Investigators say bad maintenance practices at a Georgia air force base missed a deteriorating propeller blade that broke off six years later as a U.S. Marine Corps transport plane cruised over Mississippi at 20,000 feet, causing the KC-130T to break into pieces and plunge into a soybean field, killing 15 Marines and a Navy corpsman.

The report on the causes of the July 10, 2017 crash, released Wednesday, slams "consistent production errors" at Warner Robins Air Logistics Complex in Warner Robins, Georgia, saying evidence from the crashed plane shows employees missed growing corrosion on the key propeller blade during a 2011 overhaul. The report finds workers at the base did a poor job of following the Navy's specific procedure for its propellers, in part because the vast majority of blades overhauled at the base followed different procedures. The report indicates the Air Force has now agreed to adopt the Navy's more demanding overhaul procedures for all propellers.

Military officials have known of the problems since at least September 2017 and some family members had previously indicated they knew what had happened, although they declined to discuss details. In July, just before the anniversary of the crash, Anna Johnson, the widow of crew member Gunnery Sgt. Brendan Johnson told The Associated Press that "planes don't just fall out of the sky.

"It was a grave mistake, it was an accident that was most likely preventable," Johnson said then. "I don't want their deaths to be in vain. I want something good to come of it."

The report lays out 17 recommendations to prevent a recurrence. Brig. Gen. John Kubinec, commander of the Warner Robins Air Logistics Complex, told The Telegraph of Macon that the base expects to restart propeller overhauls early next year.

"When we first heard that work done here in 2011 may have contributed to the mishap, leadership and the (propeller) shop were devastated," Kubinec said. "The first thing we did was take action to ensure that processes were in place that this wouldn't happen again. That's what our commitment has been since we first heard about it."

The report says a corrosion pit eventually developed into a crack, breaking off from the propeller closest to the fuselage on the left-hand side of the plane. A number of other propeller blades on the four-engine aircraft were also found to have corrosion. The report said investigators found a protective coating had been painted over corrosion on some blades from the plane, proving that Warner Robins Air Logistics Complex workers "failed to detect, remove and repair corrosion infected blades they purported to have overhauled."

The report said inspectors visiting the base were dismayed to find workers relying on memory for how they should conduct propeller maintenance, even though they had laptops with the correct procedures at their work stations. They also said technicians did a poor job of tracking paperwork that said who a propeller belonged to, which determined whether they were supposed to use methods for the Air Force, the Navy or P-3 surveillance planes. Plus, quality inspections did not cover "the steps regarding identification and removal of corrosion."

The Air Force doesn't know which technicians inspected the blade in 2011, though, because its previous policy was to dispose of maintenance paperwork after two years. Although the Navy had the power to audit work done by the Air Force in Georgia, the report says there's no evidence any audit ever occurred since the Navy handed off the work to the Air Force in 2009.

The report also concludes that the Marine Aerial Refueler Transport Squadron 452 at Stewart Air National Guard Base in Newburgh, New York, didn't do enough to inspect propeller blades or track maintenance records. The squadron was supposed to perform an electrical current inspection on blades any time a plane didn't fly for more than eight weeks, but did not. However, investigators said that even if maintenance workers had conducted inspections they missed, they might not have found the problem.

"It cannot be concluded with any reasonable degree of certainty that the radial crack would or would not have been detected," investigators wrote.

The blade sliced through the fuselage where passengers were sitting, lodging into the interior of the right hand side of the skin. The impact affected the drive shaft of a propeller on the right side, causing that propeller to break loose, causing it to hit the fuselage and then knock part of the stabilizer off the plane. The plane, then basically uncontrollable, broke into pieces, and the area containing passengers "explosively disintegrated."

The report says all aboard suffered "shock, disorientation, inadvertent physical responses, rapid onset of below freezing conditions and near impossible crew communication." All the men died from blunt force trauma and contusions, investigators found.

Despite speculation at the time, the report found ""no evidence of inflight fire damage or ammunition discharge."

The Navy grounded its fleet of C-130Ts until propellers are replaced, with Congress appropriating $121 million to accelerate the work. However, the aging KC-130T models like the one that crashed are being phased out. C-130s have historically been one of the military's safest aircraft.

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Follow Jeff Amy at: http://twitter.com/jeffamy .

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Johnson reported from New Orleans.