An examination of a failed cable removed from a Yak 50
We recently were asked to examine a failed cable, and what we discovered
was very disturbing as it has some very serious safety implications.
We got a call from an FBO/Maintenance facility. There was a Yak 50 on the
field, and during a storm, one of the rudder cables failed. The wire rope
had litterally pulled out of the swaged fitting - the fork which attaches
to the rudder horn.
Since we had never heard of such a situation before, we requested that the
cable be sent up so we could examine it.
We should explain a bit about the swaging process. The fitting to be swaged
has to be annealed after machining so that it is as soft as possible. The
wire rope slides in, the fitting is staked so it cannot move, and then run
into the swaging machine (of which there are many types).
The cold forming process turns the wire rope and swaged shank into a
monolithic structure. The wire rope retains it's form, but is compressed.
The swaged shank is formed down around the wire rope and the bore actually
takes the form of the wire rope. The ID takes the mirror image of the wire
If you section this (both directions) you can see how all the elements (the
strands of the wire rope and the metal of the shank) have been brought
together to eliminate all spaces, but there is no crushing of the wires.
The bore of the shank section of the fork was examined with a 5X loop.
It was completely smooth, with no imprint of the wire rope at all.
The wire rope itself was somewhat compressed but undamaged.
It is clear that this particular cable was never proof loaded.
It never would have passed.
How did this happen?
We next started to examine the fittings.
Usually, the shank portion of the fitting is not completely swaged up to
the next section, so you can compare the before and after sections.
This is what we found: the unswaged section was ~.235 diameter, and the
swaged section was ~ .219.
Now, .219 indicates the use of a very common (-4) die size, but the
fittings that are run thru these dies start at .250 diameter. This is about
a 12 1/2% reduction in size.
If you start w/ .235 and bring it down to .219, this is less than a 7%
reduction in size.
The differences between a properly swaged fitting, and the failed fitting
are quite evident as shown in the photos.
What is less clear is the photo which shows the elevator cables.
One fork started at .245 diameter, and the other started at .238 diameter.
There is really only one way to tell if the cables are airworthy, and that
is a proof load.
We find it improbable that these particular cables are unique and
recommend that similar aircraft from Eastern Europe (? but we really don't
know where these cables came from, do we) be carefully examined for proper
swaging of the fittings to the wire rope.
You should carefully consider the downside of a decision about this. This
cable failed while the aircraft was tied down. What if?
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