(Having a screen in the background to write on is great, but if you could write any comment on the screen it would be even better).

Something like:
2 buttons to write directly on the screen in flight. (Best with pen)
  1st button for texts, that stays where it is written. (Shown in color 1)
    I.e. local pressure, which moves out of view as flight progresses.
  2nd key for a text that remains on screen, even if the flight continues. (Shown in color 2)
An additional button to enable/disable the editable text screen, i.e. if the overlay is blending things underneath.

Also, an easy way to remove text.
Something like (in active mode) circle a text and delete it with a swipe.
(and a recovery swipe in case you deleted something unintentionally)