Originally Posted by
jimtx
I envision two cases where the MAX's stick force non linearity might come into play. It might make a steep turn easier if you fly the attitude you want to put the performance where you want. It also might be a problem in a clean off autopilot windshear escape. Would have to see that in the airplane or simulator.
The trigger mechanisms for the original MCAS had speed (mach) and aoa conditions. Those equate to conditions such as a wind up turn, which is quite a way from a wind shear avoidance and/or recovery case.
As far as what is and what isn't acceptable in force and gradient, AC25.7D covers the requirements, examples and suggested methods to ascertain § 25.173 Static longitudinal stability. Para C is of interest but so is A, B and D. For the amended MCAS, § 25.145 Longitudinal control may have been part of the compliance issue that led to the unfortunate turn of events in the repurposing of the MCAS from a high speed/manoeuver SAS to a low speed SAS system.
The regs provide for an average stick force gradient to be met, but there is additionally a requirement on “local” reversals in the stick force versus airspeed relationship over the range of airspeeds tested. That is shown in the AC, in figure 7-1 (page 7-4) and 7-2 (page 7-5).
Most times planes behave like darts, but they can get awkward, like the F-101 pitch up issue, the overbank of a BUFF, (where wing sweep and high bank angles with slip results in a lateral directional divergence, think
Czar 52 @ Fairchild, limitations in the Dash-1 etc. [Your T-38 got a mention in PIO history for the condition you note, NASA/WPAFB put out a great doc on the subject, and an instrumented 38 gave impressive data].
REF: AC25.7D
§ 25.145: page 5-15 to 5-19
§§ 25.173 and 25.175 pages 7-1 through 7-5