Less than 40 seconds.

That's how long Air India Flight 171 was airborne before it plunged into a densely populated neighbourhood in Ahmedabad in one of India's rarest aviation disasters in recent memory.

Investigators now face the grim task of sifting through the wreckage and decoding the cockpit voice and flight data recorders of the Boeing 787 Dreamliner to piece together what went catastrophically wrong in the seconds after take-off. Under international rules set by the UN aviation body ICAO, a preliminary investigation report should be released within 30 days, with the final report ideally completed within 12 months.

The London Gatwick-bound aircraft, piloted by Captain Sumeet Sabharwal and co-pilot Clive Kundar, lifted off from the western Indian city of Ahmedabad at 13:39 local time [08:09 GMT] on Thursday, with 242 people and nearly 100 tonnes of fuel on board. Within moments, a mayday call crackled from the cockpit. It would be the last transmission. This was followed by a loss of altitude and a crash engulfed in flames.

Captain Kishore Chinta, a former investigator with India's Aircraft Accident Investigation Bureau (AAIB), calls this "the rarest of the rare" crashes - a controlled flight into terrain just 30 seconds after take-off. "To my knowledge, nothing quite like this has ever happened," he told the BBC.