Having read through the posts and some of the observations, these are my (an old grease monkey) answers to the basic questions on Boeing aircraft and its maintenance procedures, that seems to have confused many in this thread.
Screw-jack: From the early aircraft (B707 onward and may be earlier) all screw jacks have a brake, and it is called 'No-back brake'. As the term implies, it prevents the air-load pushing the control surface in the opposite (up) direction. This brake normally has two discs, one stationary and other rotating that are kept in contact by springs and axial force of air load acting through screw-jack. The control surface is moved against this brake force. In flaps this is the commonly used brake. When flaps are lowered, the drive motor has to move it against the air-load on flap. So there is a ratchet mechanism that slips and brake is not applied. In the opposite direction the brake is active and the axial force by the air-load on screw-jack makes the two discs firmly rubbing against each other and the braking force increases. Why this brake? You have consider control (Drive tube) dis-connection. In which case the flap can move by air-load in opposite direction without a brake. What applies to flaps are also there in most stab screw-jacks with some variation.
MCAS: This aircraft model requires this for it's certification by FAA, as it has reduced pitch stability. So there is no way this can be on MEL. If something is not in MEL it automatically implies that it cannot un-serviceable. There are some MEL items that have a statement 'Must be operative'. But some aircraft MELs do not mention the mandatory items this way.
Autopilot: Some one mentioned, if the autopilot was engaged by pilot, this crash may not have happened. See Boeing's FCOM Bulletin after this crash. If there is a AoA or IAS disagreement, auto pilot disengages and cannot be re-engaged. So that is out of question.
AoA or IAS disagree, which one?: It is not like in old gen aircraft (B737 classic)where the pitot probe and static port are directly connected to the airspeed instruments. At high angle of attack of the wing the airflow near the pitot gets distorted and this affects the SPEED sensing. To correct these errors, the pitot sensed speed is modified or corrected as a function of AoA in the ADIRU. Hence if you have a very large error in the AoA sensed, there will be an error in the IAS for the same side. So this can result in both indications showing a disagree. That is only my inference from the way the system is designed to work.
Why this crash: All sort of theories are advanced in this forum. But my take is that it should not crashed at all, as it should not have left the ground in the first place! The airspeed defect has repeated 4 times, and it keeps flying? I have posted this before. Any airline worth it's salt would not let go of these defects without confirming rectification. In the airlines I have worked, after the second report of such a significant defect, the crew will refuse to accept without positive identification of defect. This is a LCC - the pressures are different. As an engineer, I would not have dared to release the aircraft without :
1. Using an air-data tester and taking the aircraft through a full range of altitude and airspeed, just to confirm instruments operate correctly. 2. Call for a test flight if I cannot be sure to have rectified the defect.
From the day one when I was certifying, one thing that was drilled into me by my seniors, was there are ordinary defects and tricky ones. Never treat two defects in the same way. Any defect in the basic T instruments and flight controls, come in this critical list. This airline has a poor maintenance management (sorry to say that). I cannot see how an aircraft that had a stick-shaker in operation through out its flight (I am assuming from what is posted here), can be released for the next flight without confirming the defect is rectified by just changing a AoA sensor.
ONCE AGAIN WE ARE SPECULATING IN THIS THREAD -THOUGH MOST OF THE POSTS ARE FROM VERY EXPERIENCED AVIATORS.