Airframe and Powerplant

Shock Absorbers

February 1, 2007

These aircraft-quality rubber bands are essential

Bungee cords are well known for keeping thrill-seeking jumpers from hurting themselves. But did you know that bungee cords were used in the Spirit of St. Louis' landing gear? The cords absorbed the punishing load as Charles Lindbergh began the takeoff roll of his epic flight from Roosevelt Field to Paris on May 20, 1927.

Bungee cords, which are sold on reels much like rope, aren't used much anymore on aircraft, but bungee rings, technically known as "endless shock cord rings," are so critical that some airplanes can't legally fly without them (see " A True Laird Swallow," page 73).

A family business

Bungee rings are used to assist the landing-gear retraction sequence on the Douglas DC-2. Bungee rings also were used on the gunner's turrets on the Douglas Dauntless dive-bomber. As recently as 1987, Aviat Aircraft chose to use bungee rings instead of air-oil-style shock absorbers to cushion and support the main landing gear on its Aviat Husky A-1. And why not? Bungee rings are readily available at a very reasonable price, they don't leak, they work well in all kinds of weather, and they can be manufactured to fit almost every need.

"Bungees provide hundreds of pounds of force, yet weigh only ounces," explains David Valentine, president of SBC Industries, of Scottsboro, Alabama. SBC is the main manufacturer of shock rings for aviation.

"My grandfather started the business in Massachusetts. He had to stay home to run the business during World War II because bungees were vital to the war effort," says Valentine.

"Working with rubber is more of an art than a science," explains Valentine. The company employs half a dozen people and most of them have all been at SBC for a long time.

Today, shock rings are manufactured using modern rubber compounds. "We used to use natural rubber. It was unstable and hard to work with. It wasn't uncommon to throw away a big tangled, rubbery ball pretty often when we used natural rubber," says Valentine.

Bungees for airplane use are made one at a time on machinery that was specifically and locally made to wind long strings of rubber. These sophisticated rubber bands were so often used during the 1930s and 1940s that the military created a specification governing their production and performance.

Military specs for bungees

Mil-C-5651D specifies minimum breaking and force generated at steps in percent of stretch for each different diameter of cord and ring. For instance, a ring of one-quarter-inch-diameter cord stretched to 100-percent length must provide 32 to 56 pounds of pull. According to Valentine, SBC has incorporated tighter tolerances into its manufacturing.

All bungee rings for aircraft use are built to the type II specification of the military specifications. Rings are built by winding a continuous length of rubber band to get the desired cord-diameter and inside-diameter measurements of the finished ring. Then the rubber strands are covered by two woven cotton braids, both of which are glazed. Glazing makes the braid stronger and also reduces friction between the rubber and the two braids when the ring is elongated.

Shock-ring fabrication is not an assembly-line process. One worker winds the continuous rubber string, installs the two braided cotton covers that are required on rings destined for aircraft use, whips the loose ends of the braids, inspects the finished cord, and then does a load test. If the ring passes all the assembly testing, it's sent on for a sizing check and cosmetic inspection before being sent on to the shipping area.

The military specifications also require that a color code indicating the shock-ring manufacture date be woven into the outer cotton braid. The code consists of colored yarns in five colors. The date coding consists of three colored segments. Two of the colored segments are the same color and these indicate the year, while a single colored segment indicates the quarter of the year in which the bungee ring was manufactured.

Here's the code: Two black threads signify a manufacturing year ending in 0 or 5; two green threads for years ending in 1 or 6; two red threads for years ending in 2 or 7; two blue threads for years ending in 3 or 8; and two yellow threads for years ending in 4 or 9.

The quarter of the year is indicated by a single colored stripe of yarn. Here's that code: Rings manufactured between January and March have red stripes; April to June have a blue stripe; July to September have a green stripe; and October to December have a single yellow stripe.

Test yourself. If you're looking at a bungee with two black stripes and a single green stripe, what does that indicate? If you answered anything other than the third quarter of 1985, 1990, 1995, 2000, or 2005, you need to give it another try.

Airworthiness directives and bungees

Valentine's family has been making shock rings and shock cord for more than three generations. The market is small and production requires specialized equipment. "Making bungees is an old process," says Valentine. "Some of my machines are more than 50 years old. All this equipment is scratch-built," he adds. Valentine declined to give production figures. It would probably surprise many owners if they realized how dependent they are on SBC for the shock rings required to maintain airworthiness.

