AP Photo

At 10:56 p.m. on April 1, last year, Southwest Airlines Flight 812, en route from Phoenix to Sacramento with 118 passengers aboard, was completing its climb to its cruise altitude of 36,000 feet, above the small town of Blythe, Calif. An air-traffic controller at Los Angeles Center had just acknowledged a routine call from the pilot. But within a minute or so of that exchange, the controller became aware that Flight 812 was in some kind of trouble. The messages were garbled until, finally, he heard the pilot clearly: “declaring an emergency we lost the cabin.”