Okay... I'm pretty sure the particular guy telling the campfire story wasn't in ATTI because he's learned from harsh and brutal experience that ATTI is dangerous on the latter part of a flight due to the individual cell power-stresses due to abrupt course changes on the very low battery. He prefers the normal smooth GPS flying - especially at the very end. But I could double check with him...
I didn't mean that I thought he was in ATTI. Only that the sideways flying described is
similar to ATTI, and should be able to be handled by an experienced pilot. I should know! One of my first pilot experiences was trying to land a P3P in high winds in ATTI mode on a narrow 8 foot wide 1000 foot long pier. It flew as you are describing, unpredictably, but by maneuvering it around, I luckily managed to get it in
between the railings about 200 feet away from me, when it finally auto-landed, instead of over the railings into the ocean. At the time, I didn't realize Auto-land descension could be prevented, nor did I know you could ascend during it. I also fogot I was in ATTI mode, which I had switched to for faster downwind flight on the return, when battery level got critical. Aircraft seemed completely out of control. I'm a better pilot now, and make sure to land in P mode with the extra stability of the downward OA, which is
disabled during Autoland! The biggest mistake you can make during Autoland is to have the aircraft too far above you, where it is harder to see with your head straight up, and takes too long to descend, before the battery shuts off. Ideal is 25 feet or less above you and 10 feet out, so you can see it and walk to it, or bring it in at eyelevel for a handgrab from underneath.
Eager to hear more, after you double check with him.