Take the Comanche family of Piper airplanes. Both the single-engine and twin-engine Comanches use a bungee ring on each main landing gear to relieve loads on the electric-gear actuating-motor during the retraction cycle of the landing gear. Airworthiness directive (AD) 77-13-21 mandates that new bungee rings be installed at 500-hour intervals, or every three years of calendar time, whichever comes first. If the bungee rings aren't replaced at these intervals, it's not uncommon for the landing-gear motor circuit breaker to trip during gear retraction. Today, a set of bungee rings required to comply with the Comanche AD costs less than $40.

Bungee rings are used to assist in landing-gear retraction on Twin Commanders. The manufacturer recommends changing the bungee rings every year.

Most bungee rings are used to cushion shocks and support landing-gear loads. Bungee rings used on the landing gear are either installed to keep two sliding pieces of the airplane's shock strut from extending too much, or to cushion and restrict excessive up-and-down wheel travel by limiting movement at the opposite end of the landing-gear truss assembly.

Some of the other aircraft companies that utilize bungee rings include Aero Commander, Aeronca, Bell Helicopter, Piper, Aviat, and Taylorcraft. There are many others, but this list illustrates the importance of these rings to general aviation pilots.

Part-number identification

Freshly manufactured bungee rings are sold through large aviation parts supply houses. Part numbers consist of a four-number sequence. The part number for the bungee rings for the Comanche landing gear is 8097; the ones for many of the conventional-landing-gear Piper single-engine models are the 1280 and 1280HD.

The first two numbers indicate the cross-sectional-diameter cord of the ring in one-sixteenth of an inch. Based on this information the first two numbers — 80 — in the Comanche ring part number signify a one-half-inch-diameter cord and the 12 in the Piper part number indicates that a three-quarter-inch-diameter cord is used in the 1280 ring. The second set of numbers indicates the inside diameter of the ring — in a perfect circle — in inches and eighths of an inch. Using this information we can deduce that the ID of the Comanche ring is nine and seven-eighths inches and the Piper part number designates an eight-inch inside-diameter ring.

The 1280HD bungee rings obviously use the same-diameter bungee cord and have the same ID as the 1280. The HD signifies that the bungee is stronger than the 1280 and is built to the high end of the specification range. All bungee-ring braided coverings are whipped with natural-color whipping thread except for the HD rings, which are whipped with brown-color whipping thread.

Bungee installations — at least the ones performed by average-strength people — always require some sort of a tool. The installation of the gear-assist bungee rings on the Comanche requires that the airplane be placed on jacks, that an inspection panel located just outboard of each main landing gear be removed, and that the gear be partially retracted while a special bungee removal and installation tool is held in position up in the wing. The inboard end of the bungee settles onto the tool, and then the bungee pulley and bracket on the outboard end are unscrewed from the threaded retainer fitting. Installation of the new bungee on the installation tool requires that the bungee be stretched to fit onto the tool. I stretch the bungee rings over the installation tool with a hydraulic engine hoist.

Cutting a bungee to remove it may seem like a good way to speed the remove-and-replace cycle along, but it must be remembered that even the most raggedy bungee can still pack a heck of a wallop when cutting permits the loose end to suddenly retract. The half-inch-diameter cord diameter bungee rings test to between 350 to 500 pounds of pull when elongated by 100 percent. The power in a bungee ring should not be taken lightly.

Damaged or worn bungee rings are pretty easy to spot. If the bungee has been overstressed — because of a very hard landing or overloading — the rubber strands will break. When this happens, the cotton braid will neck down and look thinner at the point where the strands are broken. It's pretty easy to see. If the bungee rings have weathered and deteriorated because of exposure to sunlight, dirt, or chemicals, they will lose strength.

A loss of bungee strength is also easy to identify. It depends on how they are used, but in some airplanes a soft bungee will cause the main landing-gear wheel track to widen and the airplane will look like it's squatting. If only one bungee has lost its strength or been damaged, the wing tip on the damaged-bungee side will be closer to the ground than the wing tip on the other side. For the most part, bungee-ring troubleshooting is a visual art.

Bungee cords are still used on airplanes for the reasons David Valentine mentioned earlier — they supply relatively large amounts of force at the cost of a few ounces of weight and for less cash than dinner for two at a good restaurant. Quite a few single-engine and a few twin-engine airplanes would be immediately consigned to the airplane salvage yard if the supply of these unsung parts ever dried up. For now, though, Valentine and his crew at SBC Industries will ably fill this important niche in our aviation world.

E-mail the author at steve.ells@aopa.org.