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Control Cable Tension An important piece of the puzzle BY BRIAN AND CAROL CARPENTER

MAINTAINING PROPER CABLE TENSION on your aircraft’s control system TOO MUCH TENSION is vital for flight safety. In recent years the problems related to the While there is potential hazard of under-tenZodiac CH 601 XL have highlighted the significance of proper flight- sioned cables, there are also downsides to control cable tensioning. A National Transportation Safety Board over-tensioning the cables. Over-tensioned cables are often indicated by a stiff or heavy (NTSB) letter dated April 14, 2009, regarding Zodiac CH 601 XL aircraft accidents reveals that improper cable tensioning may have feeling in the corresponding flight control. been a contributing factor to the introduction of flight-control flutAn airplane with an over-tensioned control cable can act as though the airter leading to the in-flight breakup of several aircraft. craft’s natural stability is Over-tensioned cables are inhibited. With properly tenAlthough the link to control cable tension was not definitive in sioned control cables, the often indicated by a stiff these accidents, it was revealed controls float and respond to that there were other incidences or heavy feeling in the the aerodynamic loads imposed of in-flight flutter that did not on them. When in level flight result in significant problems, but corresponding flight control. the dihedral of a wing tends to that were, in fact, corrected by return the aircraft to level flight readjusting and proper tensioning of control cables. anytime that turbulence upsets its normal The Zodiac CH 601 XL—and many other aircraft not built with flight path. If the aileron cables are over-tenmass-balanced flight-control systems—relies primarily on control sioned, the controls will exhibit stiffness and cable tension as protection from flutter. Tension in the control cables the aircraft will tend to continue a roll in one adds stiffness to the control system, providing for a higher natural direction as though the aircraft had neutral frequency of the system. This in turn raises the airspeed at which or negative static stability. flutter may occur. In aircraft such as these, proper cable tensioning The positive pitch stability inherent in becomes even more significant. Tension is the amount of pulling most aircraft can be upset by over-tensioned force applied to the cable, and is usually achieved through the use of elevator cables, making it difficult to trim for turnbuckles within the control system. a specific speed or altitude. It is a

92 Sport Aviation January 2011

PHOTOGRAPHY BY CAROL CARPENTER

You may find even a quarter turn of a turnbarrel can take the cable tension outside acceptable limits. fine balance between over-tensioning and under-tensioning and, as a result, many aircraft require the assistance of a tensiometer, a precision cable tension measuring device. USING A TENSIOMETER

There are many types of tensiometers, and they all have different requirements. Read the instructions. Most tensiometers come with operating instructions placarded on the inside of the box or on the tool itself. Calibration: Like any tool, a cable tensiometer also needs to be calibrated. Many tensiometers come with a calibration bar that can be used to validate calibration. You can also validate your tension by hanging a calibrated weight from a cable of the identical size and checking your tensiometer against that. Temperature: Cable tension charts are normally supplied by the manufacturer of the aircraft and are graphed as a function of temperature. The stainless steel or steel cables have a different coefficient of expansion than does the aluminum or composite airframe and are often specific to each of the flight controls as well as the trim tab control systems. Control Cable Size: Ensure that you are using the tensiometer to match the cable size. Cables are measured at the largest cross-sectional area. Use a micrometer or calipers to get a good read on the size of cable. Also consult the aircraft maintenance manual to verify correct cable diameter. Tensiometer Placement: As a general rule of thumb, take the cable tension reading no closer than 6 inches to any turnbuckle, end

fitting, or connection. This can be rather difficult on some aircraft simply because of the access available to place the tensiometer. Cable systems are normally used simply because they are able to be run in tight quarters and out of the way. A good manufacturer will have thought this through and will spell out the proper location to take the measurement of the cable tension. Adjusting: Remove the tensiometer and recheck after each adjustment of the turnbuckle. Built-up friction on the risers can give an erroneous reading. Floating: Float the tensiometer on the cable so that you are not applying any load from pushing or pulling on the tensiometer tool, which would result in incorrect readings.

