Know when to change them
Article And photos by h.G. FrAutschy
arly on in the history of avi- A serviceable cord
ation it became clear that a should have a
landing gear with some “give” smooth outer cover-
or shock absorption capabili- ing, with no bumps
ties was desirable. The early or disruptions to the
Bleriot and other pioneer aircraft used woven cover. This
thin tires set on wire-spoked wheels, and one looks good, but
while they were great for clearing the age and repeated
humps and bumps of the local pasture, landing cycles have
they didn’t soak up the bumps too well, taken their toll.
so a second level of shock absorption
was added—rubber shock cording.
For decades, woven fabric-covered shock cord (often called
bungee cord) was the most common of all shock absorbers.
In fact, it was so ubiquitous, used on so many different civil-
ian and military aircraft, that it was even the subject of a mili-
No, it’s not some new exotic
pasta dish, or an undiscov-
ered sea creature. The outer
rubber strands show signs
of environmental aging, as
ozone and airborne con-
taminants (including engine
exhaust chemicals) attack
the rubber. You can see how
the inner strands are less
affected. Again, this shock
cord didn’t appear to be ex-
cessively worn when viewed
from the outside; the damage
was only obvious once the
covering was cut away.
30 AUGUST 2006
This cord shows obvious signs of damage to the outer cover-
This same cord showed some signs of abrasion damage to ing and to the rubber strands inside. The bumps and breaks
the woven covering where the shock cord was in constant in the covering indicate many broken strands of rubber in-
contact with the landing gear structure. side the shock cord. This cord must be replaced.
Most normal humans tary specification, MIL-C-5651D. It’s still
would not be able to commonly used in the military and for
pull on a 3/4-inch shock civilian use. Double-covered shock cord
cord and be able to is Type I; shock rings with a double-
stretch it, but this cord braided cover are Type II.
had deteriorated to such These cords are made with a core of
an extent that it was rubber threads and a layer of woven cot-
possible for me to do so! ton cording to protect the easily dam-
aged rubber. Shock cord used in aviation
applications has a pair of woven outer
layers. The actual diameter of the rub-
ber strip bundle is not the only factor
that controls how much force is needed
to stretch a cord. Made of heavy cotton
thread, the woven cover serves another
Shock Cord Dating System
not-so-obvious function—it controls
The color code repeats on the level of force required to stretch the
a five-year cycle.
cord (the cord’s “modulus”). The tighter
First color stripe:
the cover is woven over the rubber strip,
the greater the force required to elon-
1998—Blue gate the cord. Manufacturers can also
1999—Yellow vary the ratio of rubber strip and the
2000—Black cover yarn to control the modulus of
2001—Green the cord. That’s why it’s important to
2002—Red keep the cover intact; it not only pro-
2003—Blue tects the rubber strip from premature
aging, but also is required to maintain
the shock cord’s strength.
The rubber itself is high-grade rubber
2006—Green strip, either natural latex rubber or syn-
2007—Red thetic rubber, similar to that used when
golf balls were made with a wound rub-
The second stripe indicates the quarter of ber strip core, only wider. (There are no
manufacture: longer any manufacturers of wound rub-
January-March—Red ber core golf balls in the United States!)
April-June—Blue Some aircraft use multiple shock While it exhibits great elasticity and du-
July-September—Green cords. This is the upper end of the rability, the rubber is susceptible to en-
October-December—Yellow landing gear of a Fokker Super Uni- vironmental damage. When exposed
Some manufacturers add a versal. A similar herringbone arrange- to air, ozone and other pollutants will
third color stripe not required ment was used on the Ryan NYP, Spirit quickly deteriorate it. So will exposure to
by MIL-C-5651D. engine oil and other chemicals, such as
of St. Louis.
32 AUGUST 2006
exhaust residue. Keeping the outer cover
clean goes a long way toward keeping
landing gear seem excessively soft?
