Boeing Made MAX MCAS More Aggressive In Late Development


As Boeing tweaked final development of the 737 MAX, it made the MCAS autotrim system more aggressive and ultimately riskier, according to a new report in The New York Times. Boeing failed to inform even its own test pilots and the FAA about the revisions.

MCAS—Maneuvering Characteristics Augmentation System—was added to the 737 MAX to improve control feel in an extreme corner of the flight envelope at rearward center of gravity and high angle of attack. MCAS rolls in nose-down stabilizer trim to mimic the stick forces pilots were used to in previous 737 models.

However, as originally conceived, MCAS relied on multiple sensors including angle-of-attack vanes and accelerometers sensing load factor to prevent the system from activating erroneously. It also had a less aggressive trim schedule than the airplane was ultimately equipped with. The initial MCAS design could trim 0.6 degrees nose down in 10 seconds. The release software allowed 2.5 degrees in 10 seconds and was capable of applying full nose-down trim. During that same design change, MCAS’s primary data source was reduced to a single AoA indicator. “To me, it seems like somebody didn’t understand what they were doing,” an engineer who assessed the system’s sensors told the Times.

Nor did Boeing inform the FAA in detail of the last-minute changes to the MCAS design, according to the Times report. As the airplane was moving through final certification, Boeing asked the FAA if it could remove a description of the system from the documentation provided to customers for training and support. That was done and pilots transitioning to the MAX from other 737 models were unaware of MCAS, much less its intended function.

MCAS activates only when the airplane is being hand flown with flaps up and at high AoA or load factors. It was added because the MAX’s heavier CFM LEAP-1B engines are mounted further forward and the nacelles produce lift at high AoAs. This apparently reduced pitch-up stick forces that Boeing attempted to alleviate with nose-down trim provided automatically by MCAS.

The Times interviews of multiple Boeing employees revealed a compartmentalized approach to the MAX development flow, a process which left them without a complete understanding of the risk MCAS represented.

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  1. I don’t agree with Boeing’s decisions regarding documentation of MCAS, but any innuendo of sinister motives makes no sense either. What could Boeing have hoped to gain by excluding MCAS from pilot documentation? I read somewhere that one should never attribute to malice what is easily explained by incompetence. And let’s not forget that the investigations are not complete.

    As I understand it, unwanted MCAS activation should appear to a pilot as runaway stabilizer. I get that the accident crews were getting conflicting annunciations–in the case of the Ethiopian air crew, an impending stall AND overspeed–but FLY THE AIRCRAFT is the watchword for professional pilots. During simulator training crews face compounding emergencies, system failures, and bad indications precisely so they may learn to prioritize aircraft control ahead of everything else. With that trim wheel spinning away while yoke forces built–for ten seconds each time!–there shouldn’t have been any question what was happening or what to do to stop it, especially for the Ethiopian Air crew who were briefed on the Lion Air crash and the FAA directive. You can argue that a crew shouldn’t need to know about MCAS to properly recognize and respond to runaway stabilizer; the Lion Air crew that flew the accident aircraft the day before the crash proved that. You can also argue that you really don’t want the crew to take time to troubleshoot the cause of runaway stabilizer (is it MCAS, or???), you just want him/her to recognize it and respond before losing elevator authority. That seems like a more logical reason for excluding MCAS from the documentation than any of the speculative innuendo coming from media outlets like the NY Times.

    On the other hand, if Boeing HAD documented MCAS the accident crews would have known how MCAS modifies trim system operation. For example, 737 series aircraft incorporate proximity switches on the control column that stop stabilizer trim movement if the yoke is moved in the opposite direction of trim movement. This would be the first natural response to undesired trim movement. The proximity switches, however, would defeat the purpose of MCAS, and so are deactivated during an MCAS cycle. That leaves only the yoke-mounted trim switches, the stab trim CUTOUT switches, or physically grabbing the spinning trim wheel as the only means to stop the stabilizer during an unwanted MCAS activation. Could this information have helped either of these crews? Might it have been one less factor adding to the confusion? It’s doubtful we will ever know for certain, but that hasn’t stopped the media from speculating.

    Assuming we learn nothing further from the investigations, I believe these two tragedies have highlighted glaring deficiencies in crew knowledge and training regarding the stabilizer trim system, deficiencies that could have implications for ANY aircraft using an electric motor to position the stabilizer, not just the 737 Max. My hope is that all airlines and training providers the world over will emphasize runaway stabilizer procedures in future training scenarios.

    • Sinister intent and incompetence are not the only possible explanations for any intentional omission Boeing made of critical information from its training materials. Money is also a strong motivator, and convincing potential buyers that no additional training (read: cost) would be needed beyond what their existing 737 pilots already needs seems like a point that Boeing would have liked to emphasize strongly to try to secure more sales.