Accident Details
The aircraft was holding at altitude and began descending to a newly assigned altitude of 8,000 feet when it encountered an uncommanded roll excursion. This event resulted in a rapid descent and subsequent crash. The loss of control was attributed to a sudden and unexpected aileron hinge moment reversal that occurred after a ridge of ice accreted beyond the deice boots.
Contributing Factors
The manufacturer failed to disseminate adequate warnings and guidance to operators regarding the adverse characteristics of, and recovery techniques for, ice-induced aileron hinge moment reversal events. Additionally, the manufacturer failed to develop additional airplane modifications that could have prevented the accident. These failures led directly to the accident.
Regulatory Issues
The French Directorate General for Civil Aviation (DGAC) failed to require the manufacturer to take additional corrective actions, including performing further icing tests, issuing more specific warnings about the aileron hinge moment reversal phenomenon, developing airplane modifications, and providing specific recovery guidance. This lack of action led directly to the accident.
The ability of the U.S. Federal Aviation Administration (FAA) to monitor the continued airworthiness of the ATR airplanes was hampered by inadequately defined lines of communication, insufficient means to retrieve pertinent information, and the DGAC's failure to provide critical airworthiness information. The DGAC apparently believed this information was not required under the terms of the Bilateral Airworthiness Agreement.