# How to Calculate Top of Descent

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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Here’s a simple(ish) formula I devised:
`Dnm = (A-f) ÷ V ÷ 60 × Skts × 1.852 × 0.62137`
Where `Dnm` is the distance out to descend, `A` is your current altitude, `f` is your target altitude, `V` is your VS and `Skts` is your ground speed in knots.
The way it works is that it first calculates how long it will take to complete your descent in hours, then converts your speed in knots to km/h, then using that it will find `Dkm` which is your distance out to descend in km, then converts that to NM.

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i usually start to descend depending what altitude i am at, FL300-FL340 i start my descent when im 110nm away from my destination, and from FL350-FL390 i start my descent at 115nm, FL400 i start my descent at 120nm and i usually cruise at Mach 0.85. this is how i plan my descent procedure

What if you’re in an aircraft such as, say, the A330?

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What do you mean?

Replying to @HappySpartanJay. A330 cruise is around .78, so I’m wondering how that would affect his descent.

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Ah, got it. I thought a330 cruise was .85 though.

I believe that’s the 787. .85 is pretty fast, not a lot of commercial aircraft in IF have a faster (or equal) cruise speed.

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im not saying the a330 cruise is Mach 0.85, what im trying to say is tht i usually cruise at Mach 0.85 on any aircraft like B757 B777 A380, etc.

A little off topic but I found a useful chart:

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Oh and by the way, in my method I like to go down to Mach .8 from cruising speed for descent fyi.

right now im flying from tokyo(RJTT) to cape town(FACT) on the training server and im cruising at mach 0.86, sometimes i will actually fly at Mach 0.86 or 0.84

Right. @HappySpartanJay, my mistake, I thought you were going for realism. That’s why I got confused when you said you cruised at 0.85. No worries!

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Greatest and easiest way out there. I use this all the time, and it is easy to adjust because of weather or terrain. I’ll call this the best way to calculate descents. Great job dude!

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Thank you!