Flying Accident to Mig-21/C-1129 of MOFTU 'A'/11 Wg/Eac

Abstract

IBB inner raceway has pre-existing damage in the form of spalling due to contact fatigue. The raceways and balls are imprinted with short sliding abrasion marks, indicat-ing lubrication-related problem. These two forms of pre-existing damages do not correlate well with the primary failure of IBB. The dent marks formed due to ground impact but the raceways are displaced prior to ground impact.This displacement is also manifested by the rotational rub marks on the central portion of outer surface of the cage. IBB locknut threads have undergone plastic deformation and subsequent axial movement. It has hardness of 36-37 Rc and is significantly harder than the HPCR locknut. HPCR locknut shows complete flattening caused by extensive mat-erial flow in circumferential direction indicating circum-ferential movement. It has hardness of 27 Rc. The flaring up of split cone is caused by axial movement of non-threaded end of IB bush in it while they are in rotation. The split cone is made of low alloy steel having tempered martensitic microstructure. Inner surface has hardened case whereas outer surface is devoid of it. The cracking of split cone is limited to case cracking. The damages at inner surface of the IB bush are caused by fretting corro-sion. Fretting in-turn is caused by small oscillatory axial movement over the period of exploitation. The root cause of this may be either i)housing bore was not made upto specification, or ii) large nicks may be present on the housing shoulder. The accident might have been caused by primary failure of IB bush but it is not proved concl-usively. HPTR blade tip rubbing took place while in rot-ation. No evidences of overheating of blade are observed. The observed solidified metal droplets at the blade tip came from other overheated source made of nickel base superalloy. Embedding of foreign particles in the rubbed zone is a post impact phenomenon. The fracture mode of LP NGV platform is intergranular. The oxidation of fracture surface indicates that the fracture occurred prior to fire. It has experienced overtemperature close to y'solvus of the alloy. This overtemperature seems to have occurred over a short duration while the nearby rotating parts are in rotational rubbing contact. The intense rubbing at the rear of the HPTR disc occurred while it was in rotation. The disc also experienced radial movement while it was in rotation. The rub marks at the head of the locking pin is due to rotational rubbing. This rubbing is only possible when the shaft and assembly containing pin moved radially. This radial movement occurred before ground impact.