Originally Posted by
yoko1
Actually, it would be more of a problem if MCAS did not activate. Keep in mind the purpose of MCAS - to augment the control feel at high AOA. If you have a high AOA for any reason - including windshear or GPWS maneuvers - then you are in the flight regime for which MCAS was designed. The original problem occurred when MCAS activated when it wasn’t needed.
If indeed that's what MCAS only did, augment feel, I would agree. But based on everything I read that's not exactly what it does, at least not the old version. As far as I know it doesn't take into account stick force or deflection at all as an input. Which is a strange behavior for a system which officially is designed to augment stick feel, but I guess in theory it could work as designed even with this limitation.
So, from what I understand, MCAS activates when it detects an angle of attack "it doesn't like", and it keeps adjusting the stabilizer until the angle of attack returns to a value "it likes", or until it hits its hard limit of 2.5 stabilizer trim units (0.6 units at higher speeds).
We don't know exactly what the trigger point is, so I'm just guessing here, but let's assume MCAS was designed to activate before the stick shaker, and you are in a terrain escape maneuver when MCAS activates.
The result would be that the angle of attack reduces with the same stick deflection, and the pilot would have to apply more and more deflection (and force) to reach the stick shaker limit. At some point MCAS would reach its 2.5 units of trim limit and stop adjusting the stabilizer. Then, assuming the elevator has enough authority to counter those 2.5 units of stabilizer trim, the pilot will finally be able to reach the stick shaker limit.
Now, at this point, with the old version, if the pilot blipped the trim switches, 5 seconds later MCAS would reactivate, it would again not like the AoA, and it would feel free to adjust the trim another 2.5 units. Now, would anyone want up to 5 units of additional nose down trim during a terrain escape maneuver?
What I'm trying to say is that, depending on where its trigger AoA point was configured in the original version, MCAS might have been a problem even when the AoA sensors worked properly. Of course, a lot of what I said above is speculation, but MCAS doesn't smell right to me, at least not when described as a "stick force augmentation system".