First off you will need a flight plan so you know time “ETE to destination”.
If you are cruising at 20, 000 ft, you can start your descent 20 minutes before “ETE to Dest” descending at - 1,000 VS,
10 minutes before “ETE to Dest” descending at - 2,000 VS.
You will always come in early and will have to reduce your VS as your aircraft slows. Which is what you want, since you never want to come in to high.
I use it all the time between waypoints as well.
Here is an example;
When I am cruising at 20,000 ft towards KPDX, I know I have to be above 12,000ft when crossing waypoint KRATR and I have to be at exactly 5,000 ft Max 210kts at LIQWD.
Therefore four minutes - 04:03- before KRATR I start descending at - 2000 VS to lose 8,000ft (screenshot).
(As you can see in the first screenshot, “ETE to Dest” is also 10 minutes (10:38) so starting descent at - 2000 VS will get you there as well. Of course it will take longer to arrive because as the descend your speed and VS will decrease as well.)
On my way to LIQWD, I continue at - 2000. As I get closer to the waypoint I am reducing my speed to below 210kts so my vertical speed will reduce as well. Just keep an eye on your “ETE to Next” to adjust VS accordingly. By the time I arrive at LIQWD I am usually around - 1,000 VS.
he means as you descend from cruise level you do not go faster than 280kts and then when below no faster than 250kts when descending.
depending on the aircraft I might even plan to go slower in order to not go to fast. Example on the B777 I tend to plan to decend at 260kts when above 10000ft and when below set to 240kts…then slow down further as I get closer to landing!
What I do is your current altidute (eg. 30,000) minus your target altitude (eg. 125) divided by 0.0524 (tan3°) That number is in feet so turn it into miles! This is an exact number… Probably the ones above are a lot easier.
I use a more complicated one. (ALT - (Field Elevation+2500)) *3=X/1000+10
Basically you take your altitude and minus that with the Field Elevation of your arrival airport, plus 2500, which is the intercept altitude. Then you multiply the result from the earlier calculation (ALT-(FLD ELEV+2500)) with 3. That gives X. The result for that is X. But since your result is then in the high thousands, and in case you are too lazy to remove three "0"s, you divide by 1000. The number you get then you plus with 10, as you want to intercept the localizer 10nm out.
Now, lets say you´re cruising at 35000ft, and Approaching KATL. KATL has a field elevation of about 1000ft.
So the calculation will be: (35000 - (1000+2500)) *3=94500/1000=94,5+10=104.5. Your Top of Descend will then be 104.5 nautical miles out of the airport.