V-22 Osprey program officials are searching for the cause of a hydraulics failure that forced one aircraft to land immediately on Aug. 4, The DAILY has learned.

One of two primary hydraulics systems that power the aircraft's flight controls failed on aircraft number 21 during a routine flight above Marine Corps Base Quantico, Va. But a backup system automatically activated and the pilot landed the aircraft safely, V-22 program spokesman Ward Carroll said.

Government and industry experts have been assembled to find the cause of the failure, but the program already has ruled out two previous system flaws: hydraulic line chafing and defective titanium tubes, Carroll said.

The V-22 program has battled a series of hydraulic system failures, including a leak that led to a December 2000 crash in North Carolina which killed four Marines. In March, the V-22 fleet was grounded for eight days after the program discovered a manufacturer had supplied defective titanium tubes used in the hydraulics system (DAILY, March 19).

In this case, however, V-22 officials pointed to the safe landing of the aircraft as a sign that the program's efforts to improve the design of the hydraulics system are working.

"The airplane performed as advertised with regard to pilot warning indications and the number three [hydraulics] system picking up the load to effect a safe landing," Carroll said, adding it was a "good news" story for the program.

One prominent critic of the program disagreed.

"What I would still wonder, though, is, 'What is the cause of this particular incident?'" Philip Coyle, former director of the Office of Test and Evaluation, told The DAILY. "How is it after 2.5 years of work here that we're still having this problem? That, I think, is the most important question. What does it say about the developmental work?"

A month before the North Carolina crash, Coyle's office published a report noting that "failures related to the [V-22] hydraulic system deserve special mention." The system had suffered 170 failures during an 804.5-hour operational evaluation, according to the report.

Those conclusions and the results of the crash investigation prompted the V-22 program to redesign the spacing of the hydraulic lines and electrical wiring within the two engine nacelles.