The bleak picture is compounding an already dire financial situation for the airlines, which are burning through cash and talking to the Treasury Department about grants. Newly revised federal rules will let the companies cut some routes by as much as 90% through September and eliminate others altogether to avoid flying nearly empty planes.

A carrier that served a city less than five times weekly would need to provide only one flight a week under final Transportation Department rules issued Tuesday on minimum domestic flying levels through Sept. 30. A company with more than 25 weekly flights would be able to scale back to only five. On some routes, the drop in service could be about 90%.

The regulations open the door to major service reductions as airlines gird for a lean summer, when planes are usually jammed and the industry collects its largest profits. Underscoring the urgency of the cuts, the number of people flying in the U.S. dropped to below 100,000 on Tuesday, 95% below the level a year ago.