Operators of piston-engine aircraft operating under part 91 of such title, excluding those operating under subpart (K) of such part. (x) Operators covered by subpart (K) of part 91 of such title. (xi) Operators covered by part 133 of such title. (xii) Operators covered by part 136 of such title. (xiii) Operators covered by part 137 of such title. (xiv) Operators of public aircraft that qualify under section 40125. (xv) Operators of aircraft that neither take off from, nor land in, the United States. (B) Additional segments The Secretary may identify and include additional segments of air traffic users under subparagraph (A) as revenue and air traffic services cost data become available for that additional segment of air traffic services users. (4) Definitions For purposes of this subsection: (A) Air traffic services The term air traffic services means services— (i) used for the monitoring, directing, control, and guidance of aircraft or flows of aircraft and for the safe conduct of flight, including communications, navigation, and surveillance services and provision of aeronautical information