Originally Posted by BOAC
- yes, but that does not explain the logic, merely the mechanism. Why is it so designed that it operated in cruise? Why did the a/c need "High AoA protection" whilst in a high altitude cruise, and such that it would pitch the a/c up into a ...........High AoA environment?
The function of the High AoA protection is to keep the airplane out of the stall. So it is entirely logical that this function is activated at an AoA that is sufficiently close to alpha-max so as not to hinder normal operations, yet far enough below it to ensure correct operation in all reasonably expected operating conditions. These considerations govern the selection of alpha-prot.
What might be considered illogical are certain aspects of its implementation. The AoA vanes obviously cannot distinguish between the 'mean AoA' and the short-period fluctuation caused by turbulence. These fluctuations can be mitigated by suitably filtering the AoA signal, but that would make it less sensitive and hence increase the necessary margin between alpha-prot and alpha-max.
Obviously you wouldn't want to filter out the 'real' increase of AoA that occurs when the airplane flies into the powerful updraft that some posters on this thread theorize about. The designers have made the opposite choice by applying "phase advance", which amplifies the sensitivity to turbulence. I guess you need to ask the designers to elaborate the pros and cons of their choice.
The other illogical element in my view is that alpha-prot law is not cancelled when AoA decreases below alpha-prot. Again I suspect that that choice has been made after intensive investigation of all the "what if"-s, but you have to ask the designers.
Every design involves compromises between conflicting requirements ...