Bell Boeing officials have taken issue with US General Accounting Office (GAO) findings that cutbacks in V-22 flight testing could have contributed to the fatal crash last year of a US Marine Corps tiltrotor as the result of vortex-ring-induced loss of control.

The GAO asserts that the original V-22 flight test plan "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. To meet the cost and schedule targets, the actual testing conducted was less than a third of what was originally planned."

The V-22 flew 2,800h of testing during the initial full-scale and subsequent engineering and manufacturing development. The programme last April suffered the first in a series of setbacks with the crash of a MV-22B at Marana, Arizona.

Investigators attributed the crash to pilot error and a descent rate far in excess of the maximum 4m/s (800ft/min) and 40kt (74km/h) airspeed stipulated by the flight manual. If one of the V-22's two proprotors passes through its own tip vortex, asymmetrical thrust and loss of control can result. "These conditions are extreme and outside the flight envelope of any aircraft," says a USMC source.

The GAO says flight control system flying-quality testing during development, which included vortex ring state investigations, was scaled back from 103 test conditions to 49 to cut cost and time.

Of these only 33 were completed. The omitted flights included tests at 0kt, 40kt and 80kt at high gross weight. Programme officials counter that the original flight test plan provided for every contingency and, like all development schedules, was modified in the light of data collected. Follow-on tests have focused on duplicating the crash conditions, but flying has been suspended since December's unrelated crash of a second MV-22.

"We made a reasonable judgement about what was required for operational evaluation and to test the envelope. We found nothing and don't expect to find anything to say that the V-22 is any more susceptible to vortex ring state than any other aircraft," says a source.

The USMC has delayed the report from the latest crash until the Department of Defense Inspector General completes an investigation of falsified maintenance records. This and a V-22 programme review, is expected to be completed by early April.

Source: Flight International