On Wednesday, 23 September 2015 08:14:15 UTC+2, Jonathan St. Cloud wrote:
I fresh idea. I shall paint a half dollar size green dot in the wheel down and locked position and a red square in the wheel retracted position. Less confusing than arrows or just color, square does not roll.

What happens when you forget what a square shape represents during the heat of the moment or someone else flies your glider?

The words UP and DOWN (or EXTENDED and RETRACTED) cannot be confused and since language is usually one of the last things one loses when your brain starts failing, it's probably the safest bet.
If my brain can't interpret UP/DOWN then in all likely hood I've already lost all my motor skills and a crash is inevitable.