How VNAV Works?

Does anyone know how TOD in IF is calculated? As in is it based off a descent angle or descent rate?


There is a mini section on the bottom of the screen where you could select vnav and it will count down to zero and then you arm vnav and it will decent to the bottom altitude I normally arm it one minute before Tod


It involves some math

( it calculates your cruising altitude - final altitude divided by 1000 times 3 )

EX: going from 35,000ft to 10,000ft is 25,000ft to lose

25,000 divided by 1000 is 25

25 times 3 is 75 so 75NM from that altitude restriction is your TOD!

( I could be totally wrong )


No you are completely correct now I’m not a real world pilot but I have heard real pilots explain it like that there’s also the 3 to 1 rule which states that for every thousand feet yo travel three nautical miles


Thanks for the responses, but I’m wondering how Infinite Flight calculates TOD when you use VNAV. I know there are several ways to calculate TOD on your own, so I was curious as to if anyone knew which method IF uses. Maybe a developer knows?


Maybe it uses the methods we’ve mentioned above only thing I don’t like about vnav is that sometimes the plane would nosedive but yeah maybe a developer could explain how it works


I tested two points so take this with a grain of salt but it seems like it’s angle based to a certain extent (at least for determining an initial TOD). At 494 knots from FL340 it had TOD at 159 nm @ 19:20 ETE Dest and at 529 knots it had TOD at 160 nm @ 18:08 ETE dest. Upping this to FL400 it was 188 nm, leading me to believe it calculates off a descent angle of 212 feet/nm, or about 2 degrees descent.

This seems to hold true as long as you’re not going fast enough to need more than -3000 FPM to maintain that (which would be pretty difficult as you would need to be traveling 850+ knots), and obviously if you’re not constraining VNAV to any preselected altitudes.

I’ll test this a bit further and get back to you.

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Update: seems to hold true for most/all jets, and works whether the point is up high or an airport. 212 ft/nm, 2 degree descent.


Very interesting, this is what I was looking for. Thanks!

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Doesn’t that depend on speed though? If you are going to get from A to B in which are 1000 feet vertically separated and the distance is 3 nm, and it will take 3 mins to get there, then you would descend at 1000 fpm. but if you were going half the speed, it would take 6 minutes, so it would be 500 fpm


It does but honestly 1000 feet per 3 nm is quite an ambitious goal at any speed, that’s 3.2 degrees.

When I first time using VNAV,I clicking all the stuff and the VNAV works like normal.Now I know how to use VNAV but remembering first time when using it,it made laugh a bit.


So it does go over when a normal decent rate is 3 degrees

Yeah I mean, if you try to descend from FL340 from 102 nm I think you’re gonna find it pretty difficult energy management wise, especially in slippery heavies (737s you probably will get away with it). It’s quite steep especially because you need to slow down as well.