After you have programmed your legs page for a descent,connecting the arrival/approach without any discontinuities, you must have a bold speed, and altitude on your legs page, at a point, at the commencement of your initial approach segment,preferably in level flight.This has now instructed the FMC that you intend to arrive at that geographical position, at a specific speed and altitude. The next step is to program the variables into the FMC. Go to the descent page and select forecast. On the forecast page, enter your cruise altitude with the wind direction and speed (just before descent). Do the same for a medium altitude( during descent) and your destination altitude. e.g.If cruise was at FL350, enter the altitude,wind direction and speed.
Flight level. Wind SPD. Direction
FL350 260. 30
Fl160. 220 40
00300. 180 15
If you are unable to get winds at the desired altitudes, you can enter another altitude. It doesn't matter if it is an even or odd altitude. Neither does it matter if the altitude ends in 0 or any other numeral.
This allows the computer to calculate a descent path, considering the winds(Headwind or Tailwind). The final part is this.The computer calculates the descent path to arrive at the position mentioned earlier on, with the "hard" speed and altitude.The only thing is that it assumes that altitude is with a standard QNH(Altimeter setting) of 29.92 or 1013 mb.You now have to apply the variation from the standard setting, on the same descent forecast page, i.e. temperature variation from i.s.a. and the actual altimeter setting at the destination. You will see some green circles on your legs page.They indicate where the Aircraft has to change it's pitch/descent path, for a change in speed.The first two closely spaced dots represent when the Aircraft starts to slow down to 250kts, just before 10,000 ft.The second on represents when the Aircraft is slowed down to 250kts,the resumes the descent path.
The next dot represents when the Aircraft starts to slow down, in order to achieve the hard speed,and altitude programmed earlier. As the Aircraft is approaching the clean maneuvering speed + 10kts, or as shown in the FCTM/FCOM, the flaps are lowered to the initial setting. Once the initial flap setting is achieved, the speed bug automatically moves to the maneuvering speed for the initial flap setting.This process is continued for each stage of flaps, with speed or glide slope as the trigger for the next flap setting, as shown in the FCTM/FCOM.
If the FMC is programmed properly with the previous data(accurate wind direction/speed etc) it will give you the required performance. Remember "garbage in, garbage out"
If you do have any questions,feel free to ask.