MAX8 Tragedies Implicate Flight Controls

The 737MAX 8 was developed from the 737NG and certified by the FAA on March 8, 2017.  The aircraft is powered by CFM LEAP-1B engines which are 12% more fuel efficient than the CFM56-7B engines used on the 737NG.  The airframe has a length of 129 ft. 6 in. (39.47m), a wingspan of 117 ft. 10 in. (35.9m), and a maximum payload of 46,040 lbs.  It includes a newly designed winglet (the 737 MAX AT Winglet) which maximizes the efficiency of the wing. The MAX 8 offers seating capacity of up to 210.  Undiscounted sticker price? $121.6 million.

The 737 ‘Classic’ was originally designed in the 1960s and over time was revised to increase range and passenger capacity.  In 1991 Boeing developed the 737 NG (“Next Generation”) to compete with the Airbus 320. Boeing has delivered 350 of the 737 MAX 8 since 2017 and has an order backlog for 5000 more.  The more fuel efficient MAX 8 engines are larger and heavier.  To accommodate their larger diameter they needed to be moved slightly forward under the wing and higher up to keep them out of the way of the landing gear (and for ground clearance).  The repositioned engines changed the handling characteristics of the aircraft. As an aircraft takes off the angle of attack (“AoA”) of the wings is higher than at cruise. Because the engines had to be repositioned forward the nacelles that hold the new LEAP-1B engines are actually ahead of the aircraft’s center of gravity.  This forward placement creates greater lift at higher angles of attack (the engine thrust on takeoff is downward which adds more lift forward of the center of gravity as the aircraft takes off).  It’s this additional lift created by the engines that produce a pitch-up effect that further increases the angle of attack the moment the pilot releases pressure on the yoke.  The result can send the aircraft into a stall. To compensate for this pitch up effect, Boeing introduced the Maneuvering Characteristics Augmentation System (“MCAS”) which causes the aircraft to tilt its nose downward.  It is the aircraft design and engine placement that requires the installation of the MCAS to automatically trim against this extra lift effect.  Why? So that the plane and pilot population could fly the aircraft without the time or investment needed for a new type certification and additional pilot training.

During manual flight, as in takeoff, if the angle of attack is too high (or if the AoA indicator is faulty) the MCAS will trim the aircraft stabilizer ‘nose down’ a maximum of 2.5° (for a maximum of 10 seconds).  It can be interrupted by the flight crew by using the electric stabilizer trim which will stop the MCAS. However, after the MCAS has been stopped once, it will reactivate in 5 seconds and requires additional pilot manual trim commands.  To prevent constant manual trim (‘runaway trim’) the stabilizer trim cutout switches must be moved to ‘cutout’ to resolve a false or a continuing high angle of attack indication.  If the cutout switch is not used the MCAS will not reset the trim tabs from their current position and will continually pitch the nose down in additional increments of 2.5%, ultimately pitching the nose down as steeply as it can.  Without adequate training a flight crew’s reaction would be to pull back on the yoke and increase engine thrust which would not act to overcome ‘runaway trim’ but would result in an accelerating nose down dive.  Until the flying public regains confidence in Boeing’s correction of this MAX8 flight control issue other 737 models will experience a short term period of higher demand similar to what we witnessed with other widebody aircraft during the 787 Lithium ion battery issue a few years back.  Most importantly the MAX8 tragedies remind the airline industry that safety never stops being job number one.