The definition of "acceleration" is the changing of velocity, therefore not constant.
Acceleration can be and often is constant.
If I drop a ball in a long vacuum tube - i.e. no air resitance - then it will accelerate downwards at approximately 10 metres per second per second.
Whilst the balls downward velocity is changing (0 m/s at 0 s, 10 m/s at 1 s, 20 m/s at 2 s, etc. following v = u + at) ...
The accelaration - the rate of change of velocity - is always the same.
Or: whilst dS/dt changes, dv/dt or dS^2/dt does not.
That constant accelration is either due to gravity in RET, or "The Universal Accelarator Field" in FET.
And Einstein says that the two are equivalent (Equivalence Priniciple).