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Guest Gerhard

LVL CHG Descend

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Descend from cruise with LVL CHG result in a steep descend (5000 ft/min ).Why ?Gerhard

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Gerhard,why not ? LVL CHG controls your selected speed via the pitch (and not the autothrottle) - and in an idle descend you might need 5000ft/min to keep the selected speed.HansJuergen

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The info as I have it from Mel Ott (retired 744 captain): VS descent to FL180, then one can get into FLCH from there, especially below 10000 unless there are specific descent gradients that call for VNAV.FLCH is Idle power descent with airspeed controlled by pitch to maintain the Speed as set on the MCP.VS uses both pitch and power to maintain the speed and rate as shown in the MCP. If circumstances are right, this can also be idle thrust.VNAV descents are rare, wheras VNAV climb is the norm. This also depends on a specific airline SOP. LNAV/VNAV makes flying DP/STAR's easier in complex airspace, and somce countries are very unforgiving of deviations from the procedures (Japan).The entire system is amazing IMHO. Think about it for a bit and you'll see hw incredible it is comapred to simple VOR nav or even teh old radio range stuff.Tim

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Climb: VNAV. Exceptions include, if you have to expedite through a certain level you can use level change and wind the speed back to whizz through that level, or we use V/S if you're being cleared up 1000' at a time (typical for London airports).Descent: The most efficient descent is a flight idle descent - so VNAV and level change will do the same thing. The only time you use V/S in initial descent is for an ATC induced early descent, or because you c***ed it up. VNAV is cool for stuff like "descend now, FL150, level at xxx". The level at xxx is in the FMC, so you press capture in the descend page and it does 1000 fpm until it reaches it's VNAV path, then it descends at flight idle and you're at FL150 bang over xxx. If descending in VNAV, it's best to get into level change below FL100. Likewise you need to come out of VNAV and into HDG SEL before you intercept the localiser.This is for the 737NG. Can't comment on the 747, but I suspect that it's very similar.Hope this helps.PS I know that there are 100 variations, caveats and exceptions to what I've written, but I'm trying to keep it simple!

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Well, good ol' Mel has probably cost his company tens of thousands of dollars of extra fuel used by using that method.VNAV descents are not rare at all. It is common though that people who used to fly on aircraft without FMCs are hesitant to use VNAV (which is really all in their head). Some argue that its passenger comfort, but starting an early descent, especially in a 744 can easily add 3-600 kilograms of extra fuel used on a SINGLE flight!VNAV climb really is no big deal, since it's just a speed controlled climb without any path control.

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