WASHINGTON – Shrinking budgets may force the US Air Force to revise its modernization plan by reducing aircraft buys or postponing critical programs, according to a new analysis by the Congressional Research ServiceCenter.

Although the pending expiration of sequestration caps offers some relief, the Air Force may not be able to afford its ambitious plan to modernize aging fleets, CRS’ Jeremiah Gertler wrote in a Dec. 11 report. The Air Force has several major projects in the pipeline, including the development of the Long Range Strike Bomber, the ramping-up of F-35 joint strike fighter buys, and the KC-46 replacement tanker program.

Unless the Air Force revises its topline, the service may be forced to reduce annual quantities of the F-35, defer the follow-on KC-Y tanker, or fund LRS-B through a separate budget, Gertler writes.

Together, the top five Air Force priorities — LRS-B, KC-46, F-35, C-130 cargo aircraft, and remotely piloted aircraft (RPAs) — account for $67.2 billion over the next five years, according to Gertler. But these are not the service’s only modernization requirements. The service also plans to recapitalize its ground surveillance fleet, the Joint Surveillance Target Attack Radar System (JSTARS) ground-surveillance fleet,; buildt a new Combat Rescue Helicopter (CRH),; develop a replacement for Air Force One; and procure T-X, a new advanced trainer aircraft, called T-X.

"The total investment required for these nine programs, combined with the budgetary restrictions in place as a consequence of the Balanced Budget Act of 2013 (P.L. 113-67), or BBA, poses a significant challenge to Air Force budgeters," Gertler writes.

How to fund LRS-B is a major point of contention within the Air Force. Spending for the new bomber, currently right now funded through the R&D budget, is projected to triple over the next five years, Gertler writes. LRS-B, CRH, the presidential aircraft replacement, T-X, and JSTARS recap will likely shift from R&D to the procurement budget at roughly the same time, setting up a battle for resources.

Increasing the Air Force’s modernization budget after sequestration caps expire is not a given, particularly as other Pentagon priorities, like the Navy’s effort to replace its Ohio-class nuclear submarines, come to a head. But Gertler warned that reducing or deferring spending may lead to gaps in important capabilities, and he urged the Air Force to think carefully about which programs to prioritizees.

One possible area to make cuts is Tthe F-35A conventional takeoff and landing variant, which currently represents 42 percent of the Air Force’s five-year budget for the top nine programs, is one possible area in which cuts could be made, Gertler wrote. Some say reducing the annual buy from 60 to 48 aircraft a year would free up approximately $1 billion per year for other priorities, he wrotewrites. However, the report says that this estimate does not take into account the increased cost of other F-35 models to other services and allies due to a reduction in the overall buy, costs incurred by extending the F-35 line, or capability gaps created through the delayed introduction of more modern aircraft, he wrote.

The Pentagon is currently re-evaluating the requirement for F-35 quantities in response to a congressional directive. A report accompanying the 2016 National Defense Authorization Act directed the US Secretary of Defense to submit a report within 180 days to either revalidate the current requirement, or identify a new one for the total number of F-35s needed.

US Senate Armed Services Committee Chairman and longtime F-35 critic Sen. John McCain, R-Ariz., recently suggested the total buy is out of whack with budget realities.

"We're going to have to reduce the buy," he told reporters in October. "The number they are now quoting — there's just not going to be that many."

The Air Force is in the midst of reviewing the requirement, Air Combat Command chief Gen. Hawk Carlisle said last month. However, he stressed that it is too early to make a final decision.

"They have looked at it and we certainly owe an answer back to Congress on that," Carlisle said Nov. 1 during a breakfast hosted by the Defense Writers Group. "I think [that] as we look to the future and what we're going to do, I think [that] there is a decision to be made on how many F-35s we're going to buy. It's way too early to make that decision when we're not even [at initial operational capability] yet."

Another option to get through the Air Force's modernization "bow wave" is to fund LRS-B through a non-service budget, as the US Navy has done for its Ohio-class replacement submarines, Gertler wrote. The same arguments for Congress' creation of the National Sea-Based Deterrence Fund, a Pentagon fund that is separate from the Navy's regular shipbuilding account, could be made for LRS-B, Gertler wrote.

"This was based on two arguments: (1) that the strategic deterrence mission of the SSBN(X) was a national mission, not unique to the Navy, and (2) that funding the procurement of SSBN(X)s outside the Navy’s shipbuilding budget would preserve Navy shipbuilding funds for other Navy shipbuilding programs," he wrote. "The same arguments could be applied to LRS-B.,

However, this proposal could face opposition on Capitol Hill. Ohio replacement is a unique program that represents 70 percent of the nation's strategic deterrence, Rep. Randy Forbes, R-Va., told Defense News recently. By contrast, LRS-B will carry conventional, as well as nuclear, weapons.

"When you are looking at the Sea Based Deterrence Fund, they are kind of apples and oranges," Forbes said. "We will do the funding [for LRS-B] but whether or not we have a unique funding system ... I'm not certain that will be needed."

The Air Force has not publicly pushed the US Congress to set up a separate fund solely for the bomber, in contrast to the Navy's very public insistence that the shipbuilding fund alone cannot cover the new subs.

Phillip Swarts contributed reporting.

Email: lseligman@defensenews.com

Twitter: @laraseligman

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