is needed. There are lots of things he did in my rebuild with maintenance in mind, as it should be. We proved that just this week, SWEET>>>> thanks Mike!
 
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I wish you the best. This is one of those situations where we all wish Piper had thought more about serviceability. Please let us know how it works out and any clever methods you learn as you puzzle through this repair. My fervent hope is that I will never face this repair.
 
Thanks, 8G. I appreciate it, and also recognize that you were looking after my best interests with your answer.

Great tip,Bill. Just what I was looking for. I'm a little aggravated about having to try and replace this cable with the fabric on--new PA-18 trim system installed less than 400 hours ago, during recover.

Sounds like something is rubbing on something it shouldn't.

Glenn
 
Remember, you posted on a public site that the trim was bad... now if you fly Big Brother can prove it was a preexisting condition.
 
Um guys..... the trim cable is a one piece loop. You can't cut it and pull it through like you're thinking. Sorry

Jason
 
Jason, as I was reading through this thread, I was sitting here thinking the exact same thing, wondering how he would get the one piece loop through the fairlead, etc.
 
2. Should I be worried about flying the plane while I'm waiting for the replacement cable? I won't use the trim (it is currently set at about 70). I also assume that, even if the trim cable came apart altogether, the horizontal stab would stay in its current position on the jack screw? In other words, is there any way the horizontal stab could travel to one trim stop or the other based only on air pressure from flight?
I will answer your 2nd question with an example. You can come up with your own conclusion. The Cessna 180/185 has the same type of trim system. There are 2 screw jacks with a cable/chain system which is controlled by a wheel vs a crank on the Cub. The trim wheel has a little spring loaded button/detent arraignment which helps hold the wheel steady. Normally a screw jack system is considered to be non-reversible. Once upon a time I was zipping along in my nearly new 185 in level trimmed flight. All of a sudden the trim ran to full nose up. Needless to say it took all of my effort to push the nose down which also required pulling the throttle back. I am thankful that It did not happen close to the ground and that it was a short trip back to the airport. What happened? There is a little roll pin which holds the trim wheel to it's shaft. The roll pin fell out! The fix was to replace the pin and safety it with a piece of safety wire through the pin and around the shaft. There has been no trouble since, but it could have been disastrous.

Now would I fly it without the cable connected? No way, unless I was stuck out in the boonies. Then I would find a way to safety lock the screw jack so that it could not turn. I can think of a few ways. l'll let you use your own imagination of how you would do this.
 
I have used the old cable to pull the new cable through the structure between the headliner and fabric on several Pacers and Tri-Pacers. Still have to have to get to each fairlead but I find it hard to push cable especially this small. I guess it is like that old saying about pushing a rope. ;)
 
I was overly casual in how I worded my first post. I have not flown the plane since I pulled the panels and found the frayed cable, and won't fly until the new cable is properly installed. Skywagon's story, and stories like it, are the reason I asked the question. Thanks for sharing your collective experience.

The fact that the cable is one piece is what makes the job so miserable, but the fact that the plane won't fly until we get it replaced should be enough incentive to push a rope through pulleys inside a covered fuselage.


PS--I'm with George on the Big Gubment thing. Just cuz you're paranoid doesn't mean they're not out to get you!
 
take lots of pictures of both ends, how cable is in there if double grove setup...... if not double grove set up, now might be the time to go to it.... there anothe thread on here on how I make a clamp in set of pulleys for it.... somewhere...
 
Hopefully they put a big square inspection plate on the outside fabric right by the trim handle and pulleys. That saves pulling all of the interior. With a -12, it just depends who put the trim in and did the last rebuild. If you are thinking ahead, and the rebuild is designed right, it is not a big deal to change a trim cable. Fortunately for me, Mike Skup of @Mikes MCS Repair did my -12 build, so I am all set up if such occasion of cable replacement is needed. There are lots of things he did in my rebuild with maintenance in mind, as it should be. We proved that just this week, SWEET>>>> thanks Mike!

Mike, Do you have pictures of what you do here?
 
I think it is easy on my pa 18s with stock interiors,takes time like the jack screw,might be time to do that while you are there,it is very easy nothing like bigger ac,no cable tensions to set,replace the springs,and pulleys,cheap to very easy keeps cable from slipping,if trim handle moves up and down time to put a bushing and pin.do it right it will last long and move easy.
 
Sounds like something is rubbing on something it shouldn't.

Glenn

In terms of double-groove pulleys, I have the FAD STC for the -18 trim. Looking at the diagrams, it doesn't show anything about the indicator cable, which I assume is still the stock assembly. Upon inspection, it looks like my indicator cable is tangled with the trim cable--which I assume is what caused the premature wear due to rubbing? Or, am I missing something here, and the indicator and trim cables are supposed to be wrapped together?

(Don't worry, someone who knows what they're doing will do the install. I'm just trying to puzzle through some of this for my own edification.)
 
wrap the cable around the pulleys at the jackscrew and idler pully. There are two small guide pulleys in the same location. Run cable through those. (I should have mentioned to remove the old cable from that location. Secure the new cable to the grooved pulley with a clamp or tape so it doesn't fall off. Attach new cable to the old cable and pull it through to the baggage/ cabin compartment. There shouldn't be any fairleads between the two small pulleys in the rear and the small fairlead at the baggage compartment. Make sure you don't twist the cables and from there everything is open.(after removing interior panels).Take before pictures so you know how to wrap and route cables around the double pulley in the back and the pulleys in the front. If you don't wrap the cables properly ,especially in the front , the cable will rub against itself and wear. Is this where your problem is? And no the indicator cable should not be wrapped around the trim cables at any point.

John
 
John, you nailed it. I don't think the cables were properly wrapped in the front when the original install was done. This appears to have caused severely premature wear, sawing through several strands of the trim cable. The frayed strands of the cable balled up in the fairlead and froze my trim. Amazing to me that it only took 350 flight hours (and a lot less hours of actual trim cable movement) to cut multiple strands of the trim cable.

The wear may be because of, or in addition to, the fact that the indicator cable is twisted with the trim cable forward of the baggage fairlead.
 
Darin, what's to keep you from putting an inspection hole(s) in the fuselage abeam any points where it seems impossible to do the threading? Although you won't be able to do as thorough a job as when a complete re-cover takes place(generally the inspection hole rings are covered with fabric "doilies" before painting) it will work, and will stay on if glued properly. Paint the cover the right color and it will look like it belongs there.

Andrew
 
You do not have to cut inspection holes other than whats original. Follow my procedure in my last post. I've done it several times. It works. Like the Keyocera comercial on TV: "It's not that complicated" lol.

John
 
Have my cub torn apart to fix a couple things. Here are some pics of behind the interior panels. Sorry, don't have any pics at the tail because the idiot who recovered it didn't put any inspection gromments in the fuselage.

IMG_7232.JPG


IMG_7234.JPG


IMG_7245.JPG


IMG_7246.JPG


IMG_7247.JPG
 
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