Whats fs9? 2004?
Cool so you can hopefully answer my question. PFE always descends me early from my TOD do you use level change or stay in vnav? If i stay in vnav and go to the des page the a/c descends at a shallow rate 1000 fpm till it intercepts the glide path but i always arrive way too high that way.
First thing to note is that the FMC VNAV PATH has little correlation to any ATC instructions. The only time they will be "in line" is where you have altitude restrictions in the PFE flightplan that you have also put into the FMC. You will find then that the ATC instructions will match pretty well with your VNAV PATH. PFE (just like in the real world) may ask you to descend early or late compared to your FMC calculated TOD point. It will also randomly ask you to descend "at your discretion". You can then choose to use DES NOW, or to wait until the TOD point.
If you are arriving close to your final waypoint "too high", there are a few things which you can do.
1. Before you start your flight, make sure any important altitude restrictions (especially at the last few waypoints) are the same in the PFE plan and your FMC. Your FMC will initially show
calculated altitudes for waypoints (unless they are hard altitudes for a STAR), you may wish to make these hard altitudes by overwriting them.
2. If you are flying with a weather addon, make sure you put descent altitude winds into the FMC before you descend. Strong winds will make your VNAV PATH profile incorrect if you fail to do so.
3. Descending through 10,000 feet can sometimes be problematic, as you have to both descend
and slow down. If you think you are getting too high in this process or not slowing quickly enough, then use speedbrakes.
4. Just as in the real world you will sometimes get instructed to descend quickly. The quickest (in terms of vertical speed) way to descend in this situation is to switch to LVL CHG, and fly at the maximum permitted speed, whether that be 320 kias or 250 kias below 10,000. Again using speedbrakes if needed.
So to answer your question..... if you follow the advice in 1. above, then you should stay in VNAV and use DES NOW. If further on in the descent you need to descend quicker, then switch to LVL CHG.
It may help if you give an example of exactly where during the descent you feel you "arrived too high". It may simply be that you need more experience in getting the aircraft to descend and slow down at the same time. The 737 series aircraft are well known for not wanting to do this, and in this respect you are flying a very accurately modelled addon !