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TH99999

rookie help with descending

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I am still learning this plane and reading and watching vids. I am not using Air Traffic Control yet. When I program the FMC it looks good ,and use LNAV and Vnav but when I am descending it starts with a very fast decent at over 3000 ft per minute but then levels off to about 2000 after it has gone down about 4000 ft. My problem is that when it is decending it ignores the speeds in the FMC and if I try to reduce the speed on A/T it ignores as it is trying to get to the next altitude and keeps up the speed.  This is especially true when crossing the 10k feet 250 knots. 

 

I hope I am explaining right, I am not having too much trouble taking off, just when taking off and what point to most people turn on vnav, is it around 1000 ft ? 

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Hi TH,

it is normal for the real plane in VNAV PTH to not adhere perfectly to the "set" speed - the plane will calculate a fixed geometric path in the sky that it "believes" it can fly with idle power at the calculated descent speed. But more often than not some external influences (wind, mostly) will not allow the speed. The plane will prefer to stay exactly on the calculated path and lets the speed go up or down. If this is excessive, it will nag you "drag required". If the speed gets too slow, it will activate the autothrottle and add thrust to regain speed. But there is a "window" in which speed may vary without any action at all

Our plane does not model a "smooth" intercept of the VNAV PTH at the top of descent (yet). So it will initially get a big high, then tries to catch up by descending fast to the pre-calculated path. For now there is not much you can do to avoid that, except manually starting your descent a bit prior to the T/D in V/S mode, then, once on the path, enable VNAV PTH descent.

Cheers, Jan

 

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thank you I will try that , this weekend. I read about manually descending instead of VNAV but will maybe try that. looks more complicated but keep me busy on the descend

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