Many small general aviation aircraft still depend on steel cables to operate their flight control systems. The purpose of the cables is to effectively transmit the input action of the pilot’s controls out to the external flight control surfaces in the tail section and wings. Some aircraft operate using push-pull tubes but the majority of small aircraft in the field today are equipped with good old reliable cables. The steel cable assemblies on most small aircraft are referred to as 7x7 or 7x19 flexible cable made of carbon steel or corrosion resistant steel depending on their operating environment. A 7x19 cable (as shown below) has 7 woven strands each consisting of 19 smaller strands. This construction allows the cable to be flexible enough to easily change direction (over a pulley) yet durable enough to resist gradual wear.
The cable thickness is also taken into consideration when determining the size of cable needed for a particular sized aircraft or system. A typical small Cessna aircraft uses 1/8 inch diameter cables for its primary flight control systems (ailerons, elevators and rudder) and 3/32 inch diameter for the flap system and even 1/16 inch for most trim systems. Larger aircraft utilizing cable systems require a heavier gauge such as 5/32 inch or 3/16 inch cables. The cables are finished at either end of their run with attachments such as swaged ball attachments and fork ends. These attachments usually permit secure attachment to the flight control bell cranks with removable hardware.
The reliability of aircraft flight control systems is always dependent upon the continued integrity of its primary components (the cable, pulleys, attachments and hardware). The maintenance of these systems often requires tensioning or rigging the cables to a pre-determined tension range or deflection limit. Any maintenance of a cable system (especially replacement of a cable assembly) requires a vigilant inspection process by technical personnel to ensure that the installation and rigging meets the exact specifications detailed in the maintenance manual. This is why the regulations also require the specific task of double checking the work after completion to verify routing, tension, attachment and operation. This is known as an “independent check of control systems” or simply a “dual inspection” with both terms implying that a secondary set of eyes is doing the checking and attesting to its serviceability.
The normal inspection requirements of these vital components often leads to areas of the airframe that are somewhat more difficult to access without the aid of a good inspection mirror and light source. The technician follows the path of the cable while looking for possible interference amidst a strategic arrangement of pulleys, guards and fairleads. The design and materials used in these flight control systems are a testament to their reliability and minimal need for regular servicing. The manufacturer does require that we take a close look at each 100 hour inspection in order to confirm their condition and note any potential wear patterns or actual defects. Control pulleys of various sizes are strategically placed to change control cable direction as necessary throughout the airframe. If the integrity of the pulley becomes compromised for whatever reason, it will likely permit excessive or unusual wear patterns to the cable, the pulley, or both. Fairleads are used to protect the cable from chafing on bulkheads or other unprotected metal edges. They may be formed in a variety of shapes and/or sizes depending on location and are generally needed in situations where maximum cable deflection from its main patch does not exceed 3 degrees.
Perhaps the most common attention given to cable systems is tension adjustment within the prescribed tension range for the temperature of the area. Cable tensions are often affected by temperature and may even be subject to cable stretch (over time) depending on the type of installation. Maintaining proper cable tension is verified during the regular inspection using a tensiometer and then adjusting the turn barrel (aka turn buckle) if the tension is not within the proper range specified in the manual. A turn barrel is usually found on each run of cable. It is a brass barrel machined with internal threads to fit a threaded swage end into each side of the barrel which is then turned to either loosen or tighten the set tension of each cable.
Control cables should be replaced whenever the inspection criteria points to assemblies that are found to be worn, distorted, corroded or otherwise damaged in some way. An external wear pattern beyond 40 – 50% on the outer wire strands would render the cable as unserviceable. To further this guideline, the Cessna 172 service manual states that “Individual broken wires are acceptable in primary and secondary control cables at random locations when there are no more than six broken wires in any given ten-inch cable length”. The wear pattern may be visible across 2 or more strands and appear to have a blended appearance from the wear across that section. Fraying of the cable is identified visually by one or more broken strands. Broken strands of cable can also be detected by dragging a cloth along the length of the cable to see whether it snags a stray wire break. The drawback to this old-school method is that most cable defects are likely to happen in high wear sections such as the minimal arc of contact around pulleys or in other areas perhaps where corrosion is present. In some cases, suspected cable wear may need to be inspected in greater detail by removing the cable from the system and reverse looping the suspect area to see whether any strands pop out.
According to the General Practices manual AC43.13-1B par. 7-149, “Any cable that has 1 broken strand in a critical fatigue area must be replaced. A critical fatigue area is defined as: the working length of a cable where the cable runs over, under or around a pulley, sleeve or through a fairlead or is flexed, rubbed etc…” During the inspection phase of the cable, always examine the integrity of the swaged ends, hardware or any other proper forms of attachment. Checking the attaching hardware (including clevis bolts used specifically for shear load applications) and linked shackles for wear is also critical to control system serviceability. These areas are not normally prone to defects yet they should never be ignored either. Certain attachments would also benefit from a drop of oil to help reduce wear. Further details outlining the service limitations for aircraft cables are also specified in the AC43.13 document or even in the specific service manual for the aircraft. The picture below demonstrates an actual finding on a frayed cable that was subject to the loop inspection after a few loose strands were detected in a pulley contact area.
