Blanik L-13 elevator cable replacement

Greetings, group!

Our local club has a pair of L-13s. In one of them, we recently
discovered a badly-corroded elevator cable. The corrosion area was
located was approx. 15" aft of the bellcrank attachment, under the
port-side, aft cockpit flooring -- *very* difficult to get to. It
would appear from the extent of the corrosion that this particular
spot was not noticed during annual- and/or 100-hour inspections.

First, a heads-up to other Blanik L-13 operators: you may want to take
a really careful look at the state of your elevator cables, especially
in that 15"-aft-of-bellcrank spot on the port side cable. The small,
semi-enclosed space in this area of the cable run is both A/
impossible to see without flashlights and mirrors (or an endoscope),
and B/ exposed to moisture from e.g. wheel-thrown debris and cockpit
condensation.

Second: we've been told that new cables will cost $490.00 each (!)
from Videck, and take upwards of 3 weeks to get here. I know aviation
parts are rarely inexpensive, but $500+ (with shipping) for a 20-foot,
3/16" galvanized steel cable with eyes and swages seems exccessive;
similarly-built Piper and Cessna control cables, while admittedly
somewhat shorter, tend to run about $100.

Does anyone know of a reason why a less costly option, like ordering
some custom cables from Aircraft Spruce, wouldn't be legal/safe/
practical? Are there any other suggestions that might help our little
non-profit club avoid or mitigate a potential bill of $2,000.00 for 4
cables, and a month-long grounding of our Blaniks while we wait for
them to arrive?

Thanks in advance,

Brad

On Apr 21, 10:44*am, Brad wrote:

...Does anyone know of a reason why a less costly option, like ordering
some custom cables from Aircraft Spruce, wouldn't be legal/safe/
practical? *Are there any other suggestions that might help our little
non-profit club avoid or mitigate a potential bill of $2,000.00 for 4
cables, and a month-long grounding of our Blaniks while we wait for
them to arrive?

Brad,

I would suggest that you contact McFarlane Aviation, from whom I have
sourced custom cables in the past. They also sent me a free T-shirt
with the last order, so I must have full-on BTDT.

As for the bureaucracy aspect, as usual that issue depends on who you
talk to, how, and when. My suggestion would be to get an A&P you can
work with who would be willing to install the cables as owner-produced
parts as specifically allowed under 14CFR§21.303(b)2:

http://ecfr.gpoaccess.gov/cgi/t/text...1.11.2&idno=14

As usual, my favorite article on owner-produced parts by Don Dodge
describes the process and offers a cautionary tale:

http://150cessna.tripod.com/parts.html

Your "owner's" contribution to the manufacture could be as simple as
creating the diagrams and specifications that McFarlane (or whoever).

Probably the greatest hurdle you will encounter is that the Blanik
will have metric cables and fasteners, and McFarlane or whoever will
probably only have inch-sized MS-series cable, swages, and fittings.
So you either have to bite the bullet and source the cable from a
metric-capable cable firm, or you have to develop some way of adapting
inch-sized cable fittings to horns with metric holes. If the horns
have generous edge clearance, it could be a simple as drilling 6mm
(0.236") holes up by .014" to 1/4". Or it could be much more
complicated. Regardless, that is the sort of thing that you absolutely
must first square away with the A&P or IA who signs off the
installation and subsequent inspections.

In the end, you may find it more effective and expedient to just pay
the $500 per cable.

Thanks, Bob K.

In the end, you may find it more effective and expedient to just pay
the $500 per cable.


I agree with above - Nothing is as easy as it looks especially if you
have not BTDT.
However, if one were to design your own cable, how do you go about
getting the specs and properites of the original cables and the end
fittings turnbuckles, ect? Sure, one could generously oversize the
cable so it "looks" right, and get someone to buy it off, but will it
fit the pulleys and pass thoughs in the aircraft?

