ghw78 said "
When you put in an RTA you are overwriting the performance parameters by telling the aircraft to fly at this altitude to achieve a Way Point Crossing time of xxxx. Note that it comes up with a Max Speed that it will do and to achieve RTA the big controllable item to achieve this is speed.
Speed increase or decrease affects the altitude capability at a set weight thus the calculations on the VNAV CRZ Page are removed until RTA is either removed manually or by crossing the Waypoint".
He is absolutely correct, that is the logic written into the FMC programme. Proved it only today, a fiddle here, a fiddle there, but nothing short of cancelling the RTA would restore the other parameters.
Quite logical when you think about it.
Regards,
Old Smokey