Rudder Cable Systems Used in Modern Sailplanes

There seems to be a fair amount of confusion about the springs used in almost all modern sailplane rudder cables. The springs are there to keep the pedals upright (pulling forward) and to facilitate rudder pedal adjustment through the use of an 'S' tube. Each rudder cable enters the top rear of its 'S' tube and exitsthe lower forward portion of the 'S' tube, then extends on forward another foot or so and is bolted to the square tube that both pedals slide on when adjusting the pedals forward and aft. One can simulate a broken cable by simply grasping one rudder pedal and pulling it aft about 6 inches. The spring on that side will expand and the rudder cable will go slack. The spring on the other side will slam the rudder hard over..........I just did this on my ship. The same is true for Schleicher, Schempp-Hirth, DG, LS and most all sailplanes!

Some things to check on annual inspection:

1. Nylon inner sleeve in place on both 'S' tubes and extending about an inch out both ends of the 'S'tube.(the nylon sleeve keeps the cable from rubbing metel to metal during normal rudder operation and during pedal adjustment.

On Thursday, May 10, 2012 5:54:15 PM UTC-7, JJ Sinclair wrote:
There seems to be a fair amount of confusion about the springs used in almost all modern sailplane rudder cables. The springs are there to keep the pedals upright (pulling forward) and to facilitate rudder pedal adjustment through the use of an 'S' tube. Each rudder cable enters the top rear of its 'S' tube and exitsthe lower forward portion of the 'S' tube, then extends on forward another foot or so and is bolted to the square tube that both pedals slide on when adjusting the pedals forward and aft. One can simulate a broken cable by simply grasping one rudder pedal and pulling it aft about 6 inches. The spring on that side will expand and the rudder cable will go slack. The spring on the other side will slam the rudder hard over...........I just did this on my ship. The same is true for Schleicher, Schempp-Hirth, DG, LS and most all sailplanes!

Some things to check on annual inspection:

1. Nylon inner sleeve in place on both 'S' tubes and extending about an inch out both ends of the 'S'tube.(the nylon sleeve keeps the cable from rubbing metel to metal during normal rudder operation and during pedal adjustment.

2. Springs in place and pulling equal (I measured 9# spring tension on my Genesis)

3. Adjust the rudder pedals full forward and inspect the cable at the top rear 'S' tube. (this is the most likely place to find cable wear)

4. Adjust the rudder pedals full aft and inspect for cable wear at the point where it exits the lower forward 'S' tube.

5. Inspect the weld that attaches the rudder pedal to the rotating tube at the bottom. In an accident, the pilot may have ended up standing on both rudder pedals as the ship came to an abrupt stop. I have found cracked welds due to this and several AD's have been issued on this problem.

6.Inspect the Nicopress sleeve and inner protective eye (thimble) and the bolt that attaches the cable to the rudder assembly. It must have lock-nut or castlenated nut & carter pin.

7. Inspect as much of the cable runs as you can see and don't forget to check the other end where the cable attaches to the rudder, sleeve/bolt nut or pin.

8. Check rudder stops. They may be at the rudder or on the pedals, but must stop full rudder movement before rudder strikes fin or pedals hit fuselage sides.

Most manufactures recommend not using lube on the rudder cables because it attracts dirt and can lead to unnecessary cable wear, but do lube the rudder pedals and rudder hinges. One more little tid-bit, you can adjust the angle at which the rudder pedals sit by simply making the rudder cables longer..........OK, JJ just how the hell do I do that? By inserting two 1" steel carobeaners in the cable run at the forward attach point. This will make both rudder pedals rotate forward about 30 degrees. Some pilots object to the straight up angle of the pedals in ships like the Duo-Discus. Make sure you still have full rudder movement before rudder pedals hit fuselage side..
Cheers,
JJ A&P for 40 years + owner/operator of glider repair station.

Thanks JJ!
Can rust be also an issue ? After all we can't inspect most of the rudder cable.

