# How to compute the TOD (Top of Descent) - Thumb rule

Hi There,

there is an easy thumb rule for descents, that is important to know for pilots and ATC Controllers as well:

subtract the current altitude from the target altitude, eg
30000ft - 10000ft = 20000

divide the result by 1000 and multiply with 3 and you will get the miles
20000 : 10000 = 20 x 3 = 60 miles

so you have to start the descent 60 miles away from the point where you plan to be at 10000

IMPORTANT:

• descent rate is no less than 2000ft/min
• speed is not higher than 280kts til 10000ft and 250kts below

happy landings

Clipper747PA

124 Likes

Quoted from my personal notes I keepâ€¦

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When to Descend.
Using trigonometry to calculate â€¦
1mile = 5280ft thereforeâ€¦
3deg glide = 277ft per mile.
4deg glide = 369ft per mile.
5deg glide = 462ft per mile.

Remember though that airfields quite often arenâ€™t at zero ft, so use AGL or subtract the airport altitude from your actual altitude.

Most approaches will be a 3Â° glide slope. A quick and convenient way to calculate these in your head isâ€¦
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 â€¦"â€¦" 300 feet
1/2mile â€¦"â€¦" 150 feet

Another tip on long decent and knowing when to start decending isâ€¦
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 Iâ€™m guessing to others. Might be a glitch with your browser.

33 Likes

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. Thatâ€™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.

Happy Landings

Clipper747PA

6 Likes

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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.

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Thanks have just found this thread (have book marked it) as its a big help in my flying to make it more realistic and more enjoyable!

Happy Landings

5 Likes

I use this: for example Iâ€™m at FL270 and my desired altitude based on airport elevation is 3000 feet.

27000-3000 (since i wonâ€™t be descending all the way to 0 feet) = 24000

Now divide 24 by 3 = 8, so iâ€™ll need to start descending 80NM away from the airport

As for the descent rate i use GS divided by 2 and adjust if thereâ€™s at least a 100 difference lol

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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

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It would depend on your preference tbh maybe Nikâ€™s is more correct though :)

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This is helpful but confusing at the same time. What do you mean by target altitude? And is this is for real life or for the game in general? Thanks

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Target altitude should be the final approach point for the ILS interception. ThatÂ´s what I think.

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

For example:
you are descending from letâ€™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 â€śtargetâ€ť 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.

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

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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.

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this is the list i always use:

75 nm left: 22Â´000 ft altitude
53 nm left: 20Â´000 ft altitude
40 nm left: 17Â´000 ft altitude
26 nm left: 13Â´000 ft altitude
15 nm left: 8Â´000 ft altitude
10 nm left: 5Â´000 ft altitude
6 nm left: 2Â´000 ft altitude

maybe it helps some of you

9 Likes

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.

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

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I think it might be a little easier with a step by step video tutorial made by me :)

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Whats a target altitude and what is a good descent rate? I feel -2000 fpm is too fast

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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.

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Multiply GS by 5.3 to get the FPM

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