Aneywo,
Various methods exist! Descent angle depends on your aircraft type but here is a useful tool, which results in a fuel-saving descent. This method needs two calculations - firstly the Descent angle, secondly the Rate to achieve it:
First decide on an aiming point and a height /Alt/FL to be there. This can be FL100 at 30 NM, or if, as you say you want no speed restriction at FL100, a lower point and related altitude, corresponding to the expected approach. A speed reduction from 290kts to 250kts at FL100 will take 5 miles for a short range jet up to 10 miles as quoted by Brakedwell for a heavy (no matter whether done in level flight or during the last 1500feet) so you have to aim to be at FL100, or wherever you slow down point is that much earlier.
I would recommend you to start reducing IAS latest Alt 8,000 (or FL80) anyway to be in a good manoevering position for intermediate approach. So anyway pick the aiming point first, then work backwards:
- if your aircraft descends at 3deg (DC-10, DC-9, 737, some 747s etc) then divide the difference in height of your aiming point and your present altitude by 3, giving the number of miles before the point, where the ideal Top of Descent occurs.
- if your aircraft descends at 4deg (MD-80 series, light Fokker 100) then divide by 4 instead of 3 to get the miles.
- a light aircraft will descend at a higher rate for a given IAS than a heavy one - but these are ball park figures.
Next question - how do you descend at 3deg or 4 deg? Simply multiply the Mach number by 3 or by 4 and set Rate of descent to this figure. (Don't ask me why this works, but it does! - something to do with the 1 in 60 rule...)
Follow up your descent progress each ten miles. To do this take Difference in Height between Aiming point / Present FL/Alt and divide it by the miles to run to the aiming point. The answer will be the descent angle required to hit the point - it may have changed due to wind or ATC restrictions. Suppose it comes out as 31/2 deg? Then again you multiply the (new and lower- as you descend) Mach number by 31/2 and this gives you the revised rate of descent, which you then set. You might need speedbrakes in order to avoid too high an IAS, when increasing the Rate of Descent.
Do nothing more for the next ten miles and do another follow up - etc.
At first the method is not easy but becomes routine with practice. It can also be used in climb if you have to be at a particular point at a minimum Alt/FL but be careful here with your follow up, as the Mach number increases in the climb.
Also be aware that some aircraft (MD-80 is one of them) will have cabin altitude rate problems if you descend all the way from high level at 4deg. In this case, above FL300 you should have a shallow descent of only 1 deg, which is 1000 per 10 mile and gives a steady Rate of Descent of 750 fpm at an initial Mach of .75. Again, you have to work backwards from FL300 to calculate the top of shallow descent.
Do you have an FMS? If you do, you can have fun checking your calculations against the Glide or Descent Angle calculations and Top of Descent point which it produces.
