航空业界正在努力理解为何一架客机与一架军用直升机会在半空中发生致命的相撞事故。一位专家形容，那是“全球控制最严的空域”。

LOS ANGELES, April 19 (Xinhua) -- Three people were killed after a small plane crashed into a river Friday night in the midwestern U.S. state of Nebraska, authorities said. The small plane was ...