New Delhi, July 13 -- In the final seconds of Air India Flight 171, as both engines lost power and the Boeing 787 began its fatal descent, a crucial exchange unfolded between two experienced pilots that investigators believe holds the key to understanding what went wrong. "One of the pilots is heard asking the other why did he cutoff. The other pilot responded that he did not do so," according to the Aircraft Accident Investigation Bureau's preliminary report. Those paraphrased words are at the centre of a complex investigation into whether the accident was caused by human error, mechanical failure, or an emergency procedure gone catastrophically wrong. The cockpit was staffed according to standard procedures, with first officer Clive Kunde...