Ramy

On May 10, 6:30*pm, Ramy wrote:
On Thursday, May 10, 2012 5:54:15 PM UTC-7, JJ Sinclair wrote:
There seems to be a fair amount of confusion about the springs used in almost all modern sailplane rudder cables. The springs are there to keep the pedals upright (pulling forward) and to facilitate rudder pedal adjustment through the use of an 'S' tube. Each rudder cable enters the top rear of its 'S' tube and exitsthe lower forward portion of the 'S' tube, then extends on forward another foot or so and is bolted to the square tube that both pedals slide on when adjusting the pedals forward and aft. One can simulate a broken cable by simply grasping one rudder pedal and pulling it aft about 6 inches. The spring on that side will expand and the rudder cable will go slack. The spring on the other side will slam the rudder hard over...........I just did this on my ship. The same is true for Schleicher, Schempp-Hirth, DG, LS and most all sailplanes!

Some things to check on annual inspection:

1. Nylon inner sleeve in place on both 'S' tubes and extending about an inch out both ends of the 'S'tube.(the nylon sleeve keeps the cable from rubbing metel to metal during normal rudder operation and during pedal adjustment.

2. Springs in place and pulling equal (I measured 9# spring tension on my Genesis)

3. Adjust the rudder pedals full forward and inspect the cable at the top rear 'S' tube. (this is the most likely place to find cable wear)

4. Adjust the rudder pedals full aft and inspect for cable wear at the point where it exits the lower forward 'S' tube.

5. Inspect the weld that attaches the rudder pedal to the rotating tube at the bottom. In an accident, the pilot may have ended up standing on both rudder pedals as the ship came to an abrupt stop. I have found cracked welds due to this and several AD's have been issued on this problem.

6.Inspect the Nicopress sleeve and inner protective eye (thimble) and the bolt that attaches the cable to the rudder assembly. It must have lock-nut or castlenated nut & carter pin.

7. Inspect as much of the cable runs as you can see and don't forget to check the other end where the cable attaches to the rudder, sleeve/bolt nut or pin.

8. Check rudder stops. They may be at the rudder or on the pedals, but must stop full rudder movement before rudder strikes fin or pedals hit fuselage sides.

Most manufactures recommend not using lube on the rudder cables because it attracts dirt and can lead to unnecessary cable wear, but do lube the rudder pedals and rudder hinges. One more little tid-bit, you can adjust the angle at which the rudder pedals sit by simply making the rudder cables longer..........OK, JJ just how the hell do I do that? By inserting two 1" steel carobeaners in the cable run at the forward attach point. This will make both rudder pedals rotate forward about 30 degrees. Some pilots object to the straight up *angle of the pedals in ships like the Duo-Discus. Make sure you still have full rudder movement before rudder pedals hit fuselage side.
Cheers,
JJ A&P for 40 years + owner/operator of glider repair station.

Thanks JJ!
Can rust be also an issue ? After all we can't inspect most of the rudder cable.

Ramy- Hide quoted text -

- Show quoted text -

Great post - not all have S tubes, the cirrus mechanisim uses pulleys
on a sliding rack, as does the 1-34 and perhaps others. Aircraft
cables have a parrafin based lube during mfg, and should not be
cleaned with solvents or relubricated. Rust can occur in severe
situations, but wire breakage/wear in the bends is more commom.
AC43-13 give good examples of wear, damage, repair and acceptable
limits.

I think not only is the "carobeaner" a great idea, but would make a
good name for a health food...

;-)

aerodine

On Thursday, May 10, 2012 6:30:06 PM UTC-7, Ramy wrote:
On Thursday, May 10, 2012 5:54:15 PM UTC-7, JJ Sinclair wrote:
There seems to be a fair amount of confusion about the springs used in almost all modern sailplane rudder cables. The springs are there to keep the pedals upright (pulling forward) and to facilitate rudder pedal adjustment through the use of an 'S' tube. Each rudder cable enters the top rear of its 'S' tube and exitsthe lower forward portion of the 'S' tube, then extends on forward another foot or so and is bolted to the square tube that both pedals slide on when adjusting the pedals forward and aft. One can simulate a broken cable by simply grasping one rudder pedal and pulling it aft about 6 inches. The spring on that side will expand and the rudder cable will go slack. The spring on the other side will slam the rudder hard over...........I just did this on my ship. The same is true for Schleicher, Schempp-Hirth, DG, LS and most all sailplanes!

