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Cpudan80

Question about VNAV...

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Hello Everyone: First off - would like to say that I've been flying the 737 NG for about 3 days now -- got the basics down (I think) -- except for what I'm about to ask... I can manage with the FMC up through takeoff - climb - and cruise. However, when we reach the time for descent - the problems arise. Question: If you look at the legs page - you see your position - and upcoming points --- to the right of the point name is a projected speed / altitude... If I were to use VNAV through the climb / cruise / descent - would the plane follow the speeds / altitudes listed in the legs page? Second - once I program the approach in, after clearing the DISCO, can I still use VNAV until it's time to set in for VOR LOC and APPROACH modes? So, to sum it up, If I were to use VNAV trough my entire flight - the plane would follow the speeds / altitudes listed on the legs page - up until the time for VOR LOC and APPROACH modes - is that correct?Thanks! All help is appreciated!

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Hello Dan, Descents are not as one would expect. There are various ways one can achieve a desired result but in the case of VNAV we are limited to one of two things, the first is the default PATH DEScent, in a PH DES the AFS will display a target speed but will not try to capture this speed, it's for planning or reference purposes only and VNAV is only concerned with keeping on the path. Now there are rules to this that relates to the descent speed but only in the case of overspeed or underspeed, in overspeed condition the AFS will revert to a LVL CH descent, if under a certain degree in IAS the AFS will provide some thrust but not so much as to chase the target. So when you think PATH, forget the SPEED. And finally we come to a SPEED DEScent. In a speed descent the AFS will pitch for the target speed without any regard for the calculated descent profile. Speed descent mode will adhere to wpt restrictions (Temp lvl of f until passing the wpt's *position* on the calculated path). So for a SPD DES forget the path and think of speed. If you choose to arm a *mode* of descent like SPD or Path and then hit the DES NOW prompt, in the case of PTH the DES NOW will perform a 1000 feet per minute descent until the original calcaulted path is incountered then it reverts to the normal PTH DES mode. On the other hand if you chose a SPD DES prior to pushing the DES NOW prompt a SPD DES will start immediately. As for LOC and APP modes, LOC has no bearing on VNAV, only on heading modes such as HDG SEL and LNAV. APP is another story because it is following the G/S which is controlling your vertical descent to the rnwy. So at G/S capture VNAV will disconnect and APP become active as long as you armed it. One can engage VNAV past G/S if you are not doing an ILS approach but it's beyond our discussion here. [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpg

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Thanks Randy, I actually figured it all out (I think) in a test flight using the VNAV button -- worked wonders all the way till just before landing when I went of VNAV and switched to VOR LOC & APP - the bird practically landed herself... Thanks for the help -- will try your suggestions in another flight tomorrow!

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