Why was the MCAS software installed in the first place? Boeing has put forth various answers: so the 737 MAX would handle like earlier 737 models; to relieve airlines of the expense of flight simulator training for MAX pilots; to prevent stall under “unusual” flight conditions; to address a stability problem that manifests only under those “unusual” flight conditions.
History might be a clue. The original B737 was unstable in pitch. That too took Boeing by surprise in the late 1960’s, and airlines had to restrict passenger loading to the front of the cabin. They did so with a yellow ribbon tied between seats to block the rear of the cabin. I remember it well, because I worked at Boeing at the time and often flew 737’s from Seattle to Los Angeles.
The technical term is “longitudinal instability”. An airplane is longitudinally unstable when the center of gravity is behind the “neutral point”, which is the point about which changes in angle of attack produce no changes in pitch moment. Think of it as the point where lift changes caused by changes of angle of attack are concentrated . Boeing got the pitch instability sorted out with the B737-200 and subsequent models.
Until the B737 MAX. There are two obvious ways pitch instability may have been revived. One is the enlarged and forward engine nacelles, which must have moved the neutral point forward. The other is the distribution of fuel below the cabin floor, which may have allowed the center of gravity to shift behind the neutral point with heavy fuel loads, as at takeoff. The fuel is distributed in tanks each holding in excess of 1 metric ton. The B737-500 had four such tanks, and the B737-8 MAX has nine. Whether the aircraft is stable or not could depend on which tanks are filled and in which order they empty.
Longitudinal instability, center of gravity, and neutral point were entirely familiar to aeronautical engineers in the last century, but now I am not so sure. Boeing paid an engineer to address the stability of the B737 MAX, but he confused two modes of longitudinal oscillations: the low-frequency “phugoid” mode, and the “short-period” mode which alone can cause pitch instability. Evidence from the flight recordings of the two crashed B737 MAX’s indicates that they underwent violent short-period pitch oscillations.
I cannot know for sure, but I suspect that Boeing pilots flew close to pitch-unstable load conditions during early flight tests of the B737 MAX. I further suspect (based on years of teaching aerodynamics in the last century) that Boeing engineers had no knowledge of short-period pitch instability and instead became concerned about stall. The stated purpose of MCAS was to prevent stall, not instability. Of course an unchecked pitch instability can cause stall when the aircraft pitches nose up, or a high-speed dive when the aircraft pitches down. Either way the end result is a crash. Solutions also date from the last century: increase the stabilizer area, move the fuel forward, or incorporate a proper stability augmentation system that actuates the stabilizer both up and down.