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Guest Nightflight744

VNAV descent (B744)

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Guest Nightflight744

Hi guys,I am not sure whether this has already been discussed. I used the search and were not successful.Have you noticed that when you want to descent with VNAV descent mode prior to top of descent the VNAV page in the CDU is completely deleted. I set a new altitude in the MCP then go to the VNAV descent page and press Des Now (I expect a moderate descent until capture of the FMC calculated deccent path). The problem is that then all speed restrictions are cleared. I usually then switch to FLCh mode.What do I do wrong here?? Suggestions are very much appreciated.RegardsFabian

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Guest D17S

DEC NOW will shift the FMC into descent mode and clear the CRZ page, but the Decent page should still be around. Try setting the MCP to an altitude ABOVE the descent page transition altitude before you press DEC NOW. See if that will leave that transition's speed/altitude restriction in place. Did you get the 1200 fpm descent until you captured path?

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>Did you get the 1200 fpm descent until you captured path? Hi,did you? Mine still doesn't after the 744F update. Well, it's better than before but not as it's supposed to be in other add-ons like the pmdg737 or the competitive 767. There, it's just "simple" as that: After DESC NOW activation 1200fpm until catching the VNAV path, then down on VNAV path. In my case the plane indeed starts a 1200fpm descend initially, but soon begins to increase descend to -let's say 2000fpm- decreases speed and so on ...despite being below the VNAV path, i can't explain myself why. I was 20-30nm from T/D away. This happens every time I use it... so what, I'm using vertical speed instead but it would be nice to see it working one day :)BTW: A/T override is off at the moment, but didn't work with A/T override on either.I'll try to document the events next time I fly.So long!

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Guest D17S

Watch the AFS modes. DEC NOW is using the mode-set "VNAV SPD" (pitch-right FMA) and "THR" (thrust-left FMA). This provides the identical function as the FLCH SPD/THR mode-set. VNAV SPD's entire purpose in life is to maintain airspeed by using pitch. As THR increases or decreases the airplane will speed up or slow down. VNAV SPD then has to respond by either pitching up or down to maintain airspeed. Increased or decreased pitch will "naturally" cause an increased or decreased rate or climb or descent. For instance, if the AFS (autoflight system) needs a bit more rate of descent in a DEC NOW descent, it will tell "THR" to pull back power a bit. The airplane will start to slow down. VNAV SPD pitch will respond by pitching down a bit to maintain the descent target airspeed. A greater rate of descent will happen . . . ahh natural. This is the way the DEC NOW descent maintains 1200 FPM. THR leads and VNAV SPD follows. You can watch this "dance" between THR and VNAV SPD going on. See what you can see. Clearly, the AT must be engaged and be able to control thrust. Also, in a DEC NOW descent, the ND's path scale will tell you that the airplane is below path. This is because it IS below path. I've been playng with this function all weekend and will have some interesting reporting to do in a day or two. Stay tuned!

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The descend now function commands an initial rate of descent of 1200 fpm. Once this is established the A/T goes to the HOLD mode. It is then up to you to manually control thrust to maintain that rate (if desired) until intersecting the path. If you do nothing then it is only thru luck that it maintains 1200 fpm down. Once intercepting the path the thrust will go to IDLE and then HOLD. Use of DESCEND NOW also clears all constraints in the descent up to transition.CheersSteve

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>The descend now function commands an initial rate of descent>of 1200 fpm. Once this is established the A/T goes to the HOLD>mode. It is then up to you to manually control thrust to>maintain that rate (if desired) until intersecting the path.Hi guys,thanks, that clears it up! Seems to work different in the other planes ...i love to be in command so that's fine ;)

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I have tried a few of these and you are correct in that it does not appear to work properly. It reduces the thrust OK and starts the initial descent with the thrust levers in hold but stays in VNAV PATH and reduces the rate of descent and lets the speed wash off. It should not do this. Also more than 50 ml from T/D it should show ACT CRZ DES on crz page. It doesn't remove the constraints either. The DES DIR function seems to work OK (this removes all speed/alt restraints down to the MCP ALT).Steve

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Guest D17S

Steve, I think you still have 'just' the pax version. The Descend Now function works just like you are describing with the pax airplane . . . in other words, it's broke. They fixed it with the -F, but you gotta buy it.And BTW/FYI, the new mode set for DEC NOW is VNAV SPD / THR. The DEC NOW function works great now, that is if you are within range to intercept the origional Path. Not so great if you try to DEC NOW too early.

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Gee I hate to spend $30 for basically a patch. Any chance there will be a stand alone fix for this for the PAX version?

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Guest D17S

Just between you and me, let them get this -F "revenue event" more fully realized. After all, they need to pay the bills too. Then, we'll quietly see a patch. This speaks for no one or really, nothing at all ('cept common sense.)

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Hhhhmmmmm. I have only just shelled out for the Pax version. I can live with it in the meantime. I guess a patch should not be too difficult if the code is there for the 'F'.Sam if you descend too early (greater than 50 nm before T/D)it will go into a cruise descent. Have a look at the cruise page and you will see the MCP altitude is now the new cruise altitude.Steve

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