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Dec. 20, 2010—
After an "exhaustive investigation," Air Force officials still don't have "clear and convincing evidence" as to what exactly caused a CV-22B to crash April 9 near Qalat, Afghanistan, killing four and injuring the remaining 16 on board.

The investigators did rule out enemy action, environmental brownout conditions, and vortex ring state for the loss of the tiltrotor aircraft, which was assigned to the 8th Special Operations Squadron at Hurlburt Field, Fla.

The CV-22's flight incident recorder (FIR) and the vibration structural life and engine diagnostics (VSLED) control unit—collectively known as the "black box"—as well as its right engine were destroyed by precision guided bombs sent to ensure sensitive information in the wreckage did not fall into enemy hands, according to Air Force Special Operation Command's accident investigation board report, issued Dec. 16 (full text; caution large file).

Without that information, officials were unable to determine an exact cause, but the board did determine 10 factors that "substantially contributed to the mishap." They are: inadequate weather planning, a poorly executed low-visibility approach, a tailwind, a challenging visual environment, the mishap crew's task saturation, the mishap copilot's distraction, the mishap copilot's negative transfer of a behavior learned in a previous aircraft, the mishap crew's pressing to accomplish their first combat mission of the deployment, an unanticipated high rate of descent, and engine power loss.

Complicating the matter was that the Joint Operations Center in the theater and 8th SOS did not have a published plan to react to a downed CV-22 in theater, according to the report. That left commanders and first responders to "conduct by memory" the recovery effort.

Both the FIR and the VSLED systems were included in at least one list of items to collect, but in the chaos following the crash, the lists were not consolidated and the recovery team was not asked to recover the equipment before the wreckage was destroyed.

"Although recovering the VSLED and FIR would have been difficult, the [combat search and rescue] team was willing, equipped, and prepared to recover anything on the aircraft, if asked," according to the report.

Brig. Gen. Donald Harvel, AIB president, said in the report that he "determined by a preponderance of the evidence" that the 10 factors substantially contributed to this mishap.

However, Lt. Gen. Kurt Cichowski, AFSOC's vice commander until November and the investigation's convening authority, argued in an addendum to the report that there was not enough credible evidence to support Harvel's finding that engine trouble played a role in the accident.

After reviewing the report and Cichowski's subsequent opinion, Chief of Staff Gen. Norton Schwartz re-opened the investigation and ordered Harvel to analyze two Naval Air Systems Command reports and additional video data. Upon completion of the second investigation, Harvel changed one item in the AIB: increasing the ground speed that the aircraft was traveling at upon impact to 80 knots instead of 75 knots, according to an AFSOC release on the AIB findings.

"I determined that only an aircraft performance issue could completely account for the [mishap pilot's] decision to execute a roll-on landing. During a rapid descent, it is unlikely that this very experienced and competent MP would have chosen to execute a roll-on landing on rough terrain if he had power available to go-around and set up for another approach," wrote Harvel.