MarianA -
I understand that in normal day to day operation you will expect widely varying values for that accelaration X because of wide variation in the input data making it impossible to judge by feel whether it's about right or not and will take you airborne in the available space.
Almost right, except that flex thrust
tends to even out the thrust-to-mass variation, so that the primary variation in X is due to field length and elevation.
Measuring acceleration is certainly already done by inertial systems aboard and if the data is not available GPS would propably be good enough for that. The rate of change of distance covered over time. Not a critical system, a simple add-on, collateral use of available data.
So true, as has been pointed out several times.