Originally Posted by
BDAttitude
---and we will finally learn how much stability margin without MCAS remains.
This is something I too am VERY curious about.
I have tried to make some thought experiments.
We know MCAS at low speed was supposed to administer up to 2.5 units AND trim.
We dont know from which value of AOA that AND trim was supposed to be started.
We know stall begins at approximately 14 degrees AOA and is well developed at 15 degrees AOA.
We know Boeing have said MCAS was only supposed to be activated at very unusual flight conditions, so therefore one can deduce that MCAS only should become active at high AOA.
We dont know how many units of trim the elevator can fully compensate starting from neutral elevator position, but Mentour Pilot says that the elevator can compensate for full nose down trim in a NG simulator at low to normal speeds.
Now, on to my thought experiment.
Lets say MCAS was supposed to kick in at 10 degrees AOA.
Lets say that the elevator can compensate for 3.5 units of (out of) trim.
Lets say that 70 percent of elevator authority/travel is used to reach the stall AOA on a NG starting at level flight and in trim.
Lets say that the FCC starts MCAS trim input at 10 degrees AOA and have put in full 2.5 units AND trim at 14 degrees AOA.
Lets say the NG have a linear stick force and travel all the way from level and neutral and up to stall at 14 degrees AOA.
Lets say the MAX with active MCAS have the same linear stick force and travel as the NG.
Lets say we start at 3 degrees AOA at level and straigt in this thought experiment
Using these numbers we see the following;
We need to put in 6.4 percent of available elevator travel for each degree increase in AOA. (70/(14-3))
At 10 degrees AOA we have put in 44.8 percent of the available elevator travel (6.4*(10-3)
Each unit of stabilizer trim (out of trim) needs 28.4 percent of available elevator travel (100 percent/3.5 units trim)
From 10 to 14 degrees AOA we need to put in an additional 25.6 percent of available elevator travel (6.4*4)
But at the same time (10 to 14 degrees AOA) MCAS has put in 2.5 units of nose AND trim.
That MCAS AND is worth 71 percent of available elevator travel (28.4*2.5)
So, we see, with these numbers, that we put in a positive 25.6 percent elevator travel going from 10 degrees to 14 degrees AOA, but at the same time MCAS puts in AND negative 71 percent worth of elevator travel.
The sum of these two values is 45.4 percent of nose down elevator travel, in other words, when going from 10 degrees AOA to 14 degrees AOA in a MAX WITHOUT active MCAS we need to relax the stick from a position corresponding to 44.8 percent aft position at 10 degrees AOA and push it forward to a position of 0.6 percent forward position when we reach stall onset at 14 degrees AOA.
I think it is fair to say that an aircraft exhibiting a pitch stability similar to the values in my thought experiment would be a VERY awkward plane to fly manually in a high AOA flight region.
Of cource, theese numbers is more or less pulled out of my ass. And I've made many simplifications. As I said, this is just a thought experiment trying to understand the effect on a MAX with a non-operable MCAS system.
Feel free to correct my thought experiment and/or try to come up with better values.