Even though many planes have most of their flight controls actuated by pushrods and bellcranks, for the rest of us cables and pulleys do the work of telling our airplanes where to go. Maintaining them is usually pretty easy, but a little bit of attention is required to keep everything in order and keep our planes safe. Mostly, cable and pulley maintenance consists of an annual inspection, but for those of you who fly planes on floats or have cables exposed to weather, a little more attention will be required.
Experimental airplanes will likely have one or two types of cables. Almost all will have at least a few 1/8-inch control cables to work such things as the rudder, ailerons, elevator, and possibly the flaps. Typically, these are stainless steel cables of the 7×19 design. In other words, they consist of seven bundles of 19 strands twisted into a single cable. These cables are known for their superior flexibility, making it easy for them to bend around pulleys and to form loops in their ends. This is not the strongest design, but 1/8-inch stainless steel cable has a breaking strength of 1760 pounds, making it plenty strong for the job at hand. The same cable in galvanized steel has a strength of 2000 pounds. Galvanized cable is less expensive, but less corrosion resistant than stainless steel. That said, it is certainly adequate for most applications, except those that will expose it to lots of water or harsh weather. Plain steel cables are rare these days because of their potential to corrode.
The second type of cable you may encounter is 1×19 cable, which consists of 19 strands twisted into a single cable. This type of cable is better suited for drag/anti-drag wires and bracing. Because it is not very flexible, it usually comes with machine swaged ends and is not looped and swaged with Nicopress sleeves and thimbles. This cable also comes in stainless steel or galvanized steel.
You can tell galvanized cable from stainless by appearance. The galvanized cable will have a duller gray finish as opposed to the stainless steel finish, which is shinier. They are both magnetic—the stainless steel cables are 304 alloy—so the magnet test won’t work. Stainless is preferred for any environment that is likely to produce corrosion, but the maintenance of both is the same.
You may also encounter smaller cable in conjunction with your autopilot. The main concerns there are to avoid contact with other cables that might produce binding or abrasion, and to use the correct size pulleys for the smaller cable.
Aircraft pulleys are almost always made of phenolic, but there are aluminum pulleys and Experimental-only UHMW plastic pulleys also available. The standard general purpose aircraft pulleys for controls are the AN220 (MS20220) series with sealed ball bearings, and the NAS383-series pulleys with graphite-impregnated bronze bushings. The NAS pulleys are fairly common in Experimental aircraft and are considerably less expensive than the AN220-series pulleys. The NAS pulleys are somewhat more likely to stick and thus may need the occasional application of lubricant. The sealed bearings of the AN220 pulleys cannot be lubricated. The plastic pulleys have stainless steel bushings that will also require lubrication. They are particularly well-suited for use on floats where constant exposure to water presents a corrosion risk.
If cables and pulleys are properly installed, they typically have few problems. But a misaligned pulley or a cable that is bent a little too much going through a fairlead (more than three degrees) will quickly begin to experience unsafe wear and fray, eventually leading to a total failure. Even the best of control system cables can wear out from the constant bending around pulleys, which causes internal friction and wear between the strands of the cable. Exposure to weather also hastens wear. A careful inspection every year and some preventative maintenance will extend the life of cables and catch problems before they become major safety issues.
During a condition inspection every cable should be checked for fraying or any noticeable wear, especially where cables bend around pulleys or pass through fairleads. This is where problems are most likely to occur. Cables should first be inspected by rubbing a cloth along their length. The cloth will often catch on a frayed wire if one is present. Next visually inspect the cables in critical areas, looking for fraying, broken strands, and excessive wear. Any individual strand that is worn through 40% or more is cause to replace the cable, as is any broken strand if it occurs in a critical area—where it bends around a pulley, passes through a fairlead, where it flexes or rubs during normal operation, or within one foot of a swaged end—in other words just about anywhere it is likely to wear or fray. Any lack of free movement where cables connect to control surfaces can cause cables to fray from excessive bending. Look for this condition where rudder cables connect to the rudder, for example. This should be a standard preflight checklist item. A few broken wires are permitted in non-critical sections of cable, but it is unlikely to find a broken wire where the cable never bends or rubs on anything.
