Boeing Company (BA)

[Porgie Tirebiter]

Despite media reports, MCAS has nothing to do with stall prevention. It's purpose is to meet stick force requirements of Part 25.175. With the LEAP engines installed there was a pitch up thrust vector in high bank angle, high AOA, high thrust setting conditions. The thrust vector tended to ease the amount of pull force the pilot needed to exert to put the airplane into that condition. MCAS was designed to run the stabilizer nose down to increase the aft stick force required.

Boeing's mistake was to piggy-back MCAS input on the left stall system's AOA sensor. That violated the basic design principle of avoiding 'single point of failure' scenarios. In the case of the MAX the single failure of the left AOA vane caused two problems - an erroneous stall indication and MCAS driving the trim nose down.

Airplanes tend to exhibit enhanced stability at the sacrifice of efficient performance in the forward CG range. And low stability with more efficient performance in the aft CG range.

An excellent article about pitch stability. Even though it's about Part 23 airplanes, the concept is identical: https://airfactsjournal.com/2013/05/the-weight-and-the-balance/