What the much ballyhooed system in the 737 MAX does is sense an increasing AOA when the pilot is hand flying the airplane. When the AOA reaches a point without enough stall margin, the system adds some nose-down pitch trim. That pitches the nose down and gives the pilot the stick force to know that he is pulling too close to the stall margin.This concept of adding artificial feel using the pitch trim has been around for years. It has been used to add stick force at high speed cruise where Mach effects can alter stick force as well as at higher AOA where stall margins must be maintained.
What’s critical to the current, mostly uninformed discussion is that the 737 MAX system is not triply redundant. In other words, it can be expected to fail more frequently than one in a billion flights, which is the certification standard for flight critical systems and structures.
What Boeing is doing is using the age-old concept of using the human pilots as a critical element of the system. Before fly-by-wire (FBW) came along, nearly all critical systems in all sizes of airplanes counted on the pilot to be a crucial part of the system operation.
The certification concept for relying on the human involves identification of a failure, and a reaction time. The way it works is that the pilot must be able to recognize the failure, then take three seconds to analyze what is wrong, and then take corrective action before the airplane flies into a critical condition.
If you fly an airplane with an electric pitch trim system, you are flying under this certification concept. A pitch trim system running away can obviously fly the airplane into a dangerous condition, particularly when the autopilot is engaged which masks the trim runaway for some time.