Quote:
Originally Posted by my_controls 
"A descent in VNAV started before the top of descent point is an early descent. If a path descent is planned, VNAV commands a 1000 fpm descent until the idle descent path is intercepted."
The way I interpret your post you're saying that the plane will cruise descent down to the altitude in the MCP window at 1000 fpm, and that it won't have any path information until it reaches this altitude. Is this a correct interpretation?
On a side note I still don't fully understand the difference betweet a FMC SPD descent and normal lvl change either.
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The way to accomplish what you are talking about is to select the "descend now" function in the FMC which will descend the plane at 1000 FPM until the VNAV path is reached at which time the aircraft will descend to stay on the path. Using this feature you have not changed the cruise altitude to FL200 that you did in your earlier example. If you change the cruise altitude to a lower altitude you will have no accurate path information until you reach your "NEW" cruising altitude.
Think of level change as all or nothing. It will climb at climb power at the speed commanded in the MCP (mode control panel) and it will descend at idle power at the commanded MCP speed. This level of automation does not employ the FMC in any fashion. FMC speed descent will employ the VNAV function as you may know. In a normal path descent, priority is given to the path and the speed will go to whatever is necessary to maintain the path. If you have a big tailwind, the airplane may have to descent at 320 knots even though you have assigned a 300 knot descent speed in the FMC. In a speed descent mode, you are telling the FMC that speed should take priority over the path. So in the above example, you will maintain the 300 knots but may very well go high on the path. Speed brakes may become necessary to make it all work. As you use various levels of automation and learn how it all works you will develop your individual ways to make it all happen. There are many ways to make the airplane do things and it is quite entertaining to watch various pilots display their own special techniques. As long as you get what needs to get done, there is no right answer as to how to do it.