Control Cable Repair

[bdflyer]

In my BD-4C, I have a control cable which runs from the foot pedals back to control the rudder. In this cable, I accidentally found "minor" damage, where three strands were popped out. Since the plane is still under construction, I'd like to repair it while I have easy access.

Now, this cable is Spruce 05-04300 (Stainless, 1/8", 7x19). AC43.13 suggests a Type B splice (Figure 7-12). However, this may be an overkill. Replacing the entirety of the cable is not really an option given it would need to be fished.

Big questions:

• With a three strand damage profile, is there an approach other than a cut and then a double-splice which can be undertaken at lower complexity?
• Is a B splice the better option, or should an alternative be used? Perhaps an A splice (AN667/668)? Pros/cons either way?
• Is there a part or component which would connect openly onto the cable over the minor damage and remove the need to cut it and splice it?

Thanks!

[BoKu]

For something like this, I think that I would replace the entire cable unless I was certain that there was absolutely no way of towing the new one into place with the old one. Splices have a way of hanging up on things during maintenance and operation.

Note that in order to do the B splice, you need enough extra cable to overlap the two ends by about 2.5". That means that you need to find or create 2.5" of slack somewhere in the system, or you need to replace either the forward or aft ends of the cable with a piece that's at least 2.5" longer.

As for the A splice, that is a non-starter unless someone in your area has a Kearney-type swage machine that they can fit into your airplane to the work.

The C splice is a bit ambiguous, as it appears to rely on a braiding technique that is no longer depicted in AC43.13.

Thanks, Bob K.

In my BD-4C, I have a control cable which runs from the foot pedals back to control the rudder. In this cable, I accidentally found "minor" damage, where three strands were popped out. Since the plane is still under construction, I'd like to repair it while I have easy access.

Now, this cable is Spruce 05-04300 (Stainless, 1/8", 7x19). AC43.13 suggests a Type B splice (Figure 7-12). However, this may be an overkill. Replacing the entirety of the cable is not really an option given it would need to be fished.

Big questions:

• With a three strand damage profile, is there an approach other than a cut and then a double-splice which can be undertaken at lower complexity?
• Is a B splice the better option, or should an alternative be used? Perhaps an A splice (AN667/668)? Pros/cons either way?
• Is there a part or component which would connect openly onto the cable over the minor damage and remove the need to cut it and splice it?

Thanks!

[bdflyer]

We have been talking through both options. On the B slice side, this can be done, as half of the run is in clear air. There is no risk of entanglement (aka: no material collision between systems) at this location. Still, its not the preferred path.

On the "towing" note, is there a particular procedure that would be generally successful to tow a cable through four grommets? Two are in an area with no access, so its a tough one. I have the cable to do it, I think.

[BoKu]

...On the "towing" note, is there a particular procedure that would be generally successful to tow a cable through four grommets? Two are in an area with no access, so its a tough one. I have the cable to do it, I think.

Applicable techniques are limited only by your imagination and the consequences of failure. I've seen folks have perfect success using electrical tape.

If I had to tow a new cable into place blindly through a tailcone, and the consequences of failure were cutting and slicing holes in the fuselage skin, here's what I'd do. Note that this only works where you have at least 5/32" cable clearance, and probably won't pull a cable into place over a pulley:

1. Obtain a piece of brass tubing, 2" long, 5/32" OD 1/64" wall. Probably from McMaster-Carr. Hobby shops usually have it too, but those are thin on the ground these days.
2. Completely degrease the ends of the old and new cables for a distance of at least an inch. I'd use nasty EPA-disapproved brake cleaner for that.
3. Degrease the ID of the brass tube, and scuff it with a brass barrel brush (or similar) for tooth.
4. Epoxy the ends of the cable into the opposite ends of the brass tube. I'd use EA9430 for this since I keep some at the shop and it is about the strongest epoxy there is. But any decent epoxy other than the 5-minute blends ought to do. Let cure overnight at 70F or warmer.
5. Pull-test the brass tube splice to make sure it accommodates the force necessary to cross fairleads etc.
6. Tow the new cable into place with the old one, using the brass tube and epoxy splice.
7. Cut away the cable in the brass tube splice, including the entirety of cable that you degreased. Stranded cables rely on the interstrand lubrication applied at the factory.
8. Apply field eyes to the ends using thimbles and copper compression sleeves as necessary.

--Bob K.

[bdflyer]

That's a cute approach actually. Thanks!

[Frank Giger]

Boku has the right of it.

[Sam Buchanan]

A 1/16" nico-press swager might put a pretty decent crimp on that copper tubing as well.

[weedhopr]

What we often do at work when replacing aircraft cables is to attach .025" safety wire, or wire lacing cord to the end of the old cable. Pull out the old cable, and the wire or cord follows. Then attach the wire or cord to the end of the new cable, and pull the cord or wire, pulling the new cable in place following the same route as the old cable.
Mike