When to Descend.
Using trigonometry to calculate …
1mile = 5280ft therefore…
3deg glide = 277ft per mile.
4deg glide = 369ft per mile.
5deg glide = 462ft per mile.

Remember though that airfields quite often aren’t at zero ft, so use AGL or subtract the airport altitude from your actual altitude.

Most approaches will be a 3° glide slope. A quick and convenient way to calculate these in your head is…
If you multiply your distance from the airport by 300 and add it to field elevation.
Example:
SXM airport elevation is 26 ft
3 miles from SXM you should be 900 feet
2 miles from SXM you should be 600 feet
1 mile …"…" 300 feet
1/2mile …"…" 150 feet

Another tip on long decent and knowing when to start decending is…
Multiply altitude by 3 so 22000 x 3 = 68000 remove the 0s, your then left with 68, so you would descend 68nm from your desired airport. For the VS divide your ground speed by 2 which can be found in the bottom left corner of your HUD display. so of your ground speed was say 300 KTS you would descend at -1500 FPM

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Ps your icon appears normally to me, and I’m guessing to others. Might be a glitch with your browser.

which speed shall be taken for the calculation IAS or GS?

Well, the thumb rule actually assumes, that there is no wind, so GS and IAS are basically the same. That’s why it is only a thumb rule.

If you have tailwind conditions (IAS less than GS) add more descent rate, like 500ft/m, because you make the miles towards your destination faster, thus need to descent faster.

If you have headwind conditions (IAS more than GS) you can leave it like it is, you will arrive a little sooner (with regards to miles) at your desired waypoint altitude.

Above 10.000 feet, altitude x 4 = distance needed.
Below 10.000 feet, altitude x 3 + 10 = distance needed.

Also remember, some jets like the A330 are unable to descent and lose speed at the same time. So best is first to descent and when level reduce the speed if required.

Going through some old staff here and @tamm01 i find your method easy but it contradicts to @Nik formular which is also understandable but which is which?which is more accurate

target altitude simply describes the altitude you want to level of. Could be during climb or descend.

For example:
you are descending from let’s say 24.000ft (flight level 240) on your initial approach,
but you want to slow down to 250kts before descending thru FL 100 (10.000ft),
then you would set your next “target” altitude to lets say FL120.
slow down,
then you want to descend to your ILS intercept altitude of 3000ft.
then your new target altitude would be 3.000ft.

…the Thumb rule is on of many applied by the airline pro’s, using it for a reference, e.g. to see, if the TOD shown in the FMS makes sense …jsut to be on the safe side

For the Dash 8-Q400, use 3 times your height to calculate the rough top of descent. I.e. 20.000ft. (20.000*3=60.000) which is 60nm. Descent is normally flown at ca. 230kts, -2000fpm and idle power.

You should base your NM distance in the final approach point/fix for the specific runway you want to use for your airport. And then start your descent checking your ground speed.
On normal conditions, the ATC should tell you when to start descending.

sure that´s why the difference of the altitude is getting bigger the nearer you get to your destination because of your speed+
when you´re travelling with 300knts it´s obvious that you can only decend about 2´000 fet while travelling about 20 nm

The rate of descent should be marked by you ground speed by 5.
I use as target altitude the final approach points for the ILS on each runway, but the ATC should guide you when descending.