I am intimately familiar with that incident. The airplane only had FDR recording the left PFD so they don't really know what was showing on the right one based on FDR.
The only pitch angle protection is the shaker/pusher. They didn't get that slow.
The AP runs off the Flight Director it's selected to. This is normally the FD of the pilot flying.The autopilot kicks off when the pitch attitude exceeds a certain angle (I forget what it is). The capt was PF so the AP would have been coupled to the left flight director.
There is no yoke force mechanism for disconnecting the AP. If the autopilot was in Pitch Mode and the PFD suddenly goes nose up it'll drive the elevator and follow up with trim inputs to keep the A/P servo loads in limits. But in any case the AP will be in ALT Mode, the mode you are in once you capture and are holding an altitude. It pitches to hold the altitude. If the pitch indication goes nose up, but the altitude is not changing, it won't kick off until the pitch attitude exceeds the auto-disconnect attitude. Until then the AP will be holding whatever pitch attitude keeps the selected altitude.
The scenario was, the capt was sitting there, it's black outside, like black black, up and down, no lights anywhere. He noticed his own PFD going "blue" (all sky). Either he kicked the AP off or it would have kicked itself off when that PFD exceeded the auto kickoff attitude. The airplane is just sitting there minding its own business in level cruise still.
Read the report here. Capt panics and goes into tunnel-vision mode, and thinks they are in a zoom climb so he shoves the stick forward, putting them into a "bunt" (negative G pushover) in the pitch-black-in-every-direction sky. The FO, (it was assumed his PFD was functioning normally but there was no FDR for it) would meanwhile have seen all brown on his PFD by this time (and on the Standby Attitude Indicator, but I don't think they ever looked at it).
So there you are as copilot: capt is all confused and has you both hanging in your straps with speed building and crazy roll attitudes to boot. Your PFD shows all brown - diving. It's black outside, everything that was on the floor is on the ceiling, and neither of you has a horizon; left side is all blue with the high pitch chevrons and display de-clutter happening, and the right side is all brown with the low pitch chevrons and de-clutter. The noise is building to a crazy roar and with no horizon (all blue left and all brown right) it's hard to tell where the wings are relative to the horizon. They both became hopelessly disoriented.
The did several rolls on the way down and hit the ground close to Mach. There were no large pieces.
In my opinion there was a basic breakdown of a fundamental IFR skill. When the Capt's PFD went weird, with no physical sensation to match, and no altitude and speed deviations to match, the FIRST reaction should be to do nothing, and cross check with the opposite PFD and the Standby ATT. It's a case of the old saying, "Don't just do something, sit there". Seeing the right PFD and the Standby working normally and the airplane's behaviour agreeing with what they showed, the capt should have transferred the AP to the right FD and made the copilot the Pilot Flying, turned off that display, and they would've carried on their merry way.
So bottom line is if the Capt had done nothing, they'd have been fine. Even once the AP auto-disengaged due to the erroneous pitch attitude, the airplane itself would have continued level flight as trimmed.
