 The biggest and most profitable U.S. airline just posted its first quarterly loss in more than five years. Now things are going to get really bad. Delta Air Lines Inc. reported Wednesday that it lost 534 million in the first three months of the year, when it suffered a glancing blow from the coronavirus pandemic during March. For most of the first quarter, the virus seemed like an abstraction to many Americans. Sure, it sounded dire, but it was an ocean away. Life in the U.S. unfolded normally. The economy was humming, people took business trips and leisure flights, planes were nearly full. Since then, the virus brought restrictions on travel and stoked fear of being trapped in a flying aluminum tube while sharing recycled air with people who might be infected. Travel has plunged about 95. Deltas first-quarter revenue dropped 18 from a year ago. The airline forecast a much grimmer picture for the second quarter, when the full impact of the global pandemic will be felt. Revenue is expected to plunge 90 for Delta, and things will likely to look just as bleak or worse for its competitors. It could be a long time before travel returns to pre-outbreak levels. Whether its three years or two years or four years is anyones guess, said Delta CEO Ed Bastian. Here are some vital signs to watch in the airline industry: ___ TRAFFIC HAS VANISHED If there is one chart that captures the implosion of the U.S. airline industry, it might be the number of people screened each day at the nations airports by the Transportation Security Administration. About 2.3 million people passed through security checkpoints on March 1, unchanged from the same day last year. The numbers careened sharply lower from that point on, plunging below 100,000 by early April a drop of about... 