Originally Posted by
ktcanuck
I keep trying to ask this question in different ways but the mods don't publish so I'll try another way:
I see it stated that MCAS is only there because the size and forward placement of the engines on the Max results in stick force reduction as AoA increases and that this characteristic is contrary to FAR requirements.
I also see it stated that the Max is not unstable as a result of the new physical configuration. I ask innocently: Is this known for a fact?
I ask because the MCAS, as a solution, is a brutal way to simply correct a stick force irregularity. Surely a fail passive method to apply extra force to the stick is not a difficult thing to do.
The MCAS FMECA must make an interesting read! But if you absolutely must not stall then it might be justified.
Do we know for sure this version does indeed stall and recover normally like a typical stable aircraft?
737MAX is not unstable with or without MCAS in that starting from a 1g, wings level trim condition with zero column input it will not tend to increase or decrease AOA without the pilot applying column input. (This statement assumes that thrust and flight path are also trimmed to provide steady speed.) From that starting point a maneuver flown without adjustment to the horizontal stabilizer will require pull force as AOA increases over the range of AOA that must be demonstrated (stick shaker and beyond). Relaxing the column to its detent from any point along this AOA demonstration will result in AOA returning to near its trimmed condition. Pull to get nose up, relax to return = stable.
The 737MAX issue that gives rise to the need for MCAS is that the amount of pull column to stabilize at any given AOA along the way may decrease slightly from what was required at a slightly lower AOA. You still have to pull. It will still recover to a lower AOA if column force is removed. The characteristic, however, of not needing more column pull for every increment of higher AOA over the demonstration range is what does not meet the requirements.
Another point that has been discussed in PPRUNE is the idea that MCAS is only there to make the 737MAX handle like earlier 737 models. That is not what motivated MCAS and thus it would not be permitted to remove MCAS. Regardless of the desire for common handling between 737MAX and 737NG, 737MAX handling characteristics considered apart from any relation to 737NG handling characteristics do not meet the applicable FARs (see paragraph above) and thus need modification by one means or another in order to achieve certification.
As to the idea of a stick pusher rather than the MCAS use of the horizontal stabilizer, that has been discussed at great length. The 737 does not include the necessary hardware to implement such a stick pusher so the statement that "Surely a fail passive method to apply extra force to the stick is not a difficult thing to do." is not correct.