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.
Thanks for the reply!
I've noticed a disparity between aircraft types regarding achieving a fix time and couldn't figure out why it was occurring. From responses I'm still unable to figure it out and can only put it down to winds.
As it appears as though the best way to achieve a time is through adjusting the speed until the box says that the ETA is congruent with what's being asked for, a certain airframe consistently misses the mark. If there are any F100 pilots lurking I'd be interested to hear how you do it?
As for the BEVLY query; the phraseology is born from a number of factors and the intent is the following.
"There is a gap in front of you in the sequence in which I don't believe you can catch the aircraft in front if you maintain a normal descent speed. If you get there early then you free up space behind you but I've done the figures and if I give you a high speed descent from the feeder fix point, you're most likely to catch whoever is in front of you."
By not locking you into the time, it's a bonus if you're early.