Because of high fuel expense, airlines demand airplane manufacturers like Boeing utilize the most efficient engines as they become available for a new airplane. The original 737 sits low to the ground and would not allow retrofit of the new larger diameter, more efficient engines in their “normal” location.
Instead of going to the expense of designing an entirely new clean sheet airplane with adequate ground clearance to properly utilize these larger engines; Boeing chose relocate the 737 MAX engines further forward on extended pylons and raised slightly for sufficient ground clearance.
The problem is that the engines in their new more forward location tend to cause a pitch up force, especially at full power as during takeoff.
Because of this pitch up force, it was deemed necessary to provide an automatic feature that would counter an excessive climb attitude if pilots somehow became unaware, hence the installation of the Maneuvering Characteristics Augmentation System (MCAS) that could, in some instances, override pilot control of stabilizer trim; sometimes creating such pitch control downforce that pilots were unable to wrestle control away from this automatic feature; especially once the airplane was in a dive. Even if the MCAS system was manually disconnected by the pilot, under certain conditions, it would automatically reset.
I am not even going to speak to the problem of lack of training or the failure of Boeing to provide adequate information about the new MCAS system to 737 MAX pilots.
A mechanical angle of attack sensor, such as used on Boeing’s MCAS system is mounted on the side of the fuselage, is subject to all kinds of hazards including ice, water intrusion, bird strikes, etc. and in one case, at least, someone hanging a bag on the moveable mechanical blade type sensor. Such abuse can easily cause sensor damage or binding, resulting in erroneous information provided to the autopilot.
So far, not great; but Boeing chose depend on ONE mechanical blade type sensor to provide this critical attitude information to the MCAS system, without any REDUNDANCY.
Aircraft engineers and are definitely NOT stupid people. I, for one, cannot believe that at least one engineer at Boeing did not bring this lack of redundancy of a critical mechanical sensor to management’s attention.
As for the new 737 MAX with autopilot now receiving attitude information from TWO MCAS sensors and with new software that will not allow the MCAS system to operate if a significant signal difference between the sensors is detected, I am sure the that new 737 MAX will serve the airlines and their passengers quite well, after a brief period of confidence regaining.