Some things to check on annual inspection:

1. Nylon inner sleeve in place on both 'S' tubes and extending about an inch out both ends of the 'S'tube.(the nylon sleeve keeps the cable from rubbing metel to metal during normal rudder operation and during pedal adjustment.

2. Springs in place and pulling equal (I measured 9# spring tension on my Genesis)

3. Adjust the rudder pedals full forward and inspect the cable at the top rear 'S' tube. (this is the most likely place to find cable wear)

4. Adjust the rudder pedals full aft and inspect for cable wear at the point where it exits the lower forward 'S' tube.

5. Inspect the weld that attaches the rudder pedal to the rotating tube at the bottom. In an accident, the pilot may have ended up standing on both rudder pedals as the ship came to an abrupt stop. I have found cracked welds due to this and several AD's have been issued on this problem.

6.Inspect the Nicopress sleeve and inner protective eye (thimble) and the bolt that attaches the cable to the rudder assembly. It must have lock-nut or castlenated nut & carter pin.

7. Inspect as much of the cable runs as you can see and don't forget to check the other end where the cable attaches to the rudder, sleeve/bolt nut or pin.

8. Check rudder stops. They may be at the rudder or on the pedals, but must stop full rudder movement before rudder strikes fin or pedals hit fuselage sides.

Most manufactures recommend not using lube on the rudder cables because it attracts dirt and can lead to unnecessary cable wear, but do lube the rudder pedals and rudder hinges. One more little tid-bit, you can adjust the angle at which the rudder pedals sit by simply making the rudder cables longer..........OK, JJ just how the hell do I do that? By inserting two 1" steel carobeaners in the cable run at the forward attach point. This will make both rudder pedals rotate forward about 30 degrees. Some pilots object to the straight up angle of the pedals in ships like the Duo-Discus. Make sure you still have full rudder movement before rudder pedals hit fuselage side.
Cheers,
JJ A&P for 40 years + owner/operator of glider repair station.

Thanks JJ!
Can rust be also an issue ? After all we can't inspect most of the rudder cable.

Ramy

Most aircraft cables are galvanized and should resist rust OK. I have never seen rust that became an issue. A 1/8" cable is rated at more than 1000 pounds, so a little rust is OK, but worn or broken strands are not.
JJ

On May 11, 1:42*am, JJ Sinclair wrote:
On Thursday, May 10, 2012 6:30:06 PM UTC-7, Ramy wrote:
On Thursday, May 10, 2012 5:54:15 PM UTC-7, JJ Sinclair wrote:
There seems to be a fair amount of confusion about the springs used in almost all modern sailplane rudder cables. The springs are there to keep the pedals upright (pulling forward) and to facilitate rudder pedal adjustment through the use of an 'S' tube. Each rudder cable enters the top rear of its 'S' tube and exitsthe lower forward portion of the 'S' tube, then extends on forward another foot or so and is bolted to the square tube that both pedals slide on when adjusting the pedals forward and aft. One can simulate a broken cable by simply grasping one rudder pedal and pulling it aft about 6 inches. The spring on that side will expand and the rudder cable will go slack. The spring on the other side will slam the rudder hard over...........I just did this on my ship. The same is true for Schleicher, Schempp-Hirth, DG, LS and most all sailplanes!

Some things to check on annual inspection:

1. Nylon inner sleeve in place on both 'S' tubes and extending about an inch out both ends of the 'S'tube.(the nylon sleeve keeps the cable from rubbing metel to metal during normal rudder operation and during pedal adjustment.