I have this week a need for another part fron Vitek, any way one could
request expedited shipping? Also, there are some scrapped ships I
know of - check with some aircraft salvage yards

aerodyne

On Apr 21, 12:13*pm, wrote:

However, if one were to design your own cable, how do you go about
getting the specs and properites of the original cables and the end
fittings turnbuckles, ect? *

That's actually one of the easier aspects. The design of cable-
actuated control systems is very well understood, and there is plenty
of engineering guidance in standard design and maintenance texts.

I'd probably start with AC43.13; in change 1B of that advisory
circular cables are covered in some depth in Chapter 7. The maximum
strength of the various sizes and configurations are shown in table
7-4, and paragraph 7-148(a) advises that "Swage-type terminals,
manufactured in accordance with AN, are suitable for use in civil
aircraft up to, and including, maximum cable loads."

As for reverse-engineering the properties of the LET original cable,
I'd probably begin with the reasonable assumption that the strength of
the cable is typical of aircraft-grade control cable of similar
configuration and cross-sectional area. That's based on the equally
reasonable assumption that they were't making Blaniks out of
Unobtanium in Soviet-era Czechoslovakia.

But, just to make sure, I'd probably also fabricate a test article
using a piece of the original cable with at least one original
terminal and pulling it to tensile failure in my tensile test machine.
If AC43.13 change 1B table 7-4 is to be believed, I can pull to
failure any cable assembly up to and including 9/32" (0.281" or
7.14mm), and from a design perspective I'd expect the elevator cable
to be about half that size.

Thanks again, Bob K.

I don't agree...

The problem is you are leading folks down a slippery slope. There is
more to meeting the design intent of a certificated part than
conservative assumptions and good intentions. One thing I know from
parsing JAR-22 and older European design criteria is that they do
things differently, mainly safe life vs fail safe. They still have
this approach today, one modern glider requires ALL cables to be
replaced every 10 years.

Have you ever compared the countersink depth and angle from the "5"
rivet on the Blanik with a NAS1097 or
NASM20426? I did the layout, and they are not the same geometry. I
have never seen the proper factory rivets used in a Blanik repair, and
the folks doing the work are completely unaware of the nuances of
bearing properties of mismatched flush rivet in csk or dimpled
structure. They would not know the principle of bearing bypass if it
hit them upside the head.

If one does not have skills or experience, "designing" something based
on a single data point from a used, damaged old part does not meet the
intent of the FAR's or certification criteria, even if you do have a
tensile tester and know how to use it AND reduce the data to "B"
basis, ect.

You need the original design data and specs, and Blanik has extensive
overhaul, repair, and maintenance manuals. BTW, AC-43-13 cannot be
used if you can't ensure that the data therein is not contrary to the
Mfg data.

Lets encourage folks to support the Mfg and buy the right parts if
available. lf we all did that, perhaps they will stay in business and
the prices will come down.

aerodyne

On Apr 21, 5:21*pm, wrote:

...Have you ever compared the countersink depth and angle from the "5"
rivet on the Blanik with a NAS1097 or NASM20426? *I did the layout, and
they are not the same geometry...

"Aerodyne," that's the other thread. This one is about the elevator
cables.

Thanks, Bob K.

Brad wrote:
Greetings, group!

Our local club has a pair of L-13s. In one of them, we recently
discovered a badly-corroded elevator cable. The corrosion area was
located was approx. 15" aft of the bellcrank attachment, under the
port-side, aft cockpit flooring -- *very* difficult to get to. It
would appear from the extent of the corrosion that this particular
spot was not noticed during annual- and/or 100-hour inspections.

First, a heads-up to other Blanik L-13 operators: you may want to take
a really careful look at the state of your elevator cables, especially
in that 15"-aft-of-bellcrank spot on the port side cable. The small,
semi-enclosed space in this area of the cable run is both A/
impossible to see without flashlights and mirrors (or an endoscope),
and B/ exposed to moisture from e.g. wheel-thrown debris and cockpit
condensation.

Second: we've been told that new cables will cost $490.00 each (!)
from Videck, and take upwards of 3 weeks to get here. I know aviation
parts are rarely inexpensive, but $500+ (with shipping) for a 20-foot,
3/16" galvanized steel cable with eyes and swages seems exccessive;
similarly-built Piper and Cessna control cables, while admittedly
somewhat shorter, tend to run about $100.

