Amid all the uncertainty swirling around the global aviation industry, one mainstay in the US remains firmly in place - the continued strength of the market leaders, Delta Air Lines and United Airlines. Their commanding lead in US airline profitability carries on, despite headwinds created by conflict in the Middle East. The steps each company has taken to invest in product and brand have created a resilience that many other airlines are chasing; and the outcome of those efforts is unpredictable. Both Delta Air Lines and United Airlines believe their financial fortunes will continue to improve as they set clear margin targets and continue to widen their competitive gap with other US airlines, against a backdrop of structural change in the country's airline industry.