2. Springs in place and pulling equal (I measured 9# spring tension on my Genesis)

3. Adjust the rudder pedals full forward and inspect the cable at the top rear 'S' tube. (this is the most likely place to find cable wear)

4. Adjust the rudder pedals full aft and inspect for cable wear at the point where it exits the lower forward 'S' tube.

5. Inspect the weld that attaches the rudder pedal to the rotating tube at the bottom. In an accident, the pilot may have ended up standing on both rudder pedals as the ship came to an abrupt stop. I have found cracked welds due to this and several AD's have been issued on this problem.

6.Inspect the Nicopress sleeve and inner protective eye (thimble) and the bolt that attaches the cable to the rudder assembly. It must have lock-nut or castlenated nut & carter pin.

7. Inspect as much of the cable runs as you can see and don't forget to check the other end where the cable attaches to the rudder, sleeve/bolt nut or pin.

8. Check rudder stops. They may be at the rudder or on the pedals, but must stop full rudder movement before rudder strikes fin or pedals hit fuselage sides.

Most manufactures recommend not using lube on the rudder cables because it attracts dirt and can lead to unnecessary cable wear, but do lube the rudder pedals and rudder hinges. One more little tid-bit, you can adjust the angle at which the rudder pedals sit by simply making the rudder cables longer..........OK, JJ just how the hell do I do that? By inserting two 1" steel carobeaners in the cable run at the forward attach point. This will make both rudder pedals rotate forward about 30 degrees. Some pilots object to the straight up *angle of the pedals in ships like the Duo-Discus. Make sure you still have full rudder movement before rudder pedals hit fuselage side.
Cheers,
JJ A&P for 40 years + owner/operator of glider repair station.

Thanks JJ!
Can rust be also an issue ? After all we can't inspect most of the rudder cable.

Ramy

Most aircraft cables are galvanized and should resist rust OK. I have never seen rust that became an issue. A 1/8" cable is rated at more than 1000 pounds, so a little rust is OK, but worn or broken strands are not.
JJ

Have you ever heard of a rudder cable failure? Thanks

On Friday, May 11, 2012 4:23:29 PM UTC-7, bradley wrote:
On May 11, 1:42*am, JJ Sinclair wrote:
On Thursday, May 10, 2012 6:30:06 PM UTC-7, Ramy wrote:
On Thursday, May 10, 2012 5:54:15 PM UTC-7, JJ Sinclair wrote:
There seems to be a fair amount of confusion about the springs used in almost all modern sailplane rudder cables. The springs are there to keep the pedals upright (pulling forward) and to facilitate rudder pedal adjustment through the use of an 'S' tube. Each rudder cable enters the top rear of its 'S' tube and exitsthe lower forward portion of the 'S' tube, then extends on forward another foot or so and is bolted to the square tube that both pedals slide on when adjusting the pedals forward and aft. One can simulate a broken cable by simply grasping one rudder pedal and pulling it aft about 6 inches. The spring on that side will expand and the rudder cable will go slack. The spring on the other side will slam the rudder hard over...........I just did this on my ship. The same is true for Schleicher, Schempp-Hirth, DG, LS and most all sailplanes!

Some things to check on annual inspection:

1. Nylon inner sleeve in place on both 'S' tubes and extending about an inch out both ends of the 'S'tube.(the nylon sleeve keeps the cable from rubbing metel to metal during normal rudder operation and during pedal adjustment.

2. Springs in place and pulling equal (I measured 9# spring tension on my Genesis)

3. Adjust the rudder pedals full forward and inspect the cable at the top rear 'S' tube. (this is the most likely place to find cable wear)

4. Adjust the rudder pedals full aft and inspect for cable wear at the point where it exits the lower forward 'S' tube.

5. Inspect the weld that attaches the rudder pedal to the rotating tube at the bottom. In an accident, the pilot may have ended up standing on both rudder pedals as the ship came to an abrupt stop. I have found cracked welds due to this and several AD's have been issued on this problem.

6.Inspect the Nicopress sleeve and inner protective eye (thimble) and the bolt that attaches the cable to the rudder assembly. It must have lock-nut or castlenated nut & carter pin.

7. Inspect as much of the cable runs as you can see and don't forget to check the other end where the cable attaches to the rudder, sleeve/bolt nut or pin.

