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Software, hydraulics blamed in Osprey crash

Four Marines were killed in the crash of a tilt-rotor MV-22 Osprey in December  

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'Unpredicted and uncontrollable events'

Less than one-third of tests done


WASHINGTON (CNN) -- Last December's crash of a MV-22 Osprey tilt-rotor aircraft was caused by the failure of a hydraulic system component compounded by an anomaly in the vehicle's computer software, a Marine spokesman said Thursday.

Four Marines were killed when the Osprey crashed December 11 on approach to Marine Corps Air Station New River, North Carolina.

Noting that "aviation mishaps are seldom caused by a single factor," Maj. Gen. Martin Berndt said an investigation by the judge advocate general uncovered "a series of events, the combination of which proved fatal."

CNN's Eileen O'Connor reports on the cause of the Osprey crash

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The crash occurred at the end of a training flight, as the aircraft approached the airport. As its speed dropped to 160 knots, the plane began to transition from airplane mode to helicopter mode, a change that occurs automatically when the aircraft slows to a point where it might stall, Berndt said.

At that point, chafing on a hydraulic line that controls the pitch of the aircraft caused the line to rupture, he said.

Though the rupture alone would not have caused the crash, the problem was compounded when the pilot pressed a reset button that illuminated in the cockpit, Berndt said.

'Unpredicted and uncontrollable events'

Following accepted procedure, the pilot pushed the reset button eight to 10 times during the 20 seconds prior to the crash, Berndt said.

"This action started a chain of unpredicted and uncontrollable events that caused accelerating and decelerating actions of the aircraft until it stalled," Berndt said.

He blamed the software anomaly for causing thrust that led to the failure of the prop rotors. Because of the hydraulics problem, the rotors were unable to respond at similar rates, Berndt said.

"This resulted in uncommanded aircraft pitch, roll and motions which eventually stalled the aircraft," he said.

The Osprey dove into a marsh seven miles north of the airfield.

Naval Air System Command and Bell Boeing, the maker of the plane, will review the system software and the flight manual, Berndt said. It will look into the chafing problem and investigate the possibility of redesigning the planes' hydraulic systems, he said.

A June 1999 investigation had cited chafing of wires and hydraulic tubes as a potential problem in the plane. An inspection of the planes failed to find a problem with the one that later crashed, Berndt said.

"Unfortunately, the affected line that had some role in this mishap was not one of the lines identified," he said.

The fleet has been grounded since the accident.

Berndt said he did not know who designed the software. He said the crew was blameless in the accident. "The aircrew acted immediately and correctly to the in-flight emergency as they were trained to do," he said.

Less than one-third of tests done

The findings of the investigation come as the Marines seek approval to buy 360 MV-22s for $40 billion to replace its aging fleet of Vietnam-era CH-46 helicopters.

The program has been put on hold in the wake of allegations that problems with the Osprey were covered up, along with several reports critical of the plane's testing and technology.

A recently released version of the Marine report on another MV-22 crash -- which killed 19 Marines in Arizona last April -- expressed investigators' concerns about the frequent service required by the aircraft's hydraulic system.

"The frequency of servicing/maintenance requirements for the aircraft hydraulic systems -- though not causal in this mishap -- is concerning," wrote investigators in a July 2000 report.

A February report by the General Accounting Office has criticized the Marine Corps for "inadequate test and evaluation," saying fewer than one-third of the originally planned tests were carried out.

The GAO report said critical testing which could have helped identify the flight characteristics of the Osprey was skipped or simulated "in order to meet cost and schedule goals."

"This would have provided considerable knowledge of V-22 flying qualities, especially in areas where the Marine Corps states the aircraft is susceptible to a sudden loss of controlled flight," investigators concluded.

Marines turn Osprey investigation over to Pentagon
January 24, 2001
Officer admits asking Marines to falsify Osprey records, official says
January 19, 2001
Osprey commander caught on tape
January 19, 2001
Marines ground 3 aircraft, including Osprey
August 27, 2000
Investigators believe pilot error caused Osprey crash
May 10, 2000
Bodies to be recovered from Osprey wreckage
April 11, 2000

U.S. Marines

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4:30pm ET, 4/16

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