How to Calculate Top of Descent

Hey guys, today I just want to share a really easy and quick way to calculate your rate of descent. It works every time for me.

First, you want to note your cruising altitude and speed. Let’s say it’s 30,000 feet at Mach 0.80. The perfect way to capture your ILS is around 3000 feet, so add your arrival airport’s height to 3000 feet. For this example, the airport’s height will be 0 to make it simple.

Next, subtract the 3000 from your cruising altitude of 30,000 feet. You will get 27,000. Divide 27,000 by the vertical speed you want to descend at. I usually descend at -2200 fpm, but you can do whatever you want. 27,000 divided by 2200 is ~12.3.

12.3 is the amount of minutes you should start descending at. I play it safe and go for 12.5 (12 minutes and 30 seconds). As soon as it hits that, start descending!

To work in compliance with the 250 knots and under under 10,000 feet I would suggest 270 KIAS under 20,000, 260 KIAS under 15,000, and 240 KIAS under 12,000. Remember to use your speed brakes!

I know there are many tutorials on calculating your Top of Descent, usually resulting in too high or too low descents, but this one aligns perfectly. If you have any questions, please feel free to ask me below. Thank you!

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Thanks so much! I have just started doing longer flights, and I was wondering when and how to descend. You are a LIFESAVER! Thanks again! :)

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Ah, no problem! Very glad to help.

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Number of kilo feet to descend x 3. Add 10 if arrival by base or final.
Exemple 33000 ft airport at 1000 ft. 33-1=32x3=96 Nm. + 10 if straight arrival.
VS = even(abs(GS/10))x2x100
471 kts > 47,1 > 48 / 2 > 24 > 2400 ft/min
The aim is to descend at 3°. I hope that one day IF includes varie in ft/min or in degrees.
At 25 Nm you set 210 kts.
Speed M 0.80/300kts above 10000 ft. At 12500 ft you set 250 kts to be at 250 kts under 10000 ft.
I used to based my T/D on time. But I gave up for something based on distance

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This is actually very helpful! Thanks!

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No problem!

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I’ll usually start descending at 20mins or 100nm (whichever comes first) down to FL110 and adjust to 250kts. I’ll cruise from there for about to about 50nm out from then I will manually control my speed and altitude then at 10-15nm out I’ll turn off all autopilot and prepare to land.

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I am about to use this technique as I descend into London Heathrow on the training server! :)

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Awesome! Let me know if it works for you.

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It worked out perfectly. I had a crappy controller while approaching Heathrow, but all went well. :)

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Fantastic. Sorry about your controller, but I’m glad my method worked out well!

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This looks great, but just a word of warning; anything that doesn’t account for ground speed is an estimate only and may not be 100% accurate.

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@DylanHK The methodology is sound, but I do have a question. How would you account for the final approach?

I know you said “add 3000 to airport elevation” to intercept glide slope, but my question is, how would you know the flight time to the point at which you capture the ILS? The only way I can figure out is ETE to Dest, but that would put me 3000 ft AGL directly overhead the tower, which isn’t ideal. What’s your method for capturing the beginning of the cone at 3000 ft?

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Suggest that you dont need to overcomplicate it. Use the Rule of 3 …, ie Take your FL, devide by 3 which gets your distance out to start your top of descent. Add 1nm for every 10kts of IAS you are over your landing speed and there you are!

Example

38,000 ft = FL380
380/3 = 127nm

IAS = 165
LAnding Speed = 145
Difference = 20kts = 2nm to add. (dont forget to allow for head or tail winds!)

127 + 2 = 129 nm from airport = TOD.

See this vid from Capt Joe which details how proffesional pilots do the same calculations

Ref VS, as a rule of thumb i use the following as a guide:

CRUISE - FL250 = VS -2500
FL250 - FL120 = VS -2000
FL120 - 6000 AGL = VS - 1500
6000ft AGL- Landing = VS -1000 to -500 adjusted as needed.

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Nice method, just make sure you check the elevation of the airfield you are arriving into and the terrain

Ive seen quite a few people try to descend down to 4,000ft and the airport is at 5,500ft 🙃

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I do not use any methods to calculate my top of descent but I usually just see my ETE to my destination and start descending at around 20 to 30 mins ETE depending on my altitude

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*choughs * Denver.

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Ground speed changes by altitude, so any methodology with ground speed calculated is mostly inaccurate and doesn’t work.

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The whole point of my tutorial is to use ETE to destination to know when to start descending.

I’d also like to add that you should be (to be safe) add 30 seconds or so to the time. Say your result is 11.1, start descending at 11 minutes 30 just to be safe, even if it’s not exact.

I have never gotten into a situation of being overhead the airport, most times I reach 3000 feet way before the airport. The important thing to remember is that the lower your VS in the calculation is, the farther away you’ll have to descend. The final number should replicate that.

I do recommend sub 8000 feet you start making maneuvers towards the airport based on ATC instructions or by your own judgement. Be sure to use the map to judge distance to the ILS, and adjust your altitude accordingly. Even if you get to the airport early you can always cruise at 3000 feet, it’s up to you.

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This logic doesn’t really work since KIAS goes up as you go down from your cruise altitude. My method completely eliminates the confusion of ground speed and air speed calculations. Captain Joe uses the method because he’s a real pilot and in real planes the KIAS doesn’t limit to 350 or so (until you overspeed).

By the way, why are you at 38,000 feet and have a IAS of 165? What I mean is that your IAS of 165 at 38,000 feet will go up to higher numbers once you start descent, making the 165 in your calculations negligible.