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Problem of VNAV SPD

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Hello everybodyI have a problem with the 737 pmdg and with all versions.I am in CRZ mode with VNAV and LNAV engaged.30 or 40 miles before TOD i dial a lower altitude on the MCP.Imagine the SPD DES is the same as the SPD CRZ for example 290 kts.When the 737 is passing TOD, it's begin his descent with a normally rate of descent of approx. between 1900 and 2100 fts/mns.Ok for the moment all is good BUT the 737 MCP is not maintaining the SPD displayed in the DES page on the FMC.the speed decrease slowly down under the magenta sign and goes to 270 or 260 kts.I precise that no speed contraint were added to the FMC.Sometimes, all is ok again when i deconnect A/THR and reconnect it immediatly.Do you have an idea about this issue ? thanksGilles Menghetti.

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Normal. The VNAV mode will follow a predetermined descent path while keeping the engines at flight idle. The airspeed will normally drop below the "command" speed setting. What it is doing is following a path that maximizes economy based on the cost factor setting. I don't know if this is realistic but it seems alright.On the flip side, I may use VNAV initially but I almost always change to either MCP SPEED w/ V/S or (if I'm in a hurry) to SPD DES. I prefer to control the descent via the MCP when I'm crossing altitude or speed constraints due to the poor job that the NG FMC does with constraints. I've read that this is also done real world, and that the NGX will have a better working FMC with new programming.

Dan Downs KCRP

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Guest Ray51

Yea the FMC can cut it a bit close on altitude and speed restrictions. I will normally program an along track waypoint 10 nm short and assign the speed/altitude restriction there. So if I am decending on the MOTIF3 into KMDW as an example I will program in "minok/-10" to get a temporary waypoint to set my speed and altitude restriction at. To get an accurate decend profile you need to program in the winds at the differant altitudes for the FMC to really do its job.Ray

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