On the Limitations on the manual backups - someone else posted it's in the MAX training manuals including the procedure that you may need to unload the control surfaces to get them to work in certain situations and that excessive speed is bad. Which is the same as it would be on an Airbus.
The FBW has an High-speed protection, which pitches the aircraft up if it reaches a certain threshold velocity. So even if Alpha-protection pitches the aircraft down, high-speed protection would pitch it up again. As they would counteract I suggest the aircraft would change to alternate law and both protections would be disabled and replaced by the stability laws. That would result in pilot regaining command over the pitch.http://www.airbusdriver.net/airbus_fltlaws.htm
Therefore it is not possible (by design) to get a similar situation as on the MAX. The Airbus FBW system could only fly the aircraft within the speed limits into ground, or switch to direct law (EDIT: or alternate law) and then get stalled by confused pilots (AF447).
I am not trying to be argumentative - just trying to understand. See this for what I am referring too https://avherald.com/h?article=47d74074
On LH 1829 the Alpha Protection kicked in and the flight lost 4,000' of Altitude. They don't don't mention the High Speed Protection kicking in - is that because the probes were blocked and the computers confused?
I assume they never reached speeds that would have triggered High-speed protection, that is why it was also not mentioned, it was not noteworthy. The additional mention of:When Alpha Prot is activated due to blocked AOA probes, the flight control laws order a continuous nose down pitch rate that, in a worst case scenario, cannot be stopped with backward sidestick inputs, even in the full backward position. If the Mach number increases during a nose down order, the AOA value of the Alpha Prot will continue to decrease. As a result, the flight control laws will continue to order a nose down pitch rate, even if the speed is above minimum selectable speed, known as VLS.
Does not state what happens if you reach Over-speed but in general the pilots will avoid over-speed and therefore the "fight of the computer" should normally never happen. But if the gravitational acceleration exceeds the drag then the aircraft will speed up no matter how much the thrust is reduced.
It sounds like the Pilot only got control of the plane by disconnecting the second ADIRU. Until then they had no elevator authority (I'm assuming because of the full nose down excess thrust and speed over the control surfaces). It doesn't sound like the plane saved itself with High speed protection.
The first part updated at: Dec 28th 2014 22:22Z seems to be not correct as the events in the second part published at: Mar 24th 2015 20:43Z state:The aircraft however continued to pitch down, inputs to counter the pitch down remained without effect. About 45 seconds after the nose down began the first officer alerted the captain who took control of the aircraft, that at this time had reached a rate of descent of 4000 feet per minute and a pitch of -3.5 degrees. The captain provided a maximum nose up input which caused the aircraft to pitch up again and the rate of descent decreased and the aircraft entered level flight.
The captain was able to maintain altitude by providing a continuous nose up input deflecting the side stick about 50% of its travel. The autopilot could not be engaged again, and a manual nose up trim was not possible.
Which leads to the conclusion that the aircraft was controllable but the FO did not provide maximum side stick input and the captain did, what brought the aircraft back to level flight.
Disconnecting the ADIRU just made managing the aircraft easier as no more input was needed for level flight.
It also appears that the thrust was pulled way back as the Mach number comes back in the dive.
Probably, that is why no over speed occurred.
If this had happened a few thousand feet over ADD airport what would the result have been? How long would it have taken them to recover if they hadn't pulled the thrust back (assuming I'm reading the traces right and they pulled it back)?
That is really hypothetical because we do not know if normal law is disengaged if alpha protection and high speed protection is triggered the same time. But as full side stick input was able to control the aircraft there would have been no crash as I imagine if they were closer to the ground the FO would have given full input aswell and the aircraft would have lost less than 4000ft.