phollingsworth wrote:glideslope wrote:planecane wrote:MCAS, when operating properly, does not "force the plane to dive." In introduces a mis-trim to cause it to take more force to lift the nose more. MCAS, operating properly, would never have caused a plane to dive or even for a pilot to notice it existed. The issue with MCAS was the botched fault tree analysis and allowing a single sensor failure to cause a potentially catastrophic runaway.
The need for and inclusion of MCAS is not the issue. Especially since it is only need for edge cases that aren't entered into very often, if at all, in normal service. As long as MCAS is fixed to only operate when and how it was originally intended, there is no issue with it.
STS on the NG is doing something similar. It just didn't have the authority and lack of a simple cutout (the column switches for STS) to cause a problem. Actually, STS activates in normal operation all the time so it is far more active than a properly functioning MCAS would ever be.
Correct. The Max is not an inherently unstable design. It was the implementation of MCAS with poorly written code from outsourcing, lack of training, lack of communication with pilots and companies, and Boeings unwillingness to to own up and fix things. Personally I don't see how the Max can fly again if MCAS is retained without a new Type Rating. The Max simply flys different enough from an NG to warrant this.
I'm starting to agree however that it is approaching the point where Boeing may need to think about cutting the program, ask for some Govt. assistance as other companies do ( ) and design a new NB. It's the only way to truly recover from this.
Except that all the evidence says that for the Normal and Operating flight envelope the MAX flies substantially the same as the NG. AS Planecane points out the NG, at the behest of the JAA (now EASA), has an AoA driven function in the Speed Trim System to force the aircraft nose-down in certain situations. This looks to operate in a continuous fashion driving the stab to the limits at a constant rate. Further, the Elevator feel mechanism is designed to make it quite hard for the pilot to reach the aft-cutout limit. This is systems is provided by two simplex systems in an active-monitor setting with a warning light if active disagrees with monitor. Note this disagree activated in the MTC log on the two JT flights, but for some reason the indicators in the cockpit didn't work.
MCAS in its original incarnation was to add manoeuvre stability (stick force per g), as such you don't want the elevator feel mechanism making it impossible to pull back as this makes the aircraft less/un-manoueverable. So Boeing disabled the aft column cutout. However, when Boeing moved it to the 1g case with the increased rates. The fact is that the rates for MCAS are the same as those in the flaps down condition. So what is the problem.
1. Boeing removed the aft-column cutout from the 1g STS/MCAS case. Given that the authority is faster and has a greater range this will delay countering and put the aircraft into a more severe miss-trim.
2. Boeing converted the continuous STS/AoA run to a cyclical one, further delaying the ID.
My guess is the higher rate and range have to do with the fact that the stall is even less identifiable in the MAX compared to the NG. There will be less buffet and the aircraft will have less nose-down tendency. Note: stab trim does not fix the control force issue, it makes it somewhat worse. This is done by the elevator feel system. The problem here is this seems to be solely driven the airspeed. So reasonable for column force to airspeed stability, but no good for column force to AoA stability near stall (this isn't required for anything other than stall ID).
Agreed. Great overview. Thank you.