A preliminary report from investigators on the crash of Ethiopian Airlines Flight 302, as you doubtless have heard, looks to be another case of a new flight control augmentation system responding to bad data from an angle of attack sensor and putting the aircraft out of control. That system, as you surely know by now, is called the Maneuvering Characteristics Augmentation System, MCAS for short. The system is new on the 737 Max. It was developed to give the new mode handling characteristics that would allow it to meet FAA standards after the addition of new, larger and differently positioned fuel-efficient engines that were the raison d’étre behind the Max.
The report holds a couple of hugely upsetting details, the first being that the MCAS apparently once again created a flight condition the crew was unprepared to deal with. MCAS activates when the AOA sensor detects a too-high reading and, thinking the airplane is on the verge of stalling, uses the electric trim system to aggressively trim the nose down, repeatedly. In the case of the Lion Air crash of a 737 Max 8 in late October, in which the same dynamic is suspect as the cause, the crew seemed baffled by what was happening as the plane trimmed itself repeatedly more and more nose down until the point the pilots couldn’t outmuscle the controls and the airplane crashed into the sea killing all aboard.