My guess on what happened to the Lion Air Flight JT610 is that the IRS malfunctioned and the pilot attempted to turn around, did not have adequate attitude information, thought he was climbing but was descending, and crashed into the ocean at 400mph.

This is the only thing that makes sense given the data I’ve seen. It was overcast in Jakarta when this crash occurred, which matches with my above theory.

On a plane like the 737 Max the GPS only provides position information, not attitude. If you lose IRS and are in clouds and are a bad pilot, you are screwed.