The answer is FAR 25.672 (c)(2). "It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated

The answer is FAR 25.672 (c)(2). "It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system - ... The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplane configurations) which is described in the Airplane Flight Manual" MCAS is only required in high AoA, flaps up, manual flight. To get high AoA, you generally need some combination of slow speed, a steeply banked turn, and abrupt maneuvering. If you tell the pilots to keep the speed up, keep the bank angles down, and fly gently in the event of an MCAS failure, it meets the requirements. From a practical standpoint, the aircraft should never see an AoA that would trip MCAS in normal flight. The only time a jetliner would typically see an AoA over ten degrees is on takeoff and landing, when the flaps are down (and MCAS is inhibited), or momentarily in turbulence. From a risk control standpoint, you're multiplying probabilities together (of course assuming uncorrelated failures). Spitballing the failure probability chain, flights that would trigger MCAS are probably about 10^-4 to 10^-5 probability, and the lack of MCAS might cause an unrecoverable stall in perhaps 10^-1 to 10^-2 flights, so figure 10^-6 probability per flight of the airframe's poor stability leading to an accident. This is too high to be certified. If you have MCAS, but it's disabled when an AoA vane fails, figure the probability of an AoA vane failure is 10^-4, you get an accident probability of 10^-10, which is acceptable.