At 17:25 21 March 2018, wrote:
Thanks for the explanation RO. The instructor I mentioned was my
instrumen=
t-airplane instructor. Funny, that instructor never objected to my
moving
=
my head around while under the hood or in the cloud, only when
landing.
Of=
course we weren't pulling many Gs in the 172.

Getting the "leans" while IMC or under a hood can be somewhat
disconcerting, and it has killed people. (think JFK Jr) At least with the

"leans" you have control over your eyes and can focus on your
instruments to counter that form of SD. The other kind (occulagravic?
- I can't remember the exact name it was given in USAF physiology
classes) can be so bad that along with the violent tumbling sensations,
your eyes will also involuntarily twitch up and down uncontrollably.. If
you do that once to yourself while flying (and survive), you will be very
careful to not to do that to yourself ever again if you can avoid it.

Thread drift - yes, but it is the background to the vision choices I have
made which have served me well in the past, and it is one of the
reasons for the choices I will make when it comes time for cataract
replacement surgery, as I will in all probability want to continue flying
aircraft afterwards.

RO