How to Realistically Descend using VNAV (My Method)

Intro

We’ve all just pressed that VNAV button without thinking about the altitudes in your flight plan, but this often results in level segments and steep descents. In real life, there aren’t specific altitudes for every waypoint, but general restrictions on your altitude when crossing them. Because of this, Infinite Flight STAR altitudes don’t always make sense. I’ve even seen a STAR that requires you to climb just before the approach.

In this tutorial, I will teach you how to calculate your own flight plan altitudes to make VNAV work for you.

Winds

For any of the steps below, add or subtract 1 nm to or from the distance for every 10 kts of tailwind or headwind.

Calculating the Altitudes

  1. Find the waypoint closest to, but not past, where you would intercept the LOC. Set that waypoint’s altitude to the airport’s elevation + 2500 ft.

  2. Next, subtract that altitude from 10,500 ft MSL (the altitude you’ll be leveling off at to slow down), and multiply by three.

  3. Add the distance from the waypoint I mentioned earlier to touchdown, along with 5-15 nm depending on how quickly your aircraft can slow down. 737/A320, 757, and 777 will take 5; Embraer E-series and the 787 will take 10; the A220 lies somewhere in between; and the A350 will take 15. (Not gonna list every single aircraft in Infinite Flight)

  4. Anyways, take that distance and find the waypoint closest to, but not past, that distance from touchdown.

  5. Subtract that distance from the waypoint’s distance to touchdown and divide by three to get the altitude of that waypoint. It’ll make sense in the last section.

Calculating your TOD

  1. Subtract 10,500 ft from your cruising altitude and multiply by three.
  2. Add the slow down to touchdown distance.

Actually Descending

When 1-2 nm from TOD, hit VNAV. At about 11,000 ft, press VNAV again to turn it off, and set the altitude to 10,500 ft. Once your VS reads 0 and your airspeed is closing in on 250 kts resume descending via VNAV.

Notes

  • This tutorial assumes a standard ILS approach.
  • If the diffrence mentioned in step 5 is particularly large, consider adding a waypoint to your flight plan that gets closer.

Sorry if I confused you. If this seems confusing, I can add screenshots at a later time. (I would now but my iPad’s dead)

EDIT: If you have a sharp turn or U turn to line up with the LOC, set up your altitudes to have leveled off at intercept altitude earlier. And if you’re like me and would use a calculator for this, you might as well use 3.33 as the number for multiplying and dividing…

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Well. im gonna try it out thanks

Or just start slowing down early enough so you are already at <250kts when at 11000ft…

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Actually there is an other method, really more simple, just add VNAV in the information bar:

It show you how much time remaining to the TOD

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IRL, you descend at a 3-degree slope, VNAV uses a 2-degree descent. And I don’t know if it accounts for winds.

Yes, but 11,000 ft wasn’t an example. And sometimes it is hard to slow down while descending. What if the difference in step 5 was larger and the altitude was more like 15,000 ft, would it be worth the simplicity to just level off there? Either way, there’s no way to get around disengaging VNAV for the level off unless you happen to get lucky and have a waypoint 5-15 nm past.

That’s a lot of math that I kinda don’t wanna do but Ima try it anyways,

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Lol. Once you do it a few times it’s a lot easier and less time consuming. I usually do it during cruise anyway…

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Interesting tutorial, very good!

I calculate my TOD this way, and use a little trigonometry since I’m using an angle as a reference, it’s really cool!

on a recent flight I stayed level during a time when I should have been descending on the STAR I was following, as the waypoint ahead had an altitude “above 9000 below 110000” and in the flight plan the altitude was 11000ft, I think in the future flight using STARs at these altitudes it would be interesting to change the flight plan to the lowest or “middle” altitude, which in this case would be 9000ft or 10000ft.

Thanks for the Tutorial but I want to suggest the most simple method based on my own experience and that’s the Magic number of 3.5 . For Example:
A: Way point at ILS cone end ( where you catch LOC with 3K AGL)
B: Another WP with 10K-15K
C: Another WP with 20K-25K
if there is 35 nm Distance from B to A,Then consider 35/3.5=10 , 10K Altitude Difference For them. So with A at 3K, B would be fine at 13K
Also if there is 42 nm Distance From C to B, 42/3.5=12 , 12K Altitude Difference For them. So with B at 13K, C would be fine at 25K
Remember to keep appropriate speed while descending:
About 0.8 M or 300 kts From Cruise to Point C
280 kts from C to B
250 or less below 10K
200-220 on Downwind
200-180 for intercept
180-160 on Final

When possible, you can also use IFR STAR charts to plan descent.

Sometimes, a STAR calling you to climb is actually the highest altitude of a waypoint, and the minimum altitude (which you should always follow when given a ‘between’ altitude) is usually much more sensible.

For example, if waypoint EASTN gives an altitude restriction between FL180 and FL240, IF will immediately list FL240. If the lowest altitude is known (in this case FL180), change it to said altitude and it will likely make much more sense than the uploaded profile.

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Yeah. I might incorporate STAR crossing restrictions into my method sometime.

EDIT: The number for more accurate calculations is actually 3.18, not 3.33. (I couldn’t edit my topic anymore so I’m clarifying here)

@alexNine99 Love the post! Question tho, what do I do with the calculation from Step 2? Is that added to Step 3?

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Yes. 10 characters.