On some FMS you can telling an RTA (required time of arrival) for a waypoint and it will achieve that.
However, rubbishy in equals rubbish out. If the winds aren't accurate you can get some silly solutions with massive speed up and subsequent slow downs as the conditions change. Every RTA I've seen used for a waypoints not on descent has been a complete and utter cluster.
For my money I prefer to fly a 250 knot descent as that is invariably the speed ATC want after the waypoint and then adjust the cruise speed accordingly to meet the time. If that doesn't work then I'll simply insert the slower descent speed and see how close that gets me to the time. Most often I simply eyeball the profile to match.
One of the weird clearances I've experienced recently into PER is 'cross BEVLY at time 40 or earlier then normal speed'. I can't work out why the speed after BEVLY is important if I can get there at anytime prior to 40. Eg if ATC doesn't care if I get there at 35 or 39 then surely my speed after BEVLY is irrelevant? Im open to thoughts from that one. AWOL57?
The winds in the FMS depend a bit on its capability. For QF 330 ops we uplink the wind forecasts and then the FMS extrapolates between current wind and forecast wind over a disgrace before going to completely forecast wind. The 767 was a bit more generic and sometimes you needed to use a bit of rat cunning as to what the wind was likely to do in order to generate accurate estimates.
Yep the FMS in the 738 works the same way. Funny you mention about RTA Keg. I don't know anyone that uses the RTA function reliably it is a horses ass. In the dash not completely fms driven so back to the old way of adjusting the power or altitude or both and watch the time change on the screens respectively.
After going through all these creative routes, more so with holding crossing times which we would get pretty close to, atc often didn't trust us so would instruct us to turn inbound before the fms was to turn us. 😀