carry out the nuclear refueling and complex overhaul of the U.S.S. John C. Stennis (CVN–74) and U.S.S. Harry S. Truman (CVN–75). (b) Use of incremental funding With respect to any contract entered into under subsection (a) for the nuclear refueling and complex overhauls of the U.S.S. John C. Stennis (CVN–74) and U.S.S. Harry S. Truman (CVN–75), the Secretary may use incremental funding for a period not to exceed six years after advance procurement funds for such nuclear refueling and complex overhaul effort are first obligated. (c) Condition for out-year contract payments Any contract entered into under subsection (a) shall provide that any obligation of the United States to make a payment under the contract for a fiscal year after fiscal year 2020 is subject to the availability of appropriations for that purpose for that later fiscal year. 129. Report on carrier wing composition (a) In general Not later than May 1, 2020, the Secretary of the Navy shall submit to the congressional defense committees a report on the optimal composition of the carrier air wing in 2030 and 2040, including alternative force design concepts. (b) Elements The report required under subsection (a) shall include the following elements: (1) Analysis and justification for the Navy's stated goal of a 50/50 mix of 4th and 5th generation aircraft for 2030. (2) Analysis and justification for an optimal mix of carrier aircraft for 2040. (3) A plan for incorporating unmanned aerial vehicles and associated communication capabilities to effectively implement the future force design. (c) Briefing Not later than March 1, 2020, the Secretary of the Navy shall provide the congressional defense committees a briefing on the report required under subsection (a). D Air Force Programs 141. Requirement to align Air Force fighter force structure with National Defense Strategy and reports (a) Required submission of strategy Not later than March 1, 2020, the Secretary of the Air Force shall submit to the congressional defense committees a fighter force