Hi Tingtang,
In reality, and as an aside, it's not normally that much of a science. Many places we go to (in Europe and the USA at least) might ask you to descent before 'your' TOD point (as calculated by the FMC) for airspace reasons, so even if you're trying to be clever and economical and descend in VS-500 to get back on profile, they will spot it and ask you to descent faster.
However, given a free choice, and without an FMC to help you, 3 times the flight level, plus 1 mile per 10 knots back to 180, is a pretty good guide, and basically what I use on every descent. It works in the 757 and 767 - other aircraft might have different factors.
It's also useful to use standard energy 'gates': If you are at FL100, 250 kts, with 40 track miles to go, you'll probably be fine. If you're still at 300 kts, and at FL120, when you have 40 miles to go, you have too much energy and need to get rid of some.
The VNAV descent will be calculated based on an idle descent at whatever speed you have put into the computer for the descent.
This can be:
- ECON (based on the cost index... likely to be about 260-280 knots)
- A selected speed (e.g. 0.80/300 which means descending at .80 but then holding 300 knots when the IAS increases to that value)
- a couple of other ones too
If the speed is very slow, then the descent will start much earlier, as the nose will need to be a lot higher, and the ROD therefore much lower. The overall angle will then be quite shallow.
If you descend at a very high speed, the TOD point will be much further along, but when you do get there, you will stuff the nose down and descend at a much steeper angle.
Hope that makes some sort of sense - sorry about rambling a bit.
NW3