An F-16 similar to this one crashed May 4 near Hill Air Force Base, Utah, during close-air support training. An investigation board blamed a faulty fan blade in the engine that was not identified during a visual inspection. (SMSgt Thomas Meneguin / Air Force)
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The investigation into a crash that destroyed a $23.9 million F-16C in May has been amended, after Air Combat Command's vice commander questioned findings that a faulty blade in an engine should have been spotted during routine maintenance.
An imperfection on an engine part that ultimately brought down the F-16C near Hill Air Force Base, Utah, was most likely present during manufacture of the engine, the addendum to the report states.
The accident investigation board, led by Lt. Col. Thomas Olsen, found that the blade was faulty when it was installed in 2004, and eight years of use ultimately caused it to break free and destroy the entire engine. The anomaly should have been caught in a visual inspection of the engine during installation, Olsen wrote.
The pilot was flying in a two-ship formation during close-air support training when there was a loud bang and the engine began to stall. Fan speed fell and the pilot noticed a lack of thrust, radioing "looks like it's not going anywhere. I felt a pretty big bang," according to the report.
The pilot jettisoned the F-16's fuel tanks and attempted to restart the engine before ejecting about 50 miles west of Hill. The pilot was not injured.
Following release of the original AIB report in September, Air Combat Command Vice Commander Lt. Gen. William Rew directed Olsen to reconvene the board to examine the inspection of the No. 17 turbine fan blade, which came loose and damaged the engine, causing the crash. Rew wrote new evidence affected initial claims the fan blade was inspected before installation.
In the addendum to the report, released Feb. 7, Olsen wrote that the 0.215 inch by 0.641 inch by 0.739 inch anomaly on the blade was there since manufacture and could have been found during installation on the F-16, tail No. 88-0433, in 2004 at Tinker Air Force Base, Okla. However, it could have been covered by smearing or other material, and engineers typically do not inspect new blades that closely during installation.
"The probability of detection was limited due to the possibility of material transfer or smearing during machining and that feature-by-feature inspections of new blades were deemed not necessary," Olsen wrote in the Feb. 4 report.
The addendum to the investigation report also states that inspection procedures have not changed since the blade was installed in 2004. Officials at Tinker Air Force Base, Okla., where engines are installed on F-16s, told Air Force investigators that maintenance technicians do not clean and inspect blades if they are brand new and only inspect the blades if the box appears to be damaged.
"They look for damage that might have occurred during transportation, either inside or outside of the building," a propulsion chief engineer at Tinker said in submitted testimony. "In most cases, they do not have the drawings of the part so it is unlikely they would find a manufacturing defect unless it was really prominent."
The Air Force has had problems with defective turbine blades in the past. In 2008, Pratt & Whitney settled with the U.S. Department of Justice for $52.3 million for defective turbine blade replacements for F-100 engines used on F-15s and F-16s. A defective blade caused the June 2003 crash of an F-16 near Luke Air Force Base, Ariz.
The F-16 in the May 4 crash near Hill was using a different engine, a General Electric F-110.