Investigators now think they know what went wrong. Modern aircraft are operated much of the time in auto-pilot but when something goes wrong, human intervention is needed in order to stop computers from doing the wrong thing.
Flight 539 was 20 minutes from San Jose International when the fire department was alerted to prepare for a "Level 2 Emergency," a major accident with casualties. "It originally came in as a catastrophic failure electrically on the plane, and that there was a depressurization of the cabin," Capt. Mary Gutierrez recalled.
Aboard the 737-400 jet were 131 passengers and a crew of five. The loss of cabin pressure caused ears to pop. "Pretty soon, I was in pain and I looked around and everyone else was grabbing their ears, and that's what happens I guess when the plane loses its cabin pressure," passenger Dick Peck said. "And then the captain said, 'We're really sorry, but we have a problem with the cabin pressure, but it's under control now,'" passenger Carol Peck said.
The Ontario to Seattle-bound aircraft was operating routinely in auto-pilot mode when something malfunctioned. "Some sort of probably mechanical or electrical failure occurred that told the airline, 'You're really not at 25,000 feet. You're really sitting on the ground.' And all of a sudden, many sensors and systems went into a ground mode, which obviously at 25,000 feet, is not good," ABC News aviation consultant Steve Ganyard. The on-board computers might try to pull on the throttle to reduce speed which could cause the plane to descend rapidly.
In the meantime, emergency crews assembled on the runway at San Jose International Airport. As part of the plan for preparing for aircraft disasters or earthquakes, hospitals are also put on alert to receive multiple victims. Passengers said they were not given details of the emergency. Oxygen masks did not deploy. They were not instructed to prepare for a crash landing. "It was scary because we heard a sound. Babies were crying and we all knew something was going on, and they told us we're landing in San Jose," passenger Roslin Richardson said.
Alaska Airlines says the pilots manually depressurized the cabin in case the electrical system didn't work because otherwise, the imbalance of pressure would not allow the aircraft door to open once on the ground. The 131 passengers were put on other flights while the 737 stayed in San Jose for inspection.