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them. The MCAS uses a single, non-redundant AOA sensor input to trim nose down at high AOA – and the amount of nose down trim authority given to this autonomous, single-source system is very significant. Additionally, the single AOA sensor input may have a reliability issue. Boeing does offer the option for a second AOA input to the MCAS and an AOA disagree warning light for the system. Two power-shutoff switches on the center console (almost every turbine airplane, including the 737, has had these for a thousand years) are designed to disable the MCAS system either way, which will then allow a manual re- trim of the aircraft through a hand-crank trim wheel by the captain’s right knee and another at the FO’s left knee. Malfunctions in a system that operates silently in the background, like the MCAS, can be a challenge when they mal- function especially if the failure mode mimics a normal, day-to-day behavior of the system.
As the Trim Wheel Spins
The 737 has several modes of stabilizer
trim, only one of which is the MCAS. The mode everyone is familiar with is activated by the switches on the yolk and we use it all of the time – and it spins the trim wheel. Another mode trims the aircraft when the autopilot is off. Called the Speed Trim System (STS), it’s designed to improve flight characteristicswithalowgrossweight,aftcenterofgravity and high thrust when the autopilot is not engaged (i.e. during takeoff).Thissystemspinsthetrimwheelaswell.Another mode is the autopilot-on mode, which trims in the same
manner as we do when hand flying – and it also spins the trim wheel. And yet another mode is the now infamous MCAS which, yes indeed, spins the trim wheel.
My point is this: 737 pilots are accustomed to seeing and hearing the trim wheel spin. When we’re fast, slow, autopilot on, autopilot off, when we expect it to trim and when we don’t expect it to trim, the trim wheel spins. Is it possible that we are desensitized to trim wheel movement? Absolutely yes, at least potentially. May that be the cause of a delay in our response if the MCAS fails and runs away with our trim? Absolutely yes, at least for a few seconds. And how many seconds of trim wheel spinning does it take to make the nose really, really, really heavy? A couple of crews have found the answer to be less time than they imagined.
If the Trim Misbehaves
Required by AD 2018-23-51: In the event of uncommanded horizontal stabilizer trim movement (which I just told you happens routinely with the STS, MCAS, and the normal autopilot-on mode), combined with any of the following potential effects or indications resulting from an erroneous AOA input, the crew must execute the runaway
stabilizer procedure in the operating manual:
• Continuousorintermittentstickshakerontheaffected
side only;
• Minimumspeedbar(redandblack)ontheaffected
side only;
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May 2019 / TWIN & TURBINE • 29