Assuming you were in VNAV PTH before selecting RTA, the FMC was flying a constant speed - FMC SPD regardless of wind changes. After you select RTA, the FMC now computes the speed required to make the RTA good and you see the A/T moving as and when required to compensate for wind changes. This is why sometimes with the selected RTA, you get the message RTA UNACHIEVABLE as the headwind/tailwind increases/decreases and the selected RTA moves out of the RTA bracket. From climb basics, OPT/MAX altitude both depend on the excess power available = (power available-power required). In case of a changing power setting, the FMC is unable to calculate the excess power available and hence no figures are available for OPT/MAX altitude.
Last edited by Guru8904; 2nd Nov 2014 at 19:00.