# VNAV might be failing

Hey Everybody!
right now I am flying at 0.81
and my TOD is at 305 NM
but the ETE to TOD say 50 mins, that time should be there, it should say like 15 mins not 50
my winds are calm, super, just a 13 knots Tailwind, help!

and whenever I fly with a TOD that says something like 300NM the ETE to TOD is like 20 mins, not 50

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50 minutes sounds correct to me, especially if youâre flying at M.81.

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That would be about right. At M.82-M.85 youâre covering roughly 10nm per 1 minute. So 300nm out would indicate somewhere in the neighborhood of 30 mins out (slightly less than 30 mins but close). Check it out next time youâre flying, and youâll see. đ

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Learning opportunity!

And only reason its slightly less is because of this:

What ground speed does an aircraft doing M.82-.85 do in still/calm air? About 500kts. For simplicity, letâs round this to 600kts. 600kt ground speed in 1 hour. Instead of hours lets convert to minutes. So 600kts in 60 minutes still equals that 10nm per 1 minute ratio. Now, because weâre not cruising around at 600kts all the time in calm air, we can say weâre likely doing closer to 8nm per 1 minute or so.

i didnât exactly do the exact calculations but that my best âestimate and use of rule of thumbsâ mental math.

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all right! thankyou DeerCrusher
makes sense now!

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DeerCrusher? No, math genius.

This was quite a helpful topic even though I didnât have the question, thank you for explaining!

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Thereâs no real way to practice. I will say, if youâre ever doing a road trip and youâre bored, try converting your miles per hour (kilometers per hour) to feet per second (meters per second) mentally. Kills a bunch of time. đ

Same kind of concept applies somewhat here

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Hm, Iâll have to put the Netflix down next time and try that, thanks!

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