Here is the evidence trail;
1. Recorder data shows the aileron movement in both accidents, the hinge movement was towards the direction of roll, not against which would be the pilots natural reaction.
2. Certification records from flight testing show cause that aileron hinge reversal occured at low AOA on these aircraft to the point where modifications were carried out (only sufficient for flight in clear or light to moderate icing).
3. During the investigation into the first incident by the ATR & DGAC in 1988 the engineers concluded that aileron hinge reversal could be the cause. This was not promulgated to the NTSB at the time.
4. There was intensive wind tunnel testing of the aerofoil which showed that in extreme icing a ridge would form sufficient to cause aileron hinge reversal which created a force of 50lbs or greater. There was also the famous flights behind the tanker, but these flights could only simulate freezing drizzle not freezing rain. On the second runs made in europe they did find some hinge reversal occured but not to the extent found in the wind tunnel.
5. The most important evidence was from the numerous pilots who had experienced and survived the situation. This had occured over the preceding 10 years where pilots recounted stories of the autopilot disconnecting and the aircraft rolling suddenly with large force required to regain aileron control in icing conditions. After the 1988 incident there was a pilot ban on flying the aircraft in icing but the union gave in when ATR came up with the so called fix.
6. The aircraft was never tested in freezing drizzle or rain conditions as the new european certification rules did not require it to. This was an oversight when the aircraft was put on the FAA register.
The NTSB never created the aileron hinge reversal theory, ATR had known about it the entire time (and pilots of the type). The NTSB was finally able to get at the truth about what happened.
All turbo-props are different and suffer slightly differently in icing, know your aircraft!
Last edited by 43Inches; 6th Jul 2010 at 01:35.