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johndrago

VNAV - DES NOW Question - 747 Pax

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Hi Guys,Hopefully a quick question. When ATC commands me to descend from CRZ earlier than my T/D, I use the DES NOW function. Per the normal function, the A/C will start a 1000fpm descent until it reaches its normal path.My Question: When I use DES NOW to descend early, can I tell the FMC that I want it to start its path at the point I initiate it, rather than having it wait for the T/D point?Thanks in advance,John


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- John Drago

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>My Question: When I use DES NOW to descend early, can I tell>the FMC that I want it to start its path at the point I>initiate it, rather than having it wait for the T/D point?>>Thanks in advance,I don't think so. A simple reason is that FMC knows of only one descent path (one solution) that meets all VNAV constraints and leads to the destination. You can't move this whole path in space (advance to earlier time) and expect that FMC will somehow compute something that can't be computed. Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg


Michael J.

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>Hi Guys,>>Hopefully a quick question. When ATC commands me to descend>from CRZ earlier than my T/D, I use the DES NOW function. Per>the normal function, the A/C will start a 1000fpm descent>until it reaches its normal path.>>My Question: When I use DES NOW to descend early, can I tell>the FMC that I want it to start its path at the point I>initiate it, rather than having it wait for the T/D point?>>Thanks in advance,>>JohnJohn,It is a 1250ft/min descent that will be initiated.To answer your question : i agree with Michael. One of the reasons is that the descend path calculated by the FMC is calculated with a descend rate of +/- 2000 fpm and even more, and you start your earlier desc at a rate of 1200 fpm.Now, no problem, the aircraft will sooner or later joining it's calculated descend path, and you can follow this progress on the vertical deviation indicator in the ND.


Regards,

William Vrielynck

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Guest D17S

Yea, I agree too. The path is based on an idle thrust descent at a fixed airspeed. It's been argued this is a path descent. True 'nuff, but a fixed thrust (idle) that uses a variable pitch to maintain a fixed airspeed (still) sure seems to me like a speed-on-pitch maneuver. Just like the climb speed on pitch modes, only down rather than up! When you initiate an early "descend now," watch thrust. It will not come all the way back to idle. The 1250 ROD is less than the path's calculated ROD. This means the airplane has to use thrust to compensate for the negative pitch it cannot (yet) use. Now watch the ND's path indicator. As the path indicator finally moves to 'on-path,' watch thrust pull back to idle-hold and the nose roll over. The airplane just substituted more down-pitch for the thrust it pulled out. Now you are back on the FMC's pre-calculated idle thrust, speed - determined - by - pitch, glide path. AKA, Speed on Pitch. That thing's quite a calculator.

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