The Fatigue Life of Cables in Aircraft Flight Control Systems

The cable flight control systems are commonly used for the control of small airplanes. In these systems, the cables are the only elements transmitting loads from the pilot to the control surfaces. During a flight the cables are moving through pulleys and are subjected to variable loads. A simple analysis of stress in the cable shows that the stress generated by the cyclical bending on the pulleys causes the fatigue of the wires. This phenomenon was noticed on a military aircraft of the M28 family during periodic maintenance inspection in 2007. The endurance tests of KSAN cables of the diameter equal to 3.5 mm and 1.8 mm were performed at the PZL MIELEC. The tests showed the limited fatigue life of the cables due to a progressive increase in the number of broken wires.