[PMan74]

"An Air Accidents Investigation Branch report said warnings alerted the pilot, who fixed the error at 928ft (283m)."

It's odd, you'd think that the sensation of dropping 500ft (152m) in 18 seconds would have alerted him, especially after take off. It's hard to visualise what that would feel like.

[lbriner]

You are expecting some change in G-force due to changing to AP, you don't necessarily feel the start of the drop and once descent is consistent, it won't feel that alarming, add to that the fact that you think the AP is correctly set and you have confirmation bias and won't notice anything wrong.

Just goes to show that we still don't really know if we should "trust the plane" or "trust the pilot".

[xaedes]

Regarding who to trust; both: trust the whole cyber-physical system. And like OP said, the system worked as designed. The bigger system now even learns from this and improves the existing cyber-physical system to handle such cases even better / earlier or avoid them: "Flybe implemented remedial actions quickly in response to the incident and our training and procedures have been amended to minimise the risk of a reoccurrence".

[kss238]

I mentioned this is another comment but

If you read the UK Government report that is not what happened. The plane was at 1500 ft when it began to descend. At 1300 ft the warning sounds activated and the pilot reacted but it took another 300 ft of descent before the pilot was able to fully recover from the descent.