Five years ago, Delta Air Lines made the unusual decision to buy a refinery, to protect against the risk of high crack spreads -- the difference between the cost of crude oil and the cost of refined products like jet fuel. Many energy-market analysts thought Delta was making a big mistake. Yet while Delta's Trainer, Pa., refinery has had its ups and downs, it has earned a good rate of return overall. Most importantly, it has proved effective at insulating Delta from fluctuations in crack spreads. In the wake of Hurricane Harvey, Delta is set to capitalise on its refinery bet once again. The hurricane has completely changed the economics of Delta's refinery, albeit temporarily. South Texas is the epicentre of the US refining industry, and catastrophic flooding in the region has taken about a quarter of US refining capacity offline. <br/>