Does anyone know of a reason why a less costly option, like ordering
some custom cables from Aircraft Spruce, wouldn't be legal/safe/
practical? Are there any other suggestions that might help our little
non-profit club avoid or mitigate a potential bill of $2,000.00 for 4
cables, and a month-long grounding of our Blaniks while we wait for
them to arrive?

Thanks in advance,

Brad

I have no special insights other than a well-developed wallet nerve
about aircraft ownership costs. There are general design rules about
control surfaces: one that comes to mind is the design load specified
for rudder pedals (for example): shall withstand 100 lb force on each
pedal, with a 150% design factor (if my memory serves).

If a person wanted to select the appropriate aircraft cable and the
appropriate terminals and found a competent Yacht chandler and gave
EXACT measurements or preferably provided the old cables with a stretch
allowance, AND above all, carried out a proof test on the manufactured
cable AND kept it off the books, then costs would be reduced.

Thankfully, I can simply ask my A&P to fit the replacement rudder cables
he is ordering for me, when they arrive. As you note - for popular types
the costs are not out of line for approved components.

Introducing a worry factor of this kind, when the pilot is going to
seek out bumpy conditions would not be a happy prospect, to my way of
thinking.

Brian W

On Apr 21, 11:44*am, Brad wrote:
Greetings, group!

Our local club has a pair of L-13s. *In one of them, we recently
discovered a badly-corroded elevator cable. *The corrosion area was
located was approx. 15" aft of the bellcrank attachment, under the
port-side, aft cockpit flooring -- *very* difficult to get to. *It
would appear from the extent of the corrosion that this particular
spot was not noticed during annual- and/or 100-hour inspections.

First, a heads-up to other Blanik L-13 operators: you may want to take
a really careful look at the state of your elevator cables, especially
in that 15"-aft-of-bellcrank spot on the port side cable. *The small,
semi-enclosed space in this area of the cable run is both A/
impossible to see without flashlights and mirrors (or an endoscope),
and B/ exposed to moisture from e.g. wheel-thrown debris and cockpit
condensation.

Second: we've been told that new cables will cost $490.00 each (!)
from Videck, and take upwards of 3 weeks to get here. *I know aviation
parts are rarely inexpensive, but $500+ (with shipping) for a 20-foot,
3/16" galvanized steel cable with eyes and swages seems exccessive;
similarly-built Piper and Cessna control cables, while admittedly
somewhat shorter, tend to run about $100.

Does anyone know of a reason why a less costly option, like ordering
some custom cables from Aircraft Spruce, wouldn't be legal/safe/
practical? *Are there any other suggestions that might help our little
non-profit club avoid or mitigate a potential bill of $2,000.00 for 4
cables, and a month-long grounding of our Blaniks while we wait for
them to arrive?

Thanks in advance,

Brad

Sorry Brad, all I can offer is tea and sympathy. I went through this
a year ago with one of our club Blaniks, but the cost was higher
because we had to do both rudder and elevator cables. The problem
with Blaniks is that time and metal are not our friends. None of the
L-13's is less than thirty years old. I've come to believe that the
reason that they are so cheap to buy is that the market has factored
in the cost of maintaining them. I haven't done the calculations, but
its my general impression that Blanik parts prices have about
quadrupled since 2005. And as one aircraft mechanic pointed out to me,
they were designed in the 50's when labor was cheap, so no thought was
given to minimizing repair labor effort.
At least your parts are in stock. We had one damaged in a landing
accident and the parts required had to be made up by the factory. The
order was placed in mid September and we received them in mid
February.
I suspect that a lot of this has to do with the fact that when the
Czech republic came out of a communist system, parts prices were
artificially low, and now that they fully realize the price required
to sustain a profitable manufacturing and inventory operation, they've
swung all the way the other way to excessive prices.
When you also look at what's happening with DG, you start to wonder if
the cost of sustaining legacy aircraft isn't a serious long term issue
that constitutes yet another threat to the viability of our sport.
Sandy Stevenson