Originally Posted by
Clandestino
AF447 crew walked the narrow path through what is completely uncharted territory. We have no way of finding out whether the astoundingly stable behaviour of their 330 at extreme alpha is general rule or whether they have through sheer chance found the power, weight, CG and control input combination that made their aeroplane fall in parachute-like manner instead of departing into violent oscillatory spin.
Well, we know that the airframe design was intended to have relatively benign stall characteristics - meaning that at the stall boundary there would be a vibration aspect (which, in the AF447 case, may have been misdiagnosed as vibration due to overspeed), followed by a relatively stable "mushing" descent profile. Modern wing designs seem to have these aspects anyway - the Birgenair B757 was similarly stable in aerodynamic stall, but eventually entered a spin due to asymmetric thrust following an airflow-induced engine compressor stall and failure.
Flight testing on modern types involved an unprecedented ability to capture data that could be fed back into the computers, allowing for extrapolation of that data to determine airframe behaviour beyond what would be considered safe in terms of a physical test. Older types were tested up to and beyond the stall boundary, but there was no way to capture the physical data in the same manner, and thus no way to feed that into simulated behaviour.