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When an airplane is over stable, it is easy for it to fly in a straight line, but harder to maneuver. When an airplane is under stable, it is hard for it to fly in a straight line, but easier to maneuver. This is why airliners, which need to fly safely in a straight line, tend to be made slightly over stable. This is different than fighter planes, which need to turn very quickly, so they tend to be under stable.