How to Calculate Top of Descent

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.

Read my tutorial again. I mention how you should add the elevation of the airfield and 3000 feet. 3000 feet is optimum for intercepting the glide slope.

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Yes, you basically described my whole tutorial. You use ETE to destination to calculate when to descend. How you descend is a different matter, of which I explained.

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No, because ground speed is the speed at which you actually cover ground, so if you use air speed in your calculations (or no speed at all) it will not be 100% accurate. You’re right that it changes with altitude, but certain calculators account for that.

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Well, that’s where it gets tricky. If you remove ground speed and different variables you get a much simpler but effective method. My method eliminates those variables, thus requiring you to do less math and a more relaxed descent.

I have found that most online TOD calculators mess me up someway or another, mostly because they do not account for G/S changing with altitude.

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Well my logic works very well thank you, since it is the same as used by professional pilots…

At FL380 it is possible to have IAS of 165kts, whilst your GS is 500kts +. This is because the higher you are, the air has lower density. Yes your IAS will increase as you descend but the initial calculation allows for this. A pilot should check every 10,000ft that stil “in the pipe” so they can correct as necessary.

When I first started using IF I made myself a cheat sheet showing DTG, vs the height and speed I should be so that I didn’t overshoot.

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Brother this is actually a very very very good post. I personally use a Third-party calculator that comes with the checklists that I use but with now I know how to calculate it and the truth is that it’s more interesting me doing it.

Do you mean 12.3 (or any that I calculate) from where I am to my destination right?

Again, thank you. Really helpful 👍🏻👍🏻👍🏻

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Yes, the 12.3 is the minutes to your destination. So make sure to use ETE to Destination. Glad you liked it.

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