There are many types of tensiometers, so be sure to read the instructions for yours. The tool will need to be calibrated prior to use. Many tensiometers come with a calibration bar that can be used for this. As a general rule, don’t take a cable tension reading closer than 6 inches to any turnbuckle, end fitting, or connection.

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H   ANDS ON HOMEBUILDER’S HANDBOOK

PROPER TENSIONING

Entirely too much emphasis is placed on a correct initial cable tension reading. Invariably, just through the process of safety-wiring a turnbuckle, the cable tensioned will be adjusted by the pulling and tugging on the safety wire. Initially, we recommend adjusting the cable tension to the high side of the limits and then rechecking once you are ready to finalize the safety wire. Depending on the cable length, you may find even a quarter turn of a turnbuckle can take the cable tension outside acceptable limits. The shorter the cable, the more sensitive the cable tension adjustment will be when rotating the turnbuckle. It is important when tensioning a cable assembly to ensure all of the twist is out of each cable and that the turnbuckle is engaged simultaneously on both threaded ends during the tightening of the turnbuckle. Both threaded ends need to remain stationary while the turnbarrel is being tightened. This is relatively easy to do in an area with adequate access to the turnbuckle assembly. However, when a turnbuckle is located in tight quarters, it may be necessary to employ the use of a turnbuckle adjusting tool, also called a turnbuckle chain tool. This tool is nothing more than two C-clamps with a chain welded between them. For most aircraft builders this will be simple to make. This tool allows the tightening of the turnbuckle while

Invariably, just through the process of safety-wiring a turnbuckle, the cable tensioned will be adjusted by the pulling and tugging on the safety wire. maintaining the position of the two threaded ends relative to each other. This tool normally is left in place while tensioning as well as safety-wiring. Installation and removal of the tool can be accomplished with one hand, and it can be fished out through even the smallest of access panels. SECURING THE TURNBUCKLE

If a turnbuckle is located in tight quarters you can use a turnbuckle adjusting tool, which allows for tightening of the turnbuckle while maintaining the position of the two threaded ends relative to each other. The double-wrap spiral method is preferred when safety-wiring a turnbuckle.

94 Sport Aviation January 2011

Of the methods using safety wire to secure the turnbuckles, the double-wrap spiral method is preferred in primary flight-control systems. It is the best method for preventing rotation of the turnbuckle. If done properly, the double-wrap spiral method will generally allow less than one-eighth of a turn when trying to rotate the turnbuckle by hand. Most manufacturers of new aircraft have converted to using the MS21251 turnbuckle and MS21256 locking clip. This is a significant improvement of the old double-wrap spiral safety-wire method. The barrel and the terminal ends have a machine groove down each thread, and a safety pin is inserted to prevent rotation. The new MS locking clip method allows for no rotation of the turnbuckle whatsoever. When cable tension is critical and tolerances are limited, the MS locking clip system is the preferred method. Whatever locking mechanism you are using, make sure to go back after your final adjustment and operate the control system several times. Check to ensure that your cables are routed correctly and

PHOTOGRAPHY BY CAROL CARPENTER

there is no binding. After you are satisfied that you have completed the rigging correctly, recheck the cable tension to see that it falls within the manufacturer’s tolerances. Movement of structures over the life of an aircraft and wear on pulleys and linkages will naturally allow for reduction in tension. Cable tension will change over time, so it is important to routinely recheck the tension. Remember, proper control cable tension is an important element in a properly rigged control system. It is simple to accomplish and takes only a few minutes. Check your cable tension today—there’s no better time. Brian and Carol Carpenter, owners of Rainbow Aviation Services, teach light-sport repairman maintenance and inspection courses. Brian, EAA 299858, is an airframe and powerplant aircraft mechanic with an inspection authorization, a designated airworthiness representative, a sport pilot examiner, and a certificated flight instructor. Carol, EAA 678959, is a sport pilot instructor, a private pilot, and an FAA Safety Team representative. For more information on the repairman courses, visit www.RainbowAviation.com. For a

Once you are satisified with your rigging, recheck the cable tension to make sure it falls within the manufacturer’s tolerances.

link to the Hints for Homebuilders video on adjusting cable tension, visit www.SportAviation.org.

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