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

Use your ETA to destination, “ETE to Dest”.

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,
OR
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.

4 Likes

Exactly my cup of tea

2 Likes

I usually just multiply the number of feet I want to descend by 3 then divide that by 1000. This gives you the number of nautical miles out to begin your descent.

For example, if I wish to descend from FL200 to sea level, I simply multiply 20000 by 3 then divide by 1000. So I would begin my descent 60 nm away.

To calculate your vertical speed during your descent, divide your ground speed by 2 and then multiply it by 10.

For example, if during my descent my gs is 300, I divide it by 2, then multiply it by 10. So my vertical speed would be -1500 f/m.

This should work fairly well.

2 Likes

surely a quicker way is just to divide your Flight Level by 3?

FL200 / 3 = 66

2 Likes

http://aerotexas.com/desplan/desplan.php
This is a nice tool online.

2 Likes

I’m a bit confused about the speed bit… [quote=“Clipper747PA, post:1, topic:920”]
speed is not higher than 280kts til 10000ft and 250kts below
[/quote]

1 Like

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!

1 Like

So when below 10000, you have to descend slower? Can you still cruise at the crushing speed?

Thanks

1 Like

there are two aspects to remember when planning your descent :

1. Air Speed :- As mentioned 260-280 Kts when above 10000ft.
2. Vertical Speed: This is how fast you tell the Auto pilot to descend at. As a guide, above 10000ft I descend at -2500 ft a min and when below 10000 I then descend at about -1500 ft a min.

There is a correct formula to use based on your ground speed, but above is a ‘rule of thumb’.

3 Likes

Okay, thanks a lot.

1 Like

regards cruising speed this is usually about 300-310 kts depening on the aircraft. Above Fl280 this is M0.76-M.082

1 Like

Okay, thanks a lot!

There’s a little mistake…there is a zero too much when you devide 20000 by 1000(you wrote 10000) ;-)
You wrote 20000-10000=20
but it has to be 20000-1000=20
;-)

…thanks anyway for the tutorial

Is there any tutorial “how high to climb” /“how to chose correctly the right cruising altitude depending on the distance between 2 airports”;-)

/Ronny

1 Like

There’s a post somewhere about that, but I don’t remember where.

Here’s what I’ve written down from that post:

Take your total route distance, divide by 2, subtract 10, divide by 3 and multiply by 1000.

Ex: 300NM / 2 = 150 - 10 = 140 / 3 = 46.666 x 1000 = 46,666 ft. (obviously unrealistic to fly that high, but use it as a reference to how high you can go in order to descendin time.)

1 Like

I found the comment! Hope this helps. :)

1 Like

Thanks!;-)

1 Like

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.

Thanks for posting! I had run across this a couple of times in various places but now couldn’t remember the rule nor where I’d read it.

While very clear and just for the sake of clarity for others, I believe there is a typo in step 2 of the example, where 20,000 would be divided by 1,000, not 10,000 (I.e. 20000 / 1000), right?

Thanks again!

Too make things easy I use the FL devided by 3 as this gives you the answer without need for further maths!

Eg FL320 / 3 = 106 nm to go.

1 Like

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.

Hope it was not too complicated. :)

3 Likes