Boeing Company (BA)

[Porgie Tirebiter]

The first paragraph is nonsensical. As an aircraft accelerates it is completely normal, and the pilot should fully expect, to add nose down trim to around 250 knots. Most will climb to altitude at that speed until the low thirty thousands where, depending on temperature, 250 knots will equal .7 mach and transonic flight begins. As speed increases past that point an aircraft will become longitudinally unstable. This is generally countered by a mach trim system engaging and trimming nose up to avoid overspeeding the aircraft. Kind of the polar opposite of MCAS. And just like MCAS, mach trim is disabled with the autopilot engaged.

Nosediving the plane to keep the engines running is not "stall prevention". In the case of Lion Air it's apparent the pilot fought with MCAS for several minutes while the co-pilot was fumbling with manuals. About a minute before the crash the pilot gave control to the co-pilot and started to look through the manuals himself. All it took was step #4 in the memory items checklist for runaway stabilizer to have saved that aircraft.

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