Replies to post #2615 on Boeing Company (BA)

[Porgie Tirebiter]

Despite what the media keeps saying over and over again, MCAS is not designed to prevent aerodynamic stalls.

During certification flights the MAX did not meet the stick force curve requirements under certain adverse loading conditions, high thrust settings and high bank angles. Specifically 14 CFR 25.175(1):

(1) With the landing gear retracted at high speed, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 VSR1, nor speeds greater than VFC/MFC, nor speeds that require a stick force of more than 50 pounds), with—

(i) The wing flaps retracted;

(ii) The center of gravity in the most adverse position (see §25.27);

(iii) The most critical weight between the maximum takeoff and maximum landing weights;

(iv) 75 percent of maximum continuous power for reciprocating engines or for turbine engines, the maximum cruising power selected by the applicant as an operating limitation (see §25.1521), except that the power need not exceed that required at VMO/MMO; and

(v) The airplane trimmed for level flight with the power required in paragraph (b)(1)(iv) of this section.

Boeing's fix was MCAS which receives angle of attack information from the left side angle of attack sensor (the unit that apparently was improperly repaired in Florida). The fundamental flaw with that design though, is that it sets up a possible scenario where the failure of one single unit (the AOA sensor) caused two separate problems. That kind of engineering logic runs counter to the concept of designing aircraft systems to avoid introducing "single points of failure" at all, much less single points that produce two problems (erroneous stall indication and firing of the MCAS simultaneously).

Apparently Boeing engineers discounted this scenario. But Murphy's Law prevailed when the design was tested in a combination involving faulty maintenance, lack of complete systems testing after maintenance, pilots who displayed poor basic airmanship skills, and a company culture that allowed an aircraft to be dispatched on a passenger flight with a known and dangerous scenario that had just been experienced on that plane's previous flight.

There is lots of blame to pass around, and Boeing's design philosophy does deserves to take a chunk of it.