Medical emergencies are rare, but hardly unprecedented in commercial flights. It's far less common when they involve the plane's pilot -- and that pilot dies. That's what happened Monday on an American Airlines flight between Phoenix and Boston. Captain Michael Johnston, 57, was piloting the plane when he fell ill somewhere en route. The co-pilot landed the jet safely. An autopsy and preliminary toxicology tests "showed that the death was the result of natural diseases," the O-non-daga County, New York, medical examiner's office said -- without offering specifics.