How do you know when to start descending?

Hi all, quick question. How do I know when to start my descend? Sometimes i descend to maybe around 6000 and I’m cruising at 6000 for 25 minutes. Please help me and thanks in advance

I usually use VNAV to descend and it puts all the altitudes that it needs if you use airport procedures. All you got to do is use the airport procedures then when you get to cruise, then click VNAV under the autopilot tab. Hope this helps you out!

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Oops, I forgot to mention that I put in my altitudes kind of Nilly Willy . I’ll try and not do that, thanks for your help!

There’s a VNAV button; activate it around 200nm from dest as long flight plan is set up correctly in a timing manner. Usually it starts top of descent (TOD) around the ballmark of 165nm.
I personally activate VNAV TOD 125-130nm from dest, but if center is active then best practice to request permission to descent vis STAR or manually as you deem fit and appropriate, depending on altitude/distance😎

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Thanks, one other thing. Do you recommend FPD (flight plan database) or simbrief? Like, which one is more realistic

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I prefer sim brief but it’s just preference pretty much.


I prefer Simbrief because they are more detailed and accurate. I agree with @CaptainRyan

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@CaptainRyan @tjb0709 @Pilot_InfiniteFlight thanks for all your help!


A nice rule of thumb for determining your ToD is this: 3 nautical miles for each 1,000 ft. of altitude to lose + 1 nautical mile for every 10 KIAS of airspeed that you need to lose before landing.

To illustrate with an example.

We are cruising along at 38,000 feet and landing at an airport with an elevation of 2,000 feet which means that we need to lose 36,000 ft. Let’s say that we will be descending at 350 KIAS (knots of indicated airspeed) and need to slow to 150 KIAS for landing:

Thousands of feet to lose: 38 - 2 = 36 thousands of feet
Tens of KIAS airspeed to lose: 35 - 15 = 20 tens of KIAS

Hence, distance for ToD = 36 x 3 + 20 = 128 NM out from your destination.

So the formula is : altitude (in thousands of feet to lose) x 3 (nautical miles) + KIAS to lose (in tens)

Hope this helps! I used to use this before the addition of altitudes and STARs to the sim and it usually was pretty spot on.


Thank you!