Hi PPRune Community,
as this is my first post, just a short introduction. I'm a just retired performance engineer having worked for more than 3 decades for a major airline.
The selection of the proper acceleration altitude has accompanied me all times in my job.
According to the regulations (FAR 25.111) an airplane must satisfy a certain minimum climb gradient at every point along the takeoff flight path starting at 35 ft until end of third segment. The requirement reads 1.2% for a twin, 1.5% for a three engine aircraft and 1.7% for a quad. There is no word about the required thrust setting during the second and third segment. So from the regulations perspective the maximum acceleration altitude is not limited by the thrust time limit rather by the climb gradient capability (for acceleration the equivalent climb gradient is used).
Most takeoff programs however have adopted the thrust time limit to calculate the maximum acceleration altitude for computation simplification reasons. One exemption is the Boeing program (e.g. B737-300) allowing a so called "extended second segment" where the acceleration and flap retraction is performed with Maximum Continuous Thrust.
Hope this helps clarifying this subject.
Regards dalan