About 150 planes were sent to long-term storage in Roswell, N.M. -- yes, that Roswell -- where the dry conditions are better suited for long-term aircraft preservation. Many others were parked at United's hub airports in and near cities including Chicago, Washington and Newark, where technicians could more easily get them back into service if needed.
Since July, United has brought back more than 150 of the planes that the airline or its regional carriers had grounded, it said on Thursday. About 450 are still stashed away, but must be maintained in a way that allows flexibility.
To get it right, Tom Doxey, United's senior vice president for technical operations, and his team consult models created by computer scientists and solicit guidance from maintenance crews. Generally, two considerations loom large: how soon a plane will need substantial maintenance and the likelihood that it will be among the first to start flying again.
"If you have an aircraft that maybe is less likely to come back soon, you kind of want it at the back of the parking lot," Mr. Doxey said. "It goes into prolonged storage and it probably goes to a desert location."