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National Transportation Safety Board officials have ruled out "wind shear" as a factor in the incident. Instead, they are looking into possible mechanical failure or crew error. It has been established that the automatic pilot system was in operation while the plane cruised at 41,000 ft. This mechanism keeps aircraft on a safe course at high altitudes, where the human eye cannot judge the angle of flight. But while a plane is operating under the system, even small variations in speed or flight angle can cause a stall or nose dive. A key question is whether Ming overrode the system while making his descent. Federal investigators will continue inspecting the aircraft and interviewing passengers and crew members for several weeks.