I actually think the RTA feature on the B737 works really really well!
I know that is not the opinion of the vast majority who try it. I agree it CAN really muck you around and cause all sorts of issues on descent such as choosing a descent speed lower than that required at the waypoint in question and/or changing the descent speed midway during the descent which can have a profound effect on the descent path (steeper/shallower). However if you keep a number of points in mind, you can consistanty make good an RTA to within 15-20 seconds (yes even those ones on descent) and if required loose around 5-7 minutes per 100 nm.
The trick is to:
1. Go to the LEGS page and enter the IAS that ATC want you to maintain from the RTA waypoint.
2. Go to the PERF LIMITS page and enter a limit on the MIN descent speed. The lower limit of IAS on descent should be equal to the speed ATC require you to maintain from the RTA waypoint (usually 250 kt). If the FMC should choose a higher descent speed than that for descent, that is fine as slowing down prior to arriving at a waypoint is handled very well by the FMC. It's when it has to speed up prior to a waypoint that it gets into trouble. i.e if you want to maintain 250 kt IAS from the chosen waypoint, and the descent speed chosen by the FMC prior to arriving at that waypoint is 265 kt, the descent profile will be fine.
However if the FMC chooses a descent speed of 220 kt to make good the arrival time at that waypoint, the descent path from that waypoint onwards will still be based on a descent speed of 220 kt!
Remember a speed entered in the LEGS page is actually an AT or BELOW speed. The FMC has chosen a speed BELOW 250 kt. Once you have passed the waypoint you would then have to enter 250 kt in the descent page and execute. This might put you high or low on the new descent path and you may have to put on thrust to accelerate and/or regain the new descent path. (I have seen descent speeds lower than 250 kt cause a descent path to be steeper than that built on a 250 kt descent.)
The fact that you have put a lower limit on the DESCENT speed forces the FMC to calculate a lower CRUISE speed than it would have otherwise chosen to make up for the higher descent speed. Remember too that it will NEVER choose a speed (Mach) less than that which equates to the best Holding Speed (IAS).
3.If the minimum speed (best holding speed) will not be slow enough to make good the required time of arrival, a CDU scratch pad message "UNABLE RTA" will appear. However this is only true for the current cruise altitude entered in the CRZ page. Remember for a given IAS your TAS decreases with decreasing altitude! With one CDU on the RTA page, the other goes to the CRZ page and enters a lower cruise altitude. Notice the effect this has on the waypoint ETA. It will be getting closer. Keep entering a lower and lower cruise altitude until the FMC indicates it can now make the required arrival time. You might have been cruising at F350 and the required level might be F300 or F250. You then simply have to request descent to that level.
4. If descent is required, do so in (Flight) Level Change or equivalent so that the descent is made using idle thrust and the TAS reduces as quickly as is possible. Make sure the MCP target speed is IAS (not Mach) and that the IAS is set to the best holding speed (found on the HOLDING page).
Remember too the effect of wind at the new level. You might be descending out of a jet stream. A decreasing tailwind will have a profound effect on the resulting ground speed. Descending out of a headwind will obviously have the opposite affect and may cancel the reduction in TAS.
Here are some numbers to keep in mind.
IAS @ FL = MN TAS 100 nm in: Time lost
254 380 (.80) 462 13.0 min -
(230) 380 .73 425 14.1 min 0.9 min
(230) 300 .62 367 16.3 min 3.3 min
(220) 250 .54 325 18.5 min 5.5 min
(220) 200 .49 300 19.9 min 6.9 min
Perhaps in busy airspace you may not have the flexibility to descend as required. Also you may not want to descend as there may be weather below you or reports of clear air turbulence.
Not sure if this answers the questions askeds but I thought it might be useful anyway.
Cheers.