Even when the cords have been well
the rubber from deteriorating. That’s protected, the interior of the shock cords LEVEL
why on a Cub or other similar landing will deteriorate over time and will need
gear systems, leather or vinyl “boots” replacement. Evidence of that wear is continued from page 1
are used to cover the shock cords. most often seen in the form of surface
If you’ve owned an airplane that is irregularities in the covering. Home, Washington, which is near Se-
equipped with shock cords as part of Bumps, tears, or other disruptions in attle. Now, I’m not going to reveal a
the landing gear, you know how impor- the smooth woven cover tell you some- lot of detail here in this column, be-
tant it is to check the cords, to avoid a thing’s amiss under that cotton wrap. Is cause there is a really neat story to be
letdown feeling when one of the cords there discoloration on the cover, indi- told here, and I am hopeful that be-
lets go with a sharp report. But what cating possible exposure to chemicals or tween Sam and H.G., our editor, you
do you look for? How do you know it’s oil, or is it dirt and grime from a lifetime will have the opportunity to read all
time for a replacement? Do you use the of living on the belly of an airplane? about this great adventure Sam was
calendar, the appearance of the cord, or Take a look at the photos in this ar- on when he arrived at SMD in Fort
the cord’s date of manufacture? ticle. These shock cords had been in ser- Wayne, Indiana.
The answer is all three! While a cord vice for 14 years on an Aeronca Sedan. But what I will reveal here is that
may look perfectly fine, if it’s been sit- A pair of rings is used on each side, for Sam had just purchased his second
ting unprotected on the shelf in a hot a total of four shock cord rings. I’d no- 1929 Bird in the Long Island, New
hangar for years, odds are the rubber ticed that the gear seemed rather soft York, area nearly six weeks earlier.
strip inside has deteriorated to such when the wings were rocked up and You will likely learn about his first
an extent that a few cycles of stretch- down, and the gear seemed splayed out Bird in the future article. So, here was
ing will create a lumpy, useless mess. more than normal. Also knowing that 60-year “Young Sam Dodge” (this is
You’ve probably seen what can happen the age of the cords was at least 14 years, how he signs his e-mails) six weeks
to a set of shock cords when an airplane I made plans to change the cords during into this adventure of getting his Bird
has been left sitting out in the open for the annual inspection. While one cord home to Home, Washington, from
years. It doesn’t seem to take very long looked pretty good, the other, manu- Long Island, New York, with all of
for the landing gear to begin to splay factured four years earlier (evidenced by 25 hours of tailwheel time in his
outwards, and before you know it, the different color-code cords in the woven logbook, forging his way across the
gear is near collapse. covering) had obvious defects. United States.
Do your best to obtain fresh shock Once the decision has been made to Now I have to tell you, this is one
cords when it comes time to change out replace the cords, caution must be ex- interesting gentleman. Sam is a for-
a cord. Each outer layer of the woven ercised to prevent injury. These shock mer Hollywood producer who left
cord made to the exacting standards re- cords can recoil with tremendous force the industry and became a stay-at-
quired for the mil spec will contain a pair once stretched. home dad for his two sons some 20
(or sometimes a trio) of colored treads, For the Piper Cub and its brethren, a years ago. Sam ended up spending
which indicate the date of manufacture. few enterprising companies have made the whole July Fourth holiday with
The mil spec defines the meaning of the special tools for stretching and install- us at SMD, and we had a great time
colored yarn. See the chart included in ing the cords. In other cases, a special together. We assisted him with some
this article for details. The military re- tool is not needed, but do avoid the use minor maintenance items on the
quires a shock cord or ring be packaged of sharp or pointed tools to lever or pry Bird and then sent my new friend on
and delivered no later than six months the shock rings in place. Disrupting the his way to finish his great adventure.
after manufacture; if stored properly, covering not only exposes the rubber to As I write this column, Sam is in Bill-
the cord can last for many years. the environment, but also can cause a ings, Montana, and he continues to
During an annual or other peri- change in the cord’s modulus in a small chronicle this great adventure. We
odic inspection, check the shock cord area, weakening the cord and leading to hope to share it with you soon.
first by standing back and looking at premature failure. EAA AirVenture Oshkosh 2007—
the airplane from the nose. Does it sit Once the cords have been replaced, The World’s Greatest Aviation Celebra-
high up on its landing gear? Or have go out and enjoy that “new landing tion—is coming July 23-29, 2007. VAA
you noticed that the wing seems to be a gear feeling.” But be careful—you’ll be is about participation: Be a member!
bit lower than it used to be (sometimes amazed at how stiff the landing gear has Be a volunteer! Be there! Let’s all pull
evidenced by clonking your forehead become, and you may need to adjust in the same direction for the good of
when you go to enter the cockpit)? Is your landing technique. But even if you aviation. Remember, we are better to-
the inner portion of each of the tires “sproing” a few of those first landings, gether. Join us and have it all.
wearing excessively? you’ll have the peace of mind knowing
When you rock the wings with your your bungee cords are up to soaking up
hands while on the ground, does the anything you can throw at them!
VINTAGE AIRPLANE 33