Not wishing to dissuade you from your quest to find an app, but there is that old saying that NASA spent millions developing a pen that could write in space, whilst their Russian counterparts simply used a pencil.
I suspect that if you truthfully compare the time it takes to ....
get the iPhone out of your pocket
tapping in your pin code to unlock it
opening the correct app
tapping the button
Compared to ...
Scribbling a few numbers on your kneeboard
You may understand what I'm getting at.
penciling in the takeoff time while trying to fly the plane
Surely the correct sequence of events is ... line up, scribble down, take off... not messing around with iPhone (or pencil) during climb-out
