Complex sentence: A major instability problem with these designs was discovered during the course of flight testing and was determined to be induced by "air flow separation" whereby the air stream would become very turbulent, causing loss of control and lift.
Split into simpler sentences: A major instability problem with these designs was discovered during the course of flight testing. It was determined that induced "air flow separation", in which the air stream would become very turbulent, would cause loss of control and lift.