8. Check rudder stops. They may be at the rudder or on the pedals, but must stop full rudder movement before rudder strikes fin or pedals hit fuselage sides.

Most manufactures recommend not using lube on the rudder cables because it attracts dirt and can lead to unnecessary cable wear, but do lube the rudder pedals and rudder hinges. One more little tid-bit, you can adjust the angle at which the rudder pedals sit by simply making the rudder cables longer..........OK, JJ just how the hell do I do that? By inserting two 1" steel carobeaners in the cable run at the forward attach point. This will make both rudder pedals rotate forward about 30 degrees. Some pilots object to the straight up *angle of the pedals in ships like the Duo-Discus. Make sure you still have full rudder movement before rudder pedals hit fuselage side.
Cheers,
JJ A&P for 40 years + owner/operator of glider repair station.

Thanks JJ!
Can rust be also an issue ? After all we can't inspect most of the rudder cable.

Ramy

Most aircraft cables are galvanized and should resist rust OK. I have never seen rust that became an issue. A 1/8" cable is rated at more than 1000 pounds, so a little rust is OK, but worn or broken strands are not.
JJ

Have you ever heard of a rudder cable failure? Thanks

There are multiple accounts of rudder cable failure discussed in another thread that starts with "Crunch Alert"...

On May 11, 2:42*am, JJ Sinclair wrote:
On Thursday, May 10, 2012 6:30:06 PM UTC-7, Ramy wrote:
On Thursday, May 10, 2012 5:54:15 PM UTC-7, JJ Sinclair wrote:
There seems to be a fair amount of confusion about the springs used in almost all modern sailplane rudder cables. The springs are there to keep the pedals upright (pulling forward) and to facilitate rudder pedal adjustment through the use of an 'S' tube. Each rudder cable enters the top rear of its 'S' tube and exitsthe lower forward portion of the 'S' tube, then extends on forward another foot or so and is bolted to the square tube that both pedals slide on when adjusting the pedals forward and aft. One can simulate a broken cable by simply grasping one rudder pedal and pulling it aft about 6 inches. The spring on that side will expand and the rudder cable will go slack. The spring on the other side will slam the rudder hard over...........I just did this on my ship. The same is true for Schleicher, Schempp-Hirth, DG, LS and most all sailplanes!

Some things to check on annual inspection:

1. Nylon inner sleeve in place on both 'S' tubes and extending about an inch out both ends of the 'S'tube.(the nylon sleeve keeps the cable from rubbing metel to metal during normal rudder operation and during pedal adjustment.

2. Springs in place and pulling equal (I measured 9# spring tension on my Genesis)

3. Adjust the rudder pedals full forward and inspect the cable at the top rear 'S' tube. (this is the most likely place to find cable wear)

4. Adjust the rudder pedals full aft and inspect for cable wear at the point where it exits the lower forward 'S' tube.

5. Inspect the weld that attaches the rudder pedal to the rotating tube at the bottom. In an accident, the pilot may have ended up standing on both rudder pedals as the ship came to an abrupt stop. I have found cracked welds due to this and several AD's have been issued on this problem.

6.Inspect the Nicopress sleeve and inner protective eye (thimble) and the bolt that attaches the cable to the rudder assembly. It must have lock-nut or castlenated nut & carter pin.

7. Inspect as much of the cable runs as you can see and don't forget to check the other end where the cable attaches to the rudder, sleeve/bolt nut or pin.

8. Check rudder stops. They may be at the rudder or on the pedals, but must stop full rudder movement before rudder strikes fin or pedals hit fuselage sides.

Most manufactures recommend not using lube on the rudder cables because it attracts dirt and can lead to unnecessary cable wear, but do lube the rudder pedals and rudder hinges. One more little tid-bit, you can adjust the angle at which the rudder pedals sit by simply making the rudder cables longer..........OK, JJ just how the hell do I do that? By inserting two 1" steel carobeaners in the cable run at the forward attach point. This will make both rudder pedals rotate forward about 30 degrees. Some pilots object to the straight up *angle of the pedals in ships like the Duo-Discus. Make sure you still have full rudder movement before rudder pedals hit fuselage side.
Cheers,
JJ A&P for 40 years + owner/operator of glider repair station.

