Two fatal accidents in 2000 brought the V-22 to the edge of extinction, but the subsequent redesign of the aircraft and restructuring of the programme brought technical and operational maturity to the Osprey.

The New River, North Carolina accident in December 2000 was the result of a hydraulic line rupture, due to chafing caused by a wire bundle, compounded by a flight-control software anomaly. The software was corrected, and electrical harnesses and hydraulic lines in the nacelles were redesigned and rerouted to increase clearances. There has been no recurrence of chafing, says Taylor.

The Marana, Arizona accident in April 2000 occurred when the aircraft was flown outside the safe limit for rate of descent, and encountered vortex ring state (VRS) - uncontrolled descent caused by the rotors settling into their own downwash. During the two-year operational pause that followed the accidents, the V-22 team conducted extensive flight testing to establish the VRS safe envelope.

"Vortex ring state is a close cousin to power settling, a common phenomenon in rotary-wing aircraft," says Taylor. "The criteria for getting in trouble are exactly the same: high rate of descent, low airspeed, high gross weight and high density altitude." Flight testing after the Marana accident established the boundary for VRS, and a buffer zone was defined that provides a "huge margin" of safety.

Testing revealed the V-22 is "significantly less vulnerable" to the phenomenon than other rotorcraft, says Taylor. "The limit for safe rate of descent is the same as other aircraft, but the buffer is larger."

The limit is set at 800ft/min (245m/s), but onset of VRS is not seen until 1,600ft/min, he says, and recovery is easier in a tiltrotor - simply by rotating the nacelles forward for 2s to accelerate the aircraft out of the disturbed air.

To aid recognition of, and recovery from, vortex ring state, the V-22 has been fitted with a warning system. This does not sense VRS, says Taylor, but provides an audible cue to the pilot when the aircraft is approaching conditions conducive to VRS based on rate of descent, airspeed, gross weight and other parameters.

Source: Flight International