Originally Posted by
Mad (Flt) Scientist
re stick "nudgers" and stick pushers
Stick pushers are sometimes fitted when there are "stability issues approaching the stall" - in such a case, though, in order to fix such issues the pusher needs to be set to trigger BEFORE the adverse characteristics are manifest, with attendant likely adverse impact on stall speeds. A pusher can also sometimes be fitted when the characteristics AT the aerodynamic stall are acceptable, but the post-aerodynamic stall characteristics degrade so quickly that a "barrier" is required to prevent excursions into an unsafe zone. Bottom line, pushers can be fitted for a variety of reasons.
Regarding why no "nudger" was fitted and MCAS adopted instead: A "nudger" would operate on the elevator, not the stab. If the issue is insufficient elevator authority/power to dependably recover from a stall under worst case conditions, nudging the stick will achieve nothing. In such a case, the solution is either more elevator authority (major redesign) for "help the elevator with the stab". The latter seems to be the MCAS solution.
You are correct. In a way the stall approach, actual stall, stall recovery and the post stall behaviour are a package. But at any part of the "package" the aircraft behaviour may become non-compliant. For example is a pitchup a fraction of a knot prior to the stall a stability issue or a stall characteristics issue? It really doesn't matter, a fix is required irrespective. Then it is up to the manufacturer to develop a fix which is acceptable to the certification authority. One could be pedantic and say that there is only one reason these devices are required to be fitted - non-compliance.
The point about the stick nudger was they have been approved in the past for other aircraft types (including other Boeings). None of us know the the degree of instability of the 737MAX (if any, outside the STS speed range), but it is up to the manufacturer to present a fix to the certification authority. I would be very surprised if there was a high degree of instability considering that only one configuration seems problematic. Also, the stability may be marginal, but compliant, in all other stall approach cases. If a low authority stick nudger could be sufficient there is still nothing to stop the manufacturer presenting the MCAS system for approval. To me if a low authority device would have been adequate, I am not so sure about the engineering justification, other than that all the required equipment was in place, of using a hugely powerful control to achieve that end. However, from an airworthiness view it can still be a perfectly acceptable approach.
I have difficulty with the concept that the elevator authority was perfectly adequate for all stalls approaches except one which that requires tailplane trim assist. That implies a huge loss of pitch control authority. So I will stick with my guess that this is a claytons stick nudger solution for mild longitudinal instability, implemented in the quickest, cheapest, way possible and nothing to do with loss of longitudinal controllabillity.
Perhaps I should have said a "stability augmentation device", of which a stick nudger is but one type.
I have not previously heard the proposal of stick nudgers being used as a stall recovery device, or stall identification device, perhaps I have misunderstood.