Investigation of caving-in incident of MIG 21 aircraft

In an accident, the starboard (STBD) side of front fuselage was ‘caved in’ during ground run of the Mig-21. A post-incident inspection revealed vertical and horizontal cracking occurred at the edges of the additional air intake shutter (AAITS). The guard of the
AAITS was sucked in causing extensive damage to the aeroengine. However, port side fuselage remained un-affected. The chemical composition and evaluation of the mechanical properties of the alloy match with the Russian specification D16AT, D16ATB. It means that the duralumin alloy is in hardened and aged condition. Characterization of microstructure in optical, SEM and TEM illustrate more or less negligible variation for fresh and service exposed material. From the investigation it can be inferred, on the starboard side near some of the rivet holes at the edges of AAITS opening, primary cracks were formed owing to fretting fatigue. The structural non-uniformity of annular area on the starboard side with respect to port side raised the air pressure around the frame #5 and the skin ‘caved-in’. As engine demanded more air, under severe suction pressure, the skin containing micro-cracks
became unable to withstand the load and the guard was sucked in from outside towards the engine causing overload failure to the areas away from rivet hole and rest of the zones near the AAITS.