Metallurgical Analysis:Flying Accident to Mig-27 ML TS - 579

Abstract

At Air Force Station, Kalaikunda in September’2010, during the low level high speed run of sortie, the pilot started ejection at a height of ~750 m with a speed of ~500 Km hr-1. Suddenly it was noticed, that rear fuselage caught fire. The nose of the aircraft turned sharply downward. The aircraft crashed against ground, fragmented in pieces and got embedded deep into paddy field. The present inves-tigation aims at finding out nature and cause of the accident as well the sequence when firing took place. From the damaged components critical parts were identified and investigated. The component was made of Al alloy with Cu, Mg and Mn as major alloying elements to harden the matrix by secondary dispersion. The composition is close to the Russian specification of D16 AMTY 482-61. The microstructure of the alloy consists of equiaxed grains. The grain boundary was c learly visible. Uniform distr-ibution of second phases was found. The obtained hardness of the alloy was 111 - 115 BHN. Thus either structural or mechanical degradation was not observed. Clear evidence of intergranular failure in one of the samples, indicates stress corrosion cracking of the comp-onent during service exploitation. The stress corrosion cracking during service exploitation is the primary cause of failure. The damage to all other components is seco-ndary in nature and manifested by dimple rapture owing to overload. The fracture surface of the damaged component exhibited excessive scratch marks, tearing and folding. These may be because of the rubbing action and multiple impact of the component at the time of the accident. Formation of thick oxide film was due to the exposure of same in moist environment over some time period after the accident. As the accident was preliminary caused by damage of the component owing to stress corrosion cracking, it is not possible to identify the effect/sequence of fire and inflammable materials during accident.