Specific flight control system inspections have also been the focus of Airworthiness Directives and manufacturer Service Bulletins. One such example is from Piper Aircraft Service Letter No. 1069, A Maintenance Alert bringing attention to Flight control cable terminal fittings and turnbuckles with more than 15 years in service. This Alert was deemed a “corrosion inspection” and was an early sign of industry feedback from the manufacturer that these areas needed special attention during regular inspection activities. Piper later issued Mandatory Service Bulletin (MSB) No. 1245A to inspect the stabilator control system of all PA-28, PA-32 and similar series aircraft for cable corrosion and possible cracking of cable turnbuckles located deep in the aft fuselage. This inspection was also directed at aircraft that were at least 15 years old and that after the initial inspection, it was to be repeated after 2000 hours (time-in-service) or 7 years, whichever occurs first. This Bulletin was later upgraded to an FAA mandated Airworthiness Directive thus making compliance of the MSB absolutely mandatory for all owners of affected aircraft. The authority of the manufacturer’s “Mandatory” Service Bulletin does not carry the same weight as a government issued AD due to the provision for certain service information evaluation procedures common to aircraft operators in both the United States and Canada.
There was a flight control system AD issued in Canada as CF2000-20 (R2) directed at the Cessna 150/152 series aircraft after a training aircraft was stuck in a spin as a result of an unfortunate jammed rudder horn which subsequently prevented applied rudder movement in the opposite direction. The result was an accident involving a flight instructor and student with fatal injuries. Technical analysis of the rudder control system found that the rudder horn stop pad contact had jumped the tail cone mounted stop bolt and remained jammed in that position. Maintenance professionals in the field thought this to be a very unlikely scenario unless there was already a serious pre-existing rigging or structural issue with the rudder control system. Nevertheless, the resulting fix was an approved modification kit from the manufacturer which included larger diameter rudder stop pads and stop bolts to be installed so that recurrence of such a condition would be virtually impossible. Today, you can see these improved components installed on any Cessna 150/152 series aircraft in the field.
The robust flight control systems in light aircraft sometimes present an unintentional attitude of complacency with the integrity of these systems. While it is true that they are rarely the subject of unsafe conditions, the cumulative effect of minimal inspection during maintenance activity could leave room for unsafe conditions to develop over time, especially in hard to reach places. Maintenance crews are reminded to exercise vigilance when working on aircraft systems and even thee related aircraft structures in order to catch any developing issues sooner than later. Part of overcoming complacency during inspection is to expect the unexpected! In some cases, it may be beneficial for operators to implement a detailed cable inspection or cable replacement procedure into their maintenance schedule. We often focus on what our minimum compliance criteria dictates yet there are always opportunities to perform over and above the minimum requirements for a little more peace of mind.
My intention is to simply make us more aware of the aircraft that we fly and its associated systems. We have entered an era in general aviation where many of these aircraft are subject to additional inspection activities in order to diligently maintain the ongoing airworthiness of an aging aircraft fleet. Many of the major manufacturers have already mandated such inspections in order to preserve the integrity of their products beyond their original service expectations. This is surely a testament to the solid design and collective experience of maintenance personnel yet we are also reminded that the environmental effects such as corrosion will always threaten even the best of practices when it comes to service severity, fatigue and the specific nature of aircraft operations. The best remedy to stay the course in aircraft maintenance is to follow the latest data and standard practices endorsed by the manufacturers of these fine aircraft. Even with the development of newer aircraft designs, it is quite likely that the common legacy designs that have been around for decades will continue to thrive when maintained diligently with attention to the details.
aircraft systems and even thee related aircraft structures in order to catch any developing issues sooner than later. Part of overcoming complacency during inspection is to expect the unexpected! In some cases, it may be beneficial for operators to implement a detailed cable inspection or cable replacement procedure into their maintenance schedule. We often focus on what our minimum compliance criteria dictates yet there are always opportunities to perform over and above the minimum requirements for a little more peace of mind.
My intention is to simply make us more aware of the aircraft that we fly and its associated systems. We have entered an era in general aviation where many of these aircraft are subject to additional inspection activities in order to diligently maintain the ongoing airworthiness of an aging aircraft fleet. Many of the major manufacturers have already mandated such inspections in order to preserve the integrity of their products beyond their original service expectations. This is surely a testament to the solid design and collective experience of maintenance personnel yet we are also reminded that the environmental effects such as corrosion will always threaten even the best of practices when it comes to service severity, fatigue and the specific nature of aircraft operations. The best remedy to stay the course in aircraft maintenance is to follow the latest data and standard practices endorsed by the manufacturers of these fine aircraft. Even with the development of newer aircraft designs, it is quite likely that the common legacy designs that have been around for decades will continue to thrive when maintained diligently with attention to the details.