Nuts & Bolts

Building Basics IN AUGUST, "BUILDING BAsics" addressed fabricating cables, and now it's time to install them. After fabricating the cable with the correct terminal you may want to proof test it. Usually, this test pulls the cable to 60 percent of the breaking strength for at least three minutes. Many builders run a longer test, taking up any stretch in the cable. This isn't necessary in every case, but it can be helpful for long cable lengths because it removes the stretch before installation. Once properly rigged, cable tension doesn't change appreciably over time, unless substantial wear in the system occurs. Cable Runs

In most cases, cables don't run in a

and you should inspect them periodically for wear and damage. Fairleads can redirect the cable's direction to a small degree—no more than 5 ling and routing aircraft control cables degrees. When the cable run must make a larger H.G. FRAUTSCHY turn, use a pulley. straight line from the cockpit conPulleys are usually made of phetrol to the control surface. They run nolic material and have a permaadjacent to part of the airplane's nently sealed bearing or bushing, structure and must make bends or and the sealed ball bearing is preturns around other parts of the struc- ferred. Ensure that all pulleys are ture. When a cable runs adjacent to mounted securely to the structure bethe structure, a fairlead prevents the cause a cable's rigging tension ranges two from rubbing against each other. from 10 to 150 pounds, and moving When installing fairleads, make the control against the air load in sure you can replace them easily flight adds to the strain the pulley without removing the cable. The ca- must bear. ble rubs against the phenolic, Teflon, A pulley has a guard that prevents or other low-friction plastic fairleads, the cable from jumping off the pul-

Cable Runs Through It

FAIRLEAD

BLOCK

STEEL LOGS

CABLE GUARDS KEEP CABLE C~ JAMMING CONTROLS

FROM 3UMPING PULLEY

MAX.

2 DEGREES

DEFLECTION

TO PREVENT CABLES CONTACTING STRUCTURE,

USE FAIRLEADS

SWAGED

TERMINAL ENDS

CABLE .ROUTING 88

SEPTEMBER 2000

COMMERCIALLY AVAILABLE

FAIRLEAD

Cable Quality August's article on cable basics generated some input from representatives of several wire rope manufacturers. They said some supply houses sell cable said to meet Mll-W-83420 specification. One company's catalog notes that its cable isn't "Oft-certified," but that QPl-certjfied cable is avaiable at a higher cost QPl is the Quaified Products List that names the manufacturers the Defense Supply Center has approved to produce products under a military specification. Technically, if a company isn't on the list, its product doesn't meet the MilSpec even if it adheres to all specification procedures. To make the QPL, companies must follow the required procedures and pass a DSC inspection of their manufacturing fadties. I Not being on the QPL doesn't prevent manufacturers from making and seing (except to the government) products "that meet MIL-SPEC-XXXX." These products and companies may be capable of passing a DSC inspection, but they aren't obligated to I be up to spec. Repeated testing of recently imported "aircraft cable" made by foreign manufacturers not on the QPL has shown it to have a substantial reduced fatigue Fife when compared with MiSpec cable. I Eight manufacturers are on the "Wire Rope, Flexible, For Aircraft Control" QPL. ' Seven are in the United States, and one is in Germany. I counted Wire Rope Corporation of America and the MacWhyte Co. as one because the former now owns the latter. All the major airframe manufacturers use QPL-certified cable in their airframes.

So what does this mean to you? If you're rebutting a certificated aircraft, you should be able to trace each part to its source, and tracing cable to a QPL manufacturer satisfies the requirement that it meets the military specification. To trace cable, you can request a copy of the manufacturer's QPL certification from the supplier (the supplier may offer to send you a certification on its letterhead, but the one you want comes from the manufacture!). Twisting open a section of cable and looking for a pair of colored nylon threads is the second tracing step. Each QPL manufacturer identifies its cable with a pair of colors assigned to it For example, red and yellow threads running along the cable's core strand identifies wire rope made by Loos and Company in Pomfret, Connecticut, and red and white threads identify cable made by Wire Rope Corporation of America. To verify that cable meets MilSpec you need the QPL certification and the thread identification. The thread alone is not enough because in the late 1980s and early 1990s at least one Far Eastern manufacturer copied the colored threads of a QPL manufacturer, and this counterfeit product didn't meet MIL-DTL-83420's strength and fatigue resistance requirements. Homebuilders have more choices, and they can use QPL cable or less expensive commercial cable sold by industrial and aviation supply houses. Remember, though, you have no way of knowing its level of qualty without certification as to its manufacturing source. It's your call—HGF

tings depending on the installation, but the adjustment process is the same. Turn the turnbuckle's barrel to adjust the tension. However, no more than three threads of each of the end fittings can be showing outside of the barrel, and no more than four threads can be inside the barrel, for a total tolerance of seven threads. Swaged terminals have a threaded end fitting that must be installed before attaching the cable to the control surface, and a threaded clevis fork or turnbuckle barrel transfers the load from the cable to the control surface. Use a tensiometer to check the cable's tension. Aircraft mechanics often use a cable tension indicator. It measures the force exerted across a riser designed for a specific cable diameter, and a conversion chart reCausing tension Just as too much friction can cause veals the accurate measurement. control problems, so can too much These indicators can be pricey, slack. Two common methods for ad- around $800, but moderately priced justing the cable's length or tension units are available, such as the Borare turnbuckles and threaded swaged roughs Cable Gauge ($116) that interminals and steel lugs used in con- dicates cable tension on its dial indicator. junction with clevis pins. Turnbuckles are a simple, effective way to fine-tune cable length, and Lock Down & Inspection they come in two major varieties. Like other aircraft components that One (AN 155 barrel) is safetied with move, you must safety the cable once one or two pieces of safety wire, and it's installed and tensioned. Turnthe other (MS21251 barrel) uses a buckles are safetied in position with pair of locking clips. Turnbuckle clips or safety wire, and cotter pins seends vary from pin eye to forked fit- cure clevis pins and castellated nuts. ley and jamming against the pulley bracket, and it can change the cable's direction up to 180 degrees on one plane. If the cable doesn't enter and exit the pulley within a degree or two, the repeated off-pulley pull wears the edges of the pulley, and the cable can j u m p off it and jam the control. Careful planning keeps the number of pulleys and fairleads to a minimum and reduces the friction that can increase the force needed to move the control. Using larger diameter pulleys can also help reduce control friction and cable wear. In his book, Sportplane Builder, Tony Bingelis recommends a 3.5-inch diameter pulley when a 1/8-inch cable must turn 90 degrees.

Don't use self-locking nuts in flight controls where the two components being connected move or rotate. When installing—and inspecting—your cable installation, make sure that no fitting or connection on the cable is closer than 2 inches to a pulley bracket, bulkhead, fairlead, or any other structure. Cable-actuated controls are popular on light aircraft because they are lightweight and effective. From the smallest lightplane to many of the largest transports, cable controls will continue to serve aviation well into the age of electronics. •

Resources For more information on using aircraft cable, check these sources, which are available on the EAA website at www.eaa.org or by calling 800/564-6322. • Homebuilder's Headquarters, in the members-only section of the EAA website. Advisory Circular 43.13-1, Acceptable Methods, Techniques and Practices—Aircraft Inspection and Repair. $18.95— FOO191 Sportplane Construction by Tony Bingelis. $24.95—F01395 • Sportplane Builder by Tony Bingelis. $24.95—F30140 • Aircraft Spruce & Specialty Company catalog—800/824-1930 Sport Aviation

87