CFR > Title 14 - Aeronautics and Space > CHAPTER I—FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION > SUBCHAPTER G—AIR CARRIERS AND OPERATORS FOR COMPENSATION OR HIRE: CERTIFICATION AND OPERATIONS (parts 110 to 139) > PART 121—OPERATING REQUIREMENTS: DOMESTIC, FLAG, AND SUPPLEMENTAL OPERATIONS > SUBPART H—Aircraft Requirements (§121.151 to §121.163) > 121.159—Single-engine airplanes prohibited.