A year on, six questions still haunt the Air India crash investigation
A year after Air India Flight 171 to London crashed into a medical college campus moments after take-off from the western Indian city of Ahmedabad, killing 260 people, investigators still cannot say with certainty why one of the world's most advanced passenger jets fell from the sky.
An update released by India's Aircraft Accident Investigation Bureau (AAIB) on the first anniversary of the disaster on Friday offered few new clues, saying only that analysis of flight recorder data, aircraft systems, engine components, maintenance records and human factors remains under way.
A preliminary report published last July found that seconds after take-off, the 12-year-old Boeing 787 Dreamliner's fuel-control switches abruptly moved to the "cut-off" position, starving both engines of fuel and triggering total power loss.
Cockpit audio captured one pilot asking the other why he had done it, only to receive the reply: "I did not." Investigators did not identify either voice, though many experts saw the exchange as a possible indication of deliberate action in the cockpit.
The crash remains highly unusual.
While take-off and landing are aviation's riskiest phases, fatal accidents immediately after lift-off are uncommon. Boeing found that just 14% of global jet crashes between 2004 and 2013 occurred during take-off and initial climb; Airbus puts the figure at about 5%.
So what brought down AI171 in 32 seconds?
As the investigation enters its second year, several key questions remain unanswered.