Every airplane development is a series of compromises, but to deliver the 737 Max with its promised fuel efficiency, Boeing had to fit 12 gallons into a 10 gallon jug. Its bigger engines made for creative solutions as it found a way to mount the larger CFM International turbines under the notoriously low-slung jetliner. It lengthened the nose landing gear by eight inches, cleaned up the aerodynamics of the tail cone, added new winglets, fly-by-wire spoilers and big displays for the next generation of pilots.
It pushed technology, as it had done time and time again with ever-increasing costs, to deliver a product that made its jets more-efficient and less-costly to fly.
In the case of the 737 Max, with its nose pointed high in the air, the larger engines – generating their own lift – nudged it even higher. The risk Boeing found through analysis and later flight testing was that under certain high-speed conditions both in wind-up turns and wings-level flight, that upward nudge created a greater risk of stalling. Its solution was MCAS, the Maneuvering Characteristics Augmentation System control law that would allow for both generations of 737 to behave the same way.
MCAS would automatically trim the horizontal stabilizer to bring the nose down, activated with Angle of Attack data. It’s now at the center of the Lion Air investigation and stalking the periphery of the Ethiopian crash.