Covid-19 will impact the US airline industry even more than the terrorist attacks on September 11th. Planes have been grounded and the financial impacts of the pandemic will change the industry and air travel for the foreseeable future.

As a result of the pandemic, losses in the first half of 2020 have been unprecedented. For example, Delta Air Lines, the most profitable US airline in recent years, saw its revenues plunge 56% and its operating margin flip from a healthy 14% to negative 40%. And the current outlook for the third quarter is not much better. By contrast, after the terror attacks, Delta's revenues dropped 25% in the second half of 2001 and its margin was negative 22%.

Airlines have also slashed costs more than previously thought possible in recent months, which has reduced losses -- Delta (DAL) and United Airlines have each reported that they cut operating costs by more than 50% in the second quarter. Lower fuel prices have helped, but not much when there are fewer planes in the air. Voluntary employee retirements and furloughs are trimming labor costs, and many airlines are warning that more layoffs are coming when federal payroll aid ends September 30 -- Delta, for example, just announced that it plans to furlough almost 2,000 pilots in October.