Why is vnav so early

why is vnav so early? im 200 nm away and its descending. I think I heard something about being like that irl.

This is most likely because you are going a bit faster than normal so it will take longer to reach the desired altitude

Depends on the altitude you picked

It is early because VNAV only consider altitudes as absolutes and not be below this altitude or be above this altitude so in cases where the floor is a certain FL (which is a lot of cases) VNAV will try and reach the floor which often results in a TOD from farther out.

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VNAV descends at a standard rate Whci also doesn’t affect your speed. All of us descend at different rates which at this change we may not be used to.

And above, since some arrival altitudes may be lower than you usually are in your leg of the journey.

I also set an altitude for 18000 and it went lower than 18000

If you have a star, it is going to descend to the altitude at your next waypoint

I agree with you. I don’t think VNAV is very good. I mean, it’s great, but is way to early. I turn it on 20-30nm “late”, then I ignore the fact that it is descending slowly once I’m on the STAR because I know it’s because of the hard altitudes. I made a topic about it too

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VNAV is designed so that the aircraft, whenever descending, tries to maintains a 2 degree pitch down. In real life, it’s 3 degrees. The system calculates it based off this and decends accordingly. It’s a great feature for initial descents, but final descents, where you sometimes have to turn as well, don’t always work the best using this formula. Always make sure to monitor your entire decent as well as lateral and vertical speeds whenever possible.

VNAV will calculate it base on your current speed and the altitude it it set at. You don’have to set it so early at cruise time.

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This is incorrect.

It works out the slope to your next waypoint, If its under 2 degrees then it won’t descend until it hits that 2 degree slope. At which point it will completely forget about the slope and just get you to your next waypoint at whatever alt you picked provided you don’t go above -3000fpm.

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Wait so lets say u have a STAR and on one of the waypoints u have to be @ a specific FL when do i click VNAV 100nm out? 50? Thanks for replay :)

It is up to you. Some people turn it on early and it will descend when it needs to.

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Ok thank u

VNAV uses a 2 degree instead of 3 degree descent. You can turn it on late for three degree.
Some aircraft, like the 787, will speed up on 3 degree idle descent though so be careful

I always try to achieve an idle decent, so VNAV isn’t my go too way normally, but I use it as a great tool when having at or below altitude constraints. Remember that you can always change the set altitudes too.
Also I will just leave my comment on the other related topic below:

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Oh, my bad, I’ll change it.

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