During the recent downturn, the legacy carriers have gradually used up their financial cushion of aircraft and other assets they can borrow against. If they are forced to face a new crisis, they may fail more quickly than they have in the past.

It's just the same kind of arresting gear that they use onboard aircraft carriers.

United shutting down doesn't necessarily fix the problem for other airlines, but it helps a lot. It does divert a fair amount of revenue to other airlines -- United carries about 17 percent of the nation's traffic.

The airline's already slim chances of avoiding bankruptcy are dwindling rapidly.

Usually, one would see a ratio of less than 50 percent for industrial and transportation companies; however, that's the peak and from there, it should decline as they use free cash flow to pay down debt.

The recent surge in fuel prices and Delta's disclosure that it will have to post 750 million dollars of cash collateral to extend its credit card processing agreement indicate that the airline's already slim chances of avoiding bankruptcy are dwindling rapidly.