In page http://ardupilot.org/plane/docs/automatic-landing.html , the last sentence has probably a typo
To know the airspeed at the exact moment it stalls, check your dataflash logs (*.bin on SD card) for the airspeed (ARSP.Airspeed) when your wing loses lift and drops by comparing actual roll (CTUN.Roll) and desired roll (CTUN.NavPitch) diverge.
Should it be instead CTUN.Nav.Roll ?
Good pick up. Did you raise an issue in GitHub?
No, I have no idea how to do it .