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Subject:  Re: Boeing and self-certification Date:  3/15/2019  6:24 PM
Author:  Spiffy99 Number:  555053 of 585047

Intercst comments:

My understanding is that three switches must be thrown (in different places around the cockpit) to completely disable the MCAS, all while putting 80 KG of force on the control yoke to counteract the system as the aircraft bucks and pitches, often close to the ground.

I believe the first two "switches" are regular trim adjustment controls just like on a normal 737 (one a thumb switch on the "steering wheel", and one a large trim wheel beside the pilot). On the MAX these don't shut off MCAS but rather just temporarily override it. After a few seconds, MCAS will engage again if it thinks the plane is pitching up too much (a bad sensor can confuse the system). If the pilot pulls back on the control yoke ("steering wheel") it requires 80KG of force which also temporarily overrides MCAS. On normal 737s I understand this action turns off the autopilot. So if the pilot is not familiar with MCAS he thinks his actions have solved the problem by turning off the crazy autopilot, when in reality MCAS will re-engage in a few seconds.

The third "switch" is an actual hard MCAS on/off switch somewhere on an instrument panel. This would completely take MCAS out of the picture and allow the pilot to "fly the airplane" with full control. But the pilot would need to know how MCAS works (and how it could fail) and where the hard off switch is. After Lion Air all pilots should have been fully aware (there were emergency bulletins that are required reading). Its why there were doubts about the cause of the Ethiopia crash. Once they saw the up and down roller-coaster flight profile on Ethiopia and apparently found the stabilizer in the fully actuated "nose down" position, Canada grounded the MAX and was shortly followed by the USA.

There are two issues as I understand them :
1) The MCAS only gets data from one sensor. If that sensor goes bad, then MCAS goes bad. I believe the software fix Boeing is working on will tap into other sensors to ensure data agreement.
2) Information on MCAS was buried deep in the flight manuals (apparently you needed to jump around - eg refer to MAX supplement #23 or something - to find it). In a normal 737 if you get "runaway" trim the procedure is to first disengage the autopilot, which you can do by pulling back on the yoke (steering wheel). That doesn't work when it's MCAS causing the runaway trim (but it does appear to work for a few seconds). You gotta throw the hard switch.

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