The Marine Corps blamed a fatal crash of the V-22 Osprey on the aircraft's computer software and a flaw in the flight control system that it had been aware of for at least 18 months.
Officials said no one was at fault, though they recommended a complete review of the software and possible redesign of the hydraulic system that powers flight controls.
It's another black eye for the Osprey, the Marines' controversial aircraft designed to lift off like a helicopter and fly like an airplane, reports CBS News Correspondent Bob Orr.
Maj. Gen. Martin Berndt, commander of the 2nd Marine Expeditionary Force, told a Pentagon news conference Thursday that investigators determined the crash was caused by a rupture in a hydraulic line and a computer glitch that then made it harder to control the aircraft after the rupture.
Although the rupture was caused by the rubbing of wire bundles against the hydraulic line, and the problem of rubbing or chafing was known to the corps since at least mid-1999, Berndt dismissed the question of whether an imminent rupture had been missed by maintenance crews.
"This aircraft ... was in excellent shape," he said. "There were no maintenance problems with it. Everybody had done everything right."
Asked if maintenance had been properly recorded, he wouldn't answer because he said he didn't have the records with him.
Unnamed pilots told the Washington Post they believed the problems slipped by because the Pentagon wanted to secure full funding of the troubled aircraft program.
The Post said the pilots, who agreed to be interviewed about the investigation, spoke on condition of anonymity. They were all current or former officers in the Marines' first Osprey squadron and served on the military's "mishap board," the newspaper said.
Thursday's report covered an investigation into a Dec. 11 crash near Jacksonville, N.C., that killed four Marines. It was one of two fatal Osprey crashes last year that killed 23 and put the fate of the aircraft program in jeopardy.
Another Osprey crash killed 19 servicemen in Marana, Ariz., in April 2000. The fleet has been grounded since the December crash.
The Defense Department's inspector general, in a separate investigation started in January, is looking into allegations that service members had been instructed by superiors to falsify Osprey maintenance records to cover up problems with the aircraft.
The Marine Corps wants to buy 360 Ospreys to replace its fleet of aging CH-46 and CH-53 transport helicopters.
They are made by Textron's Bell Helicopters unit and the Boeing Co.
The $40 billion program has drawn much criticism.
Besides Thursday's probe and the one on possible record falsification, the General Accounting Office reported in February that the Marine Corps skipped some tests of the aircraft to save money and meet deadlines. Also, former Defense Secretary William Cohen appointed an independent panel to rview the program after the December crash. That review is under way.
Thursday's report said a wire bundle rubbed against a hydraulic line. The investigation cites a number of reports dating back to June 1999 that describe chafing of hydraulic lines by wire bundles, Berndt said.
Berndt said varying degrees of chafing were found on all eight remaining Ospreys during an inspection after the December crash.
The investigators Thursday recommended a review of the entire computer flight control system and associated software and the placement of hydraulic lines and wire bundles.
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