Failure analysis of MGB and TGB housings of a helicopter

A fractured MGB housing and a cracked TGB housing belonging to a helicopter were forwarded to the laboratory for analyzing the causes of failures. The components were made of Mg-alloy of RZ-5 specification. Investigation revealed that both the components have failed by fatigue. The fatigue crack initiation in the mounting lugs of the MGB housing was stress related. It is believed that the component has experienced unexpected high loads at these mounting locations, probably due to some abnormalities in the gearbox. However, this needs to be ascertained by detailed examination of the gearbox components. The fatigue crack initiation in the TGB housing was due to high stress concentration arising from a thin edge at the fillet region. The fatigue crack initiation was also facilitated because of wide difference in the length of the webs of the mounting lug, which was caused due to incorporation of a temperature sensor-locating boss at this location. It is recommended that the design aspects of the lug be reviewed for a possible relocation of the temperature sensor-locating boss to a non-critical region or isolation of the boss from the attachment lug