Calcutta, Aug.12 :    Calcutta, Aug.12:  A major disaster was averted with the chance detection of a six-foot-long crack on the left wing of an Indian Airlines Airbus 300 bound for Delhi on Saturday morning. The aircraft had arrived from Delhi on Friday evening. It had been checked by engineers on night duty and cleared for take off. On Saturday, after luggage had been loaded onto IC 264 and passengers were about to board the aircraft, engineers at departure noticed the crack. Passengers were put onto another aircraft, which left for Delhi three hours behind schedule.