Seven years ago, Amol Yadav announced to his family and friends that he would build an aeroplane on the roof of a boxy apartment building in the teeming Indian city of Mumbai. Incredulous friends and family members asked the young pilot how he planned to bring the plane down once it was complete. "I really don't know," he told them. Mr Yadav, who flies twin-engine turboprop planes for a living, is nothing if not obstinate. The five-storey building, home to his 19-member joint family, didn't have a lift, so they lugged factory lathes, compressors, welding machines, and an imported 180kg...