The gums # 346,
You are correct in considering a wider view of stability, not just aerodynamics; cg, thrust, the complete aircraft. In the purist sense stability is more precise, and certification differentiates static and manoeuvre.
There is little information about the exact nature of the Max ‘aero’ problem; it differs from the requirements for STS and Mach trim. MCAS adjusts stick-force feel for good reason, but exactly what that is, is not clear. See Ref ‘Overview’ and ‘Definitions’.
Does MCAS address both static an dynamic stability?
Some posts refer to the horiz tail stab as a control surface, which is inaccurate; elevators control, horiz stab is associated with stability. (A secondary effect of stab can be used as a control in abnormal conditions).
MemberBerry, # 347, Trim Failure
MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
‘Speed Trim Fail’ indicates that the STS function is inoperative, not that the tail trim is electrically, uncontrollably active, and can move to full travel.
MCAS should have a similar ‘inoperative’ alert, to provide awareness of a change in aircraft ‘feel’ in some parts of the flight envelope.
The AoA Disagree alerts add clarification to other consequential problems due to AoA malfunction (air data, feel diff), but without a dedicated MCAS alert, the loss of MCAS has to be deduced.
‘The AOA (angle of attack) indicator provides supplementary information to the flight crew. The AOA disagree alert provides additional context for understanding the possible cause of air speed and altitude differences between the pilot’s and first officer’s displays. Information for these features is provided by the AOA sensors.
There are no pilot actions or procedures during flight which require knowledge of angle of attack.’ (Ref ‘737 MAX Flight Deck Displays’)
N.B. last sentence; perhaps this is subject to change with MCAS II
Takwis, # 351
MCAS does not ‘take control’ of the elevator, the intent is only to apply a small adjustment.
Adding to previous descriptions, consider entering a turn.
Conventional control feel requires more back stick force when turning, increasing AoA at constant speed. If the stick is released the aircraft should pitch down.
With reduced stability margin (but not unstable), the level of stick force is less than required by regulation, the aircraft pitch response is similar but less so. The aircraft might feel a bit sloppy, loose, twitchy.
Small non-pilot nose-down trim restores sufficient feel for certification.
Re speed margin to stall; MCAS appears to operate a high AoAs, but before stick shake (Vss) and stick push (stall).
Turning with increased AoA reduces the margin to stall because the in-flight AoA is closer to Vss, the stability feel requirement should cue the pilot to this change with increased stick force; however, with reduced stability the cue is less, thus the awareness of margin to Vss is reduced.
Note change in EFIS low speed awareness display, a visual depiction, whereas MCAS is the tactile depiction.
Caution, some of the above is supposition - conjecture based on the little information available to date.
Ref:
https://www.boeing.com/commercial/73...e-updates.page