Originally Posted by
bimmerdriver
The purpose of MCAS is to make the MAX fly like non-MAX in a very small area of the flight envelope, light load and max aft CoG. The difference between the two is that the MAX requires less back pressure on the yoke.
@
bimmerdriver, i don't believe it's fully accurate to say that the purpose of the MCAS is to make the MAX fly like the non-MAX.
The purpose of the MCAS is to make the MAX compliant with the federal regulations - specifically FAR §25.173 "Static longitudinalstability". The non--MAX planes do comply with this regulation, so a side effect does result in consistency with the MAX ... but type consistency is just a side effect, not the motivation.
MCAS was Introduced to counteract the non-linear lift of theLEAP-1B engine nacelles and give a steady increase in stick force as AoA increases. The new location and size of the engine nacelle causes the vortex flow off the nacelle body to producelift at high AoA; as the nacelle is ahead of the CofG this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to further increase theback pressure on the yoke and send the aircraft closer towards the stall. This non-linear/reducing stick force is not allowable under FAR §25.173 "Static longitudinalstability".
MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps upflight at airspeeds approaching stall.
So back to the comment from @
ridefar - if the new software fix turns off the MCAS system, pilots will be left with a plane that is not compliant with FAR §25.173.
@
canadiancow brings up a fair observation that pilots are trained how to deal with planes in non compliant and non normal states (engine out being a great example) ... but that leads to two questions: (1) will the MAX pilots be explicitly trained with how to deal with an MCAS out situation - and (2) will pilots even be notified when they are in that situation?
We know that an AOA disagree indicator will now be standard on the MAX, but that's not the same as a MCAS out indication.
We're getting into speculation here, but I've read that MCAS may disable itself if it detects that too many cycles have triggered during a single flight. Suppose that happens - will the pilots be warned? Will they treat that situation with the same level of urgency that they treat an engine out scenario (or perhaps more realistically, an AOA disagree indication)?