Hydraulic, Software Failures Downed Osprey, Marines Say
By Gerry J. Gilmore
American Forces Press Service
WASHINGTON, April 9, 2001 A burst hydraulic line and defective computer software caused a V-22 Osprey aircraft to go out of control and crash in North Carolina during a training flight last December, a Marine Corps' report says.
Marine Gen. Martin R. Berndt announced the findings of his service's Judge Advocate General Manual Investigation into the cause of the crash at an April 5 Pentagon press briefing.
Four Marines were killed Dec. 11 when their Osprey tilt rotor aircraft crashed near Jacksonville, N.C. The twin engined Osprey has unique prop-rotors that can be moved to point forward for level flight like a fixed-wing plane or upward to provide helicopter-like maneuverability.
Reading from a prepared statement, Berndt, commanding general of the 2nd Marine Expeditionary Force at Camp Lejeune, N.C., said the causes of the crash were twofold and weren't attributable to aircrew error.
"Aviation mishaps are seldom caused by a single factor," Berndt said. "This one was no exception."
He said a rupture occurred within one of the aircraft's three hydraulic systems used to move its engines and adjust the prop-rotors' pitch or angle. He said bundled wire within the aircraft's left engine compartment had chafed or rubbed against a titanium tube carrying hydraulic fluid.
The report noted that "based on the location of the ruptured hydraulic line in the nacelle area, it would have been extremely difficult to see any existing chafing on the hydraulic line during a routine inspection."
"The wire bundle did not rub completely through the tubing," Berndt said, but enough chafing occurred to cause the tube to rupture under pressure. "This resulted in a total loss of hydraulic fluid in the No. 1 system in a matter of seconds. This hydraulic failure alone would not normally have caused an aircraft mishap."
Berndt said the loss of hydraulic pressure caused a flight control system warning light to go on, which was accompanied by a warning tone.
"The published procedure for responding to such a failure is to press the primary flight control system reset button," he said.
When the pilot pushed the reset button, a software flaw caused "rapid and significant changes to prop-rotor pitch," Berndt said, causing the aircraft to speed up and then slow down. The pilot pressed the reset button several times, and the aircraft became increasingly unstable with each press, he said.
"The accelerating and decelerating of the aircraft every time that button was pressed was what caused the aircraft to stall and lose controlled flight," Berndt said.
Berndt said the Marine report recommends that Naval Air Systems Command and Osprey contractors conduct complete reviews of the aircraft's hydraulic line clearances, wire bundle placement, and computerized flight control system. The report also called for an investigation into the possible redesign of the Osprey's hydraulic system.
Ospreys have been grounded since the December crash. Findings are pending for DoD inspector general crash investigation and a "soup to nuts" V-22 program study by a blue-ribbon panel formed last December by then-Secretary of Defense William Cohen
"We will fly the aircraft or not fly the aircraft depending on the results," Berndt said.