DIY - Rudder Cable Replacement

[admin]

Hey guys.... a bit off topic here.... I hope you understand that I will clean up this thread in a few days and delete a few of the messages once I know they have been received. Btw, we do have a PM button under each message

Thank you!

[Antoine]

Today I was at a maintenance shop (we're becoming well acquainted...) and we talked about the upcoming rudder cable replacement.
They had just done it on a 2006 DA40. They showed me the cables and sure enough they were worn out to the point of being unserviceable!
I couldn't believe it , but these things were flattened and polished from wear on one "side". He told me that if they were bent they would eventually break.

So - no regrets - mine are coming out. And I would suggest our DA42 friends take a hard look at this too!

[Chris B]

What a terrific post, Rick! Thank you!

I am curious why the DA40 (or DA42) is susceptible to this issue. Rudder cable technology has been around for 100+ years, and from looking at the photos and manual it is not at all obvious why the rudder cables would wear out.

Is it due to the sleeves, pedal adjustment, or what?

Chris

[ihfanjv]

I think the general consensus answer is that perhaps the FAA questioned the longevity of the cables in the DA40 during the certification process, and rather than engaging in a lengthy test and review process on the issue Diamond and the FAA agreed to a random time period for cable replacement - a time period where they mutually agreed that there would be certainty that there would be no wear issues.

On the other hand, you hear sporadic reports of some cables showing wear at the 5 year replacement intervals - although I believe that almost all owners find that the cables they are cutting and replacing look brand new. The general consensus appears to be that the cable replacement requirement is actually completely unnecessary (but still required anyway).

Unfortunately, I don't think the Diamond community as the 100% correct answer to this question.

[David B]

I have to agree, fabulous overview and thorough explaination of this replacement.

Although this will be only my second post I find this forum invaluable to the owners of this type and have extracted tons of useful information off of it since joining. To all, keep up the good work, and thank you.

With respect to the rudder cable issue, and as a structural engineer, I have taken Diamond to task on this matter for a couple of years now. Having been forced to replace brand new rudder cables with brand new rudder cables last year, it allowed me to really see how utterly ridiculous this manditory change out was. Not only is it costly, it generally takes the plane off line for longer than most of us would like.

As a Canadian pilot, I contacted Diamond directly to voice my concerns. For the most part, there is no reason minor design changes cannot be made to these cables to allow easy access for inspection. The weak point is the "S" tubes and the concern of wear in these specific cable sections which are enclosed and unable to be inspected without removal. However, for those of us, which I suspect are the majority of us, that never move the rudder pedals the wear factor is zero. Given the fact that the cable size with respect to the mechanical advantage at the rudder for even the strongest of pilots is negligible, we have a requirement that, from a pure engineering standpoint, is crazy.

Apparently Diamond is listening to us and I have recieved a letter from Will Hawkes (Customer Support Manager with Diamond) who has attempted to address this concern for me directly. Below are his direct comments to me in an e-mail dated February 2, 2012.

"Rudder Cables – You are not the only customer who has asked us to amend the requirement to change the rudder cables on the DA40-180, and you can be assured that this feedback has been received and understood by DAI. Although I don’t foresee the requirement to change the rudder cables being removed entirely, it is possible that the calendar life limit may be extended. Again, the appropriate personnel here at DAI are aware of your opinion, and we appreciate you taking the time to communicate this."

Mr. Hawkes copied this to the "head of engineering" and the "VP of Operations" at diamond. As with anything in life, the squeaky wheel gets the grease, so I would ask that the rest of my fellow DA40 owners take the time to voice your concerns directly with Diamond and if anyone needs contact info, please do not hesitate to contact me.

[Chris B]

The weak point is the "S" tubes and the concern of wear in these specific cable sections which are enclosed and unable to be inspected without removal. However, for those of us, which I suspect are the majority of us, that never move the rudder pedals the wear factor is zero.

Can you please expand a bit on this? I am still trying to figure out what can cause premature rudder cable wear.

[David B]

The "S" tubes which house the rudder control cables at your feet are the mechanizm that allows you, in a Diamond, to adjust the rudder pedals fore and aft depending on your leg length. These tubes have been designed as totally enclosed, and without any sort of friction reducing lining, such as nylon. Therefore, as the pilot, or passenger, adjust the pedals the section of rudder cable enclosed in these tubes is forced to slide internally each time the pedals are moved. This is a metal on metal movement and thus there have been reports of wear to the cables within these tubes.

The above explains the main wear point to the entire system and thus, the main reason (I assume) for Diamond to force the change out. However, and to my earlier point, for pilots like myself who own the plane and are the only pilot, the pedals are never moved and thus, there is no wear on these cables. I recognize that planes used by a multitude of pilots would undergo wear, but for the majority of us, this is simply not the case.

All that being said, I still believe there is a simple solution that would allow every inch of these cables to be inspected on a 5 year basis without the need to remove them and throw away perfectly new cables. At the very least, and for those of us who do alter the rudder pedal locations regularly, we should only be changing out the cables that travel through the "S" tubes alone and inspect the rest of the sytem.

Hope that helps clarify things....?

[Chris B]

Hope that helps clarify things....?

Yes, thank you!

I suspect that even a small amount of the right lubricant would dramatically reduce wear inside the S-tubes.
FWIW, I've had great success with Dupont Teflon Multi-Use dry lubricant, which does not attract dust.
Link to Dupont site

[Steve]

Either the Diamond Annual Checklist, or the lubrication guidelines in the AMM (I can't remember which) specifies lubricating the rudder pedal S-tubes with a drop of oil. I have done this each year. The other section of the cables which cannot be inspected is in the tail boom (but I wouldn't expect much wear in there).

Steve

[David B]

Diamond has encapsulated the tail boom cables in a nylon sheath. The sheath is opaque and you cannot clearly see the cables within. For the life of me, while changing out my own cables, I could not understand why they did it this way. The majority of the tail cables could have easily been partially enclosed in a nylon guide (or nothing at all) that would allow the entire cable to be inspected simply by moving the rudder from one side to the other. The cables in my old Cessna ran through the tail totally exposed for inspection and that technology is 40+ years old?

Bottom line, Diamond's design of these cables left us with a very expensive item to deal with, when some simple changes would have negated the need to throw perfectly new cables in the garbage.