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Initial Descent in 737-800

Old 1st Aug 2016, 19:45
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Join Date: Aug 2016
Location: Boston
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Initial Descent in 737-800

Can I ask, when at cruise and cleared to commence initial descent ahead of calculated TOD, should this be done in VNAV mode (using ALT INTV) or in LVL CHG mode? I find in my simulator that I end up being significantly below profile for most of the descent if I chose LVL CHG once cleared by ATC. Just wondering what is the correct approach.

Thanks,

Darren
Dazmundo is offline  
Old 2nd Aug 2016, 16:54
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de minimus non curat lex
 
Join Date: Feb 2001
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Keep in VNav and use the descend now function. This will achieve 1000fpm, until you intercept the "green worm" actual scheduled path. Putting in 3 forecast winds helps the FMC compute the ideal TOD.

LVL CHG simply closes the throttles to flight idle for min thrust.
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Old 2nd Aug 2016, 17:46
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Very helpful. Thank you!
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Old 10th Aug 2016, 22:45
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Join Date: Aug 2012
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VNAV is useful but you should also anticipate shortcuts. VNAV will assume that you are going to follow the full route loaded into the FMS, including any RNAV transition, and calculate your descent path accordingly. If ATC then gives you a direct track to intercept the final course or something you may find yourself above profile.

Using the 3:1 rule (around 3000ft descent over 10NM on a 3 degree glide) you can mentally calculate a rough path. If it's not busy expect shortcuts. After flying to an airport a few times you get used to what the ATC is probably going to tell you and can plan accordingly.
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