Check cable tension at each condition inspection to be sure cables have not stretched, come off of pulleys, or run into other problems. Consult the kit maker for recommended cable tension. Please note that rudder cables will only maintain the tension placed in them by the rudder springs and will therefore not have the same tension in them as aileron or elevator cables. Any noticeable increase in control drag or friction should lead you to investigate further because cables could have become twisted up in each other or a pulley could have frozen. These things need to be fixed immediately.
Cable tension adjustment should never be needed if everything is functioning properly, but if it needs to be adjusted be sure to recheck your rigging after making the adjustment. Add or release a small amount of tension to the cables that both raise and lower the elevator, for example, to avoid compromising the up and down travel required for safe operation. Make small adjustments and check travel as you go, otherwise you will need to re-rig the entire control. If you do disconnect a control cable during maintenance, be extra sure to check for proper control movement to avoid a potentially disastrous reversal of the controls.
Turnbuckles should not be bent or show any signs of deterioration during your cable inspection. They should also not have more than three threads visible on either end, and safety clips or safety wire should be in place and properly installed. If there are witness holes in the turnbuckle body, use a piece of safety wire to be sure that the turnbuckle screw goes far enough into the body so the witness hole is blocked. If safety wire is used to secure a turnbuckle, as opposed to clips, make sure that the wire terminates in four wraps around the turnbuckle. The proper methods and some good illustrations of safety wiring turnbuckles can be found in AC43.13-1B, section 7-179. Turnbuckles should not come closer than two inches to any pulley or fairlead when at the closest point in their travel.
Ball bearing pulleys seldom require any maintenance unless there are alignment problems with the cables passing through them. Pulleys with bushings may need the occasional squirt of lubricant such as LPS #2 or Boeshield T-9 to keep them turning freely, but only on an as-needed basis. Excessive lubrication should be avoided to prevent the accumulation of dust and dirt. Pulleys that are constantly exposed to water such as those found on floats will need more attention and more frequent lubrication. Pulleys on floats will benefit from a frequent application of a water-displacing lubricant such as LPS, T-9, or WD-40. Pulleys subjected to direct water spray or pressure washing during cleaning should also be treated with a water displacing lubricant after washing.
Pulleys have cable guides to prevent cables from slipping off when loads are relaxed. The most likely place for this to happen is with aileron cables that will lose their tension under high-G maneuvers as the wings deflect upwards. It can also happen when a spring breaks somewhere in the system, especially in the rudder system. In any case part of pulley maintenance should include a careful inspection of the cable guides to make sure they are not cracked or missing. Clearance should be eyeball checked and measured if there is any doubt. The manufacturer will likely have a maximum clearance specification, but absent that, the clearance between the outer diameter of the pulley and the cable guide should not exceed half the cable diameter. Glasair, for example, limits cable guide clearance to even less than that at 0.04 inch. The maker of your kit or the designer of your project may have a similarly tight clearance requirement. Since these guides are often made from fairly thin material, it is a good idea to check them for cracks annually.
Fairleads, plastic sleeves through which cables pass and/or make a slight bend (less than three degrees), can also cause cable fraying or excessive wearing of the fairlead. Check these annually to be sure everything is in good shape. Especially watch out for any cable fraying or excessive wear. These usually remain trouble-free if the cable is not deflected too much as it goes through the fairlead.
Remember, the more your cables and pulleys are exposed to weather and water, especially saltwater, the more attention they are going to need. The frequent use of water-displacing lubricants in critical areas will go very far toward extending the life of these critical components. But don’t forget that the cables and pulleys inside the airplane also need a close look at least once a year.