— Deborah Lee James, Secretary of the Air Force 
The U.S. Air Force is in the midst of an ambitious modernization program, driven primarily by the age of its curren t aircraft fleets. In FY2016, the four procurement programs (F-35A, KC-46, C-130, and RPA) account for 99% of the Air Force's aircraft acquisition budget; LRS-B is 5% of the Air Force overall research and development (R&D) budget, but 60% of the budget for Advanced Component Development & Prototypes. The current plan also includes no funding for the long-expected CVLSP helicopter replacement program. The major R&D programs offer a different picture. Raise the Topline (and/or the Aviation Modernization Share)

As noted, the BBA-mandated caps on defense spending end in FY2021. Canceling programs can lead to gaps in important capabilities. One also has to guess correctly which programs to reduce as the program delayed or deferred today may be exactly the one needed sooner should requirements, scenarios, adversary capabilities, or other factors change in the future. Noting that CRS does not endorse any particular option, some possible spending reductions or deferrals that Congress may consider include (but are not limited to) the following:

			Pusharounds/Reductions in Other Programs and Activities

Aviation modernization is just one part of the overall Air Force budget. The different sources of funds impose various costs. Reducing personnel to fund modernization could challenge the Air Force's ability to carry out its full range of missions. Retard Program Growth Further

As noted earlier, the Air Force has already deferred some new starts to keep its modernization programs within a constrained topline.
The U.S. Air Force is in the midst of an ambitious aviation modernization program, driven primarily by the age of its current aircraft fleets. Four major programs are in procurement, with five more in research and development (R&D).
The need to replace several types of aircraft simultaneously poses challenges to future budgets, as the new programs compete with existing program commitments and normal program growth under a restricted service topline. The impending expiration of caps imposed by the Balanced Budget Act coincides with when modernization programs can be expected to experience the most growth, but does not necessarily offer sufficient relief to avoid program cuts or other funding approaches.
To meet its modernization requirements, the Air Force may need to revise that topline, defer or delay other programs (including possibly reducing the quantity of aircraft already in procurement), or find other sources of funding to carry all its plans to fruition. Some specific options may include (but are not limited to)
raising the Air Force topline (and/or the aviation modernization share); pusharounds or reductions in Air Force programs and activities other than modernization; reducing annual quantities of the F-35A; further retarding the growth of R&D programs; deferring the KC-Y follow-on tanker; funding the long-range strike bomber through a non-Air Force budget.
The report examines these options in further detail.