Thanks JJ!
Can rust be also an issue ? After all we can't inspect most of the rudder cable.

Ramy

Most aircraft cables are galvanized and should resist rust OK. I have never seen rust that became an issue. A 1/8" cable is rated at more than 1000 pounds, so a little rust is OK, but worn or broken strands are not.
JJ- Hide quoted text -

- Show quoted text -

When checking the rudder cables for broken strands it is best to use a
soft tissue which will readily snag on the broken strand. Of course
you can use your fingers if you like and if you have any broken
strands they can be easliy seen by the bloody drippings on the cable.

On Friday, May 11, 2012 4:13:06 PM UTC-7, zulu wrote:
On May 11, 2:42*am, JJ Sinclair wrote:
On Thursday, May 10, 2012 6:30:06 PM UTC-7, Ramy wrote:
On Thursday, May 10, 2012 5:54:15 PM UTC-7, JJ Sinclair wrote:
There seems to be a fair amount of confusion about the springs used in almost all modern sailplane rudder cables. The springs are there to keep the pedals upright (pulling forward) and to facilitate rudder pedal adjustment through the use of an 'S' tube. Each rudder cable enters the top rear of its 'S' tube and exitsthe lower forward portion of the 'S' tube, then extends on forward another foot or so and is bolted to the square tube that both pedals slide on when adjusting the pedals forward and aft. One can simulate a broken cable by simply grasping one rudder pedal and pulling it aft about 6 inches. The spring on that side will expand and the rudder cable will go slack. The spring on the other side will slam the rudder hard over...........I just did this on my ship. The same is true for Schleicher, Schempp-Hirth, DG, LS and most all sailplanes!

Some things to check on annual inspection:

1. Nylon inner sleeve in place on both 'S' tubes and extending about an inch out both ends of the 'S'tube.(the nylon sleeve keeps the cable from rubbing metel to metal during normal rudder operation and during pedal adjustment.

2. Springs in place and pulling equal (I measured 9# spring tension on my Genesis)

3. Adjust the rudder pedals full forward and inspect the cable at the top rear 'S' tube. (this is the most likely place to find cable wear)

4. Adjust the rudder pedals full aft and inspect for cable wear at the point where it exits the lower forward 'S' tube.

5. Inspect the weld that attaches the rudder pedal to the rotating tube at the bottom. In an accident, the pilot may have ended up standing on both rudder pedals as the ship came to an abrupt stop. I have found cracked welds due to this and several AD's have been issued on this problem.

6.Inspect the Nicopress sleeve and inner protective eye (thimble) and the bolt that attaches the cable to the rudder assembly. It must have lock-nut or castlenated nut & carter pin.

7. Inspect as much of the cable runs as you can see and don't forget to check the other end where the cable attaches to the rudder, sleeve/bolt nut or pin.

8. Check rudder stops. They may be at the rudder or on the pedals, but must stop full rudder movement before rudder strikes fin or pedals hit fuselage sides.

Most manufactures recommend not using lube on the rudder cables because it attracts dirt and can lead to unnecessary cable wear, but do lube the rudder pedals and rudder hinges. One more little tid-bit, you can adjust the angle at which the rudder pedals sit by simply making the rudder cables longer..........OK, JJ just how the hell do I do that? By inserting two 1" steel carobeaners in the cable run at the forward attach point. This will make both rudder pedals rotate forward about 30 degrees. Some pilots object to the straight up *angle of the pedals in ships like the Duo-Discus. Make sure you still have full rudder movement before rudder pedals hit fuselage side.
Cheers,
JJ A&P for 40 years + owner/operator of glider repair station.

Thanks JJ!
Can rust be also an issue ? After all we can't inspect most of the rudder cable.

