PSI - Issue 60

V. Venkatesh et al. / Procedia Structural Integrity 60 (2024) 372–381 V Venkatesh et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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consists of a series of control rods which transmit the pilot inputs to movement in the rotary wings. The most common mode of failure in aircraft systems is fatigue (Findlay and Harrison, 2002). Many of the structural components used in aircraft or helicopter control systems are often subjected to machining operations during manufacture. In machined components, surface roughness plays a dominant role in initiating premature failure (Javidi et al., 2008) (Novovic et al., 2004). Machining process can inadvertently introduce notches or stress concentration points on the component's surface (Liu et al., 2021) (Gao et al., 2020). These notches can act as potential sites for fatigue crack initiation, leading to reduced fatigue life of the component. Surface finish of the aircraft components is critical for fatigue performance of the aircraft components. Proper tool selection, adopting suitable machining techniques, and post-machining treatments are crucial in minimizing the notch sensitivity and ensuring optimal surface finish (Sasahara, 2005) (Arola and Williams, 2002) (Dahlman et al., 2004). This paper discusses the failure of a non-rotating control rod that links the bell crank of the control system to the stationary swash plate of a helicopter. The non-rotating control rod is made of Ti-6Al-4V alloy. The failure of the control rod resulted in loss of control, which led to the accident. Through systematic analysis, the primary cause of failure and the subsequent events that led to the accident have been established. 2. The Accident A helicopter was involved in an accident during a planned helipad sortie. The sortie was uneventful until the approach to the designated helipad. At about 50-60 m short of the helipad, the crew heard an unusual noise followed by a downward jerk. The helicopter then encountered transition at about 20-30 meters height and began pitching down and rolling to the left. The crew attempted to control the helicopter but it did not respond to control inputs. The helicopter hit the ground in a pitch down attitude and bank to the left. The crew carried out an emergency shutdown and evacuated the helicopter. A preliminary examination of the wreckage revealed fracture in the non-rotating control rod. The function of the control rod is to transmit control inputs from the pilot to the helicopter's flight controls. The non-rotating control rod was subjected to metallurgical investigation to identify the primary failure that led to the loss Figure 1(a) shows the fractured non-rotating control rod recovered from the accident helicopter. The control rod was found to have fractured at the rod-to-eye-end transition fillet region of bearing housing at the swash plate end. After fracture, the housing deformed and opened up. Examination also revealed localized damages to the inner surface of the bearing housing at symmetrically opposite location at the collar regions (refer Fig.1(c-d)). Similar damages were also observed at corresponding locations on the bearing housing race outer surface. One such region is shown in Fig.2(a). There were no external mechanical damages to the control rod. Both the bearings were freely rotating without any restriction. There was no deterioration on the ball surface of the bearings (refer Fig.2(b) and Fig.2(d)). Figure 3(a) shows the fracture surface of the control rod bearing housing. The presence of well-defined crack arrest marks or beach marks was observed on the fracture surface, which is characteristic of progressive crack propagation. From the orientation of the crack arrest marks, crack origin region could be identified, and it was found to be on the rod-to-eye-end fillet surface. The corresponding location on the fillet surface is shown in Fig.3(b). Examination revealed that after initiation on the surface, the crack propagated progressively over about 85% of the cross-section of the housing before finally leading to the final overload fracture. Although there were no external mechanical damages at the crack region, distinct machining marks were found present on the fillet surface in and around the crack origin. of control in the helicopter. 3. Results and Discussion 3.1. Visual Observation and Stereo Microscopy