WASHINGTON — A new report from the Pentagon’s inspector general asserts there were years of accumulating signs that should have prompted the Air Force to take a closer look at the KC-46′s refueling boom, on which the service is now spending $100 million to redesign.

In the years leading up to 2018 — when the service tested the full functionality of the KC-46 boom and found it could not refuel certain military aircraft — Air Force program officials continuously reduced flight testing requirements, the Department of Defense IG wrote in a May 21 report.

This reduction in testing occurred even as prime contractor Boeing made changes to the design during the preliminary design review and the engineering and manufacturing development phase.

Ultimately, the IG found program officials “did not effectively manage the development of the refueling boom for the KC-46A tanker.”

Those mistakes led the Air Force to make an expensive concession in 2019. Even though the Air Force and Boeing had agreed to a fixed-price contract in 2011, which locked Boeing into paying for cost overruns caused by technical flaws, the service signed off on Boeing’s boom design during Milestone C in 2016.

Ultimately, the Air Force agreed to fund a redesign of the KC-46 boom using taxpayer dollars.

“Had KC-46 Program Office officials effectively managed the development and testing of the refueling boom for the KC-46A tanker, the Air Force would not have had to spend an additional $100 million for the redesign of the refueling boom to achieve its required performance,” the DoD IG wrote.

“Furthermore, retrofit of the refueling boom for the delivered KC-46A tankers is not estimated to begin until January 2024, and will result in additional undetermined costs, as well as approximately a 5-year delayed delivery of the first KC-46A tankers with fully mission-capable refueling booms.”

The problem revolves around “boom axial loads” that are too high. In essence, the KC-46′s boom is too stiff to extend or retract unless subjected to more force than certain aircraft can generate. In 2018, the Air Force labeled it as a “category 1” deficiency — the most serious technical deficiency that could impact operations or safety.

The Air Force made several decisions that negatively impacted its ability to manage the development of the KC-46 boom, the IG stated.

The refueling boom presented by Boeing during the 2012 preliminary design review “differed significantly from the initially proposed design” because “a computer control system was integral to its function,” the IG said. The KC-10 boom, a proven design on which the KC-46 boom was supposed to be based, does not use a computer control system.

“We concluded that the KC-46A refueling boom included new or novel technology,” the IG wrote. “Therefore, KC-46 Program Office officials should have ensured that the refueling boom was demonstrated in a relevant testing environment before proceeding beyond Milestone B.”

Later, in 2014, the Air Force decided to execute a reduced flight test plan for the KC-46.

The IG noted that Air Force pilots in 2016 reported that the KC-46 boom had trouble refueling C-17s during tests. Although Boeing made hardware and software updates, the program office continued with the reduced flight test plan, making further cuts to testing during the flight assessments for Milestone C later that year.

For example, the reduced flight plan allowed for the KC-46 to refuel an A-10 flying at one airspeed and altitude combination, but the test was executed with an A-10 in a clean-wing configuration that was not representative of a combat environment. An A-10 loaded with weapons and other equipment would have made the test more realistic, the IG said.

Flight tests of the C-17 and F-16 were similarly not tested at heavy gross weights or “the most critical stressing point for aircraft center of gravity,” the IG wrote.

“After successfully completing the reduced flight tests, KC-46 Program Office officials prematurely considered the refueling boom high axial load problem resolved,” thus missing a key point where it could have made corrective actions, the IG said.

The Air Force rediscovered during tests in 2018 that the boom axial load problem continued to negatively impact A-10, C-17 and F-16 receiver aircraft, the IG stated. By then, it was too late — the service had already signed off on the boom design.

Boeing is set to deliver 179 KC-46s over the program of record and has delivered more than 40 aircraft to the Air Force so far.

However, because of the boom axial load deficiency and other technical problems involving its Remote Vision System, the KC-46 is only approved to refuel certain aircraft under certain conditions, and isn’t slated to be fully operational until at least fiscal 2024.