Complex sentence: The flight that would later kill Corporal Scott was begun alone as Lieutenant Rockwell test-flew the plane to ensure proper functioning; after reaching 40 mph and assured that the aircraft was functioning properly, the Lieutenant landed and picked up Corporal Scott.
Split into simpler sentences: The flight that would later kill Corporal Scott began as a solo flight when Lieutenant Rockwell test-flew the plane to ensure proper functioning. After reaching 40 mph and assured that the aircraft was functioning properly, the Lieutenant landed and picked up Corporal Scott.