Investigation of failed components of a helicopter

Madan, M and Bhuvana, R and Sujata, M and Venkataswamy, MA and Bhaumik, SK (2009) Investigation of failed components of a helicopter. Project Report. National Aerospace Laboratories, Bangalore, India.

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A few components belonging to a helicopter, which met with a Cat-I accident on November 27, 2008, were sent to this laboratory for investigation. Examination revealed that all the four bolts connecting the tail drive shaft (TDS) with the main gearbox (MGB) have sheared off. Fractographic study confirmed that these bolts have failed by fatigue. Examination also discovered that three of the four bolts used were not as per the design specification. It was found that these bolts were longer than that of the recommended one, and they are actually meant for use with the intermediate gearbox (IGB) casing. There was mix-up of bolts and the wrong ones were used in this particular case. The shank length of the IGB bolts is such that it does not provide enough threads to allow tightening of the assembly joint at the TDS connecting flange. Since the bolts cannot travel beyond the first thread from the shank, the reading on the torque wrench during this assembly would have been a wrong indication of tightness of the joint. In fact, the joint was either loose or did not have required pretension. Because of this, there was slip between the TDS connecting flange and the main rotor brake disc. This had resulted in excessive load on the fourth bolt which appears to have failed first by fatigue. Following this, the unequal load distribution in conjunction with the looseness in the joint was responsible for the fatigue failure of the remaining three bolts. Hence, the primary failure in the system was the failure of the bolts at the TDS connecting flange – main rotor brake disc assembly. The other damages seen on other components/structures of the helicopter are secondary in nature and would have occurred subsequent to the primary failure or during crash onto the ground. It is, therefore, stated that the mix-up of the bolts during assembly of the TDS was responsible for the failure and the accident.

Item Type: Monograph (Project Report)
Uncontrolled Keywords: Helicopter;TDS;Bolts;Fatigue
Subjects: AERONAUTICS > Aeronautics (General)
CHEMISTRY AND MATERIALS > Chemistry and Materials (General)
Depositing User: Ms. Alphones Mary
Date Deposited: 18 Jul 2011 05:39
Last Modified: 18 Jul 2011 05:39

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