Ramy

Most aircraft cables are galvanized and should resist rust OK. I have never seen rust that became an issue. A 1/8" cable is rated at more than 1000 pounds, so a little rust is OK, but worn or broken strands are not.
JJ- Hide quoted text -

- Show quoted text -

When checking the rudder cables for broken strands it is best to use a
soft tissue which will readily snag on the broken strand. Of course
you can use your fingers if you like and if you have any broken
strands they can be easliy seen by the bloody drippings on the cable.

Checked the rudder cables on my 27 - looking good. But as JJ described, loosening the tension on one side causes a hard over to the other side!
I would like to hear how glider manufactures defends this design! I wonder how many were killed by this design, giving many unexplained spins into the ground from higher altitude. A glider may still be landable without rudder control, but not with a full rudder. Couldn't they come up with a design with a more graceful mode of failure??

Ramy

What would be the consequence of removing the springs from the rudder pedals?

On May 15, 12:35*am, Ramy wrote:

Checked the rudder cables on my 27 - looking good. But as JJ described,
loosening the tension on one side causes a hard over to the other side!

I do not necessarily agree with JJ's synopsis of the situation. Yes,
on the ground removing the tension of one spring will cause the rudder
to go to the opposite stop. However, in the air there is airflow over
the rudder that would counteract the force of the spring. How much
effect this has depends on the strength of the return spring as well
as other aerodynamic factors. In all of the sailplanes I've flown, the
rudder springs are pretty wimpy in relation to the aerodynamic forces
involved. 14CFR23 and JAR22 dictate that the rudder circuit of light
aircraft is to be designed to react at least 150 lbs per pedal and a
combined force of 300 lbs on the pedal pair unless a lower force can
be rationally justified.

On that basis, it is my conjecture that there is an additional factor
at work in the JS1 incident besides a broken rudder cable. I am
standing by to see if such a factor comes to light.

I would like to hear how glider manufactures defends this design!

As a glider designer, I defend it so:

Cable actuation systems are a simple and effective approach to the set
of problems at hand. They are easy to inspect and service, and
problems are easy to detect. The 1/8" (3mm) cables commonly used have
about a 4x safety factor over typical maximum control forces, so they
will take a lot of abuse before failing. Every experienced A&P and IA
knows to inspect cables in their areas of tightest curvature, and
these inspections bring to light the vast majority of potential
problems long before they become critical. As typically implemented in
sailplane fuselages (including the three I have so far built), the
cable-in-tunnel system has the additionally compelling advantage of
adding increased rudder damping when the pilot applies pressure to
both pedals. This feature has been successfully used to damp incidents
of rudder flutter in quite a number of incidents that might otherwise
have eventually resulted in resonant failure of the aft fuselage.

The one issue I have with typical sailplane rudder cable systems is
that the S-tube on the side of the pedal that allows for pedal
position adjustment can cause a short-radius curvature of the rudder
cable at extremes of pedal deflection. The improvement I will try to
make in my next set of rudder pedals is to try to add an exit radius
to the ends of the S-tubes so that they look like tiny trumpet bells
in side view. This will increase the radius of curvature in the cable,
and hopefully decrease the wear and fatigue in the cable at that
point.

As a counterexample rudder actuation system, I submit the Diamant. The
makers went to heroics to reduce rudder actuation friction, using push-
pull tubes in linear roller bearings with many ball bearing pivots and
a rather complicated pedal adjustment system. What they got was rudder
flutter, and they ended up having to incorporate a hydraulic shock
absorber in order to apply damping to the system. So they started with
a complicated system and ended up having to make it more complicated
yet before it was fully functional. Think of all the things they could
have done with their energy had they just used a standard cable system
and moved on.

I wonder how many were killed by this design, giving many unexplained spins into
the ground from higher altitude.

My guess is few to none. Here in the US, crash investigations of light
aircraft, especially those of gliders, do tend to be less systematic
than those of larger aircraft. However, in my experience crash
investigators are fully competent at recognizing the signatures of
wear and fatigue failures in cable-actuated control systems. Where
such signatures are found, they are usually announced prominently in
the accident synopsis.

Thanks, Bob K.