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Guest pablo_gsxr750_aus

V-NAV & FMC descent capabilities..?

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Guest pablo_gsxr750_aus

I've been getting a problem relating to V-NAV when I enter the descent profile of the flight.ATC give me the OK to descend from FL360 to FL220, so I reduce the FL setting in the MCP, push the button and the Queen starts to head for the ground at the required speed etc etc as per the FMC Descent "ECON DES" Profile and obviously, my Flight-Plan set-up.No worries there you might say..? However, at around FL240 on route from KORD to KDFW the Queen starts to level out, for no apparent reason at all..? I've checked out everything Econ SPD, DES To, SPD Trans all OK..! I've read the FMC Descent Operations Manual and followed its instructions to the word, but the level out still continues and then after ATC start to bug me...I flick from V-NAV to FL-CH to complete my descent to FL220 and thereafter to the airport LOC as per ATC.What's causing the problem..? Surely I can use V-NAV with ATC heading and level changes until the Queen loc's onto the airport LOC and I switch to APP and land..?Any help and or comments would be appreciated...regard, Pablo

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Guest s_L_Y_F_a

This is probably due to the descend profile your FMC computed.Check the Legs page. There you will find the calculated altitudes at the respective waypoints. You can also edit your descend profile by manual assigning altitude constrains to the waypoints.Furthermore, check the deviation scale on the ND which shows you your deviation from the calculated optimum descend profile.

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Guest pablo_gsxr750_aus

OK...I'll try what you've suggested and get back to you if that's all right?But, before I go to do this...Doesn't setting a new ALT LVL on the MCP and pushing the button to activate it, remove all ALT constrains (between) automatically? Meaning that there is no reason to manually edit the ALT LVL's in the LEGS page?Bye for now...Pablo

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Pablo,>Doesn't setting a new ALT LVL on the MCP and pushing the>button to activate it, remove all ALT constrains (between)>automatically? Meaning that there is no reason to manually>edit the ALT LVL's in the LEGS page?If I recall correctly it should. But I don't remember if it was during climb or descent or both. Check your LEGS page to confirm.Cheers,


Mats Johansson
PMDG Flight Test Dept
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Guest s_L_Y_F_a

manual says this applies for both. But it confuses me a bit.Lets say you are flying a SID with lots of altitude constraints. While you are flying the SID, ATC assigns you a new (higher) altitude to climg and maintain. You select the altitude in the MCP window and press the knob, but this dumps all the SID constraints between your alt and the selected one right?Is this a legal to dump all those SID constraints and proceed directly to the assigned alt or am I missing something out?

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ATC assigns altitude based on traffic vertical separation locally and from other nearby patterns as well as provide terrain clearance. The SID is usually a guide and unless instructed otherwise by ATC those altitudes, usually maximums or minimums, apply.ATC can deviate from a SID or STAR at its discretion to meet immediate needs.


Ron Ginsberg
KMSP Minnesota, Land of 10,000 Puddles
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Guest D17S

One of the ways to initiate a descent is to dial down the MCP altitude, then just wait. Do NOT press the MCP altitude knob. When Vnav is ready to initiate its programmed Vnav Path descent, it'll just do it . . . all on its own. Pull up Vnav's cruise page and watch. You'll see a count-down till TOD. At that magic moment, Vnav will change the page from cruise to descent. Watch the FMAs. Pitch goes to VNAV PATH and the thrust FMA goes to IDLE>HOLD. Now watch the ND. The path deviation scale will appear and the diamond will remain On-Path as you cruise down the hill in an idle thrust descent.However there a couple of other ways to initiate a Vnav descent. You might have used the MCP altitude knob press method. it's a good way if you want to begin the descent before Vnav's previously calculated TOD point. This is called the "Descend Now" function. You can do this with an LSK on Vnav's descent page, but the knob press will do it too. You said you dialed down the MCP then pressed the MCP altitude knob? Yes? If so, you most likely initialed Vnav's "Descend Now" function.How can you tell if this is happening? Did the Vnav page change from cruise to descent when you pressed the knob? Check the FMAs too. In the "Descend Now" function, Pitch will go to VNAV SPD and thrust will go to THR. Now watch the ND. The path deviation scale will appear and the diamond will go Below-Path. You are diving below the FMC's previously calculated descent path. This is fine, but the Descend Now function is still a bit glitchy. You might get some odd indications during a Descend Now descent.However, if things are working OK, you will go into a gentle descent. Thrust will NOT go to idle (thrust's "THR" mode is on the job). Initially the path deviation indicator will tell you that you are going below path. You are! You elected to descend NOW, and not wait for Vnav's previously calculated TOD point to show up.But the point of the "Descend Now" is to descend more slowly than the original path (that's the reason for "THR"). You will eventually catch up with the original path . . . well that's the plan, anyway (good luck there!). As you close on the original path, watch the ND's path deviation scale. The airplane might level out a bit to capture the original path. Again, 'eagle eye' those FMAs. As you capture path, pitch will go to VNAV PATH and thrust will go to IDLE>HOLD. That path capture (attempt) might have been the "level off" you saw at FL240 (????) Ahhh, or not.The MCP knob press method (to initiate a descent) doesn't seem erase downrange altitude constraints. At cruise, I had a 3500A constraint, dialed 2000 feet into the MCP and pressed the knob. My 3500A remained untouched.Try a "Descend Now" descent and let us know what you find.

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Sam,While the mechanics might be the same, he's not doing DES NOW, he's technically doing a cruise descent that will resume the cruise phase when it completes. Same applies for climbs done in this manner too.Pablo - get to the point in this flight just before you do the attempted descent and save the flight. Mail to me at rmaziarz@precisionmanuals.com and I'll see if I can't figure out why it's leveling at 240...What does the FMA pitch section (middle) say when the plane levels at 240?


Ryan Maziarz
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Guest pablo_gsxr750_aus

OK I understand most of what's being said so far guy's and thanks for the comments, but the only way I can VNAV down without levelling out is to edit my descent profile in the LEGS page and when I do this all works out OK with a LOC and APP landing, no worries.Anyway, so that I can learn more about the Queen, FMC and VNAV, I'll send a copy of the saved flight to you at rmaziarz@precisionmanuals.com without delay.The descent profile is now the only area I'm having problems with as the PMDG doc's and this forum have sorted the climb profile issues I'd previously had with regards to ATC, VNAV, Transition and SPEED.Many thanks...Pablo

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I've had the same proplem with both the 47 and the 37. I didn't have the problem on the initial installs, only after patching and fixing over the years.I've also found out that for some reason the FMC about 99.9% of the time does not seem to be able to compute altitude and speed restrictions for various fixes on an arrival therefore I get a VNav disco message on the FMC.I do use real world weather but even with very little changes on the baro and winds I still get these messages.The only way I've got around this is to delete all the alt and speed restrictions except for the final approach fix.Even if I dial down the alt (MCP)to the final altitude of the approach fix (3000' for example) but I have an initial STAR arrival alt I have to hit VNav again to get it to continue to descend.This happens even when there is plenty of room between fixes. I'm not sure if I did something to the des page a while back. I've checked and everything seems ok.It's very weird. I haven't been able to nail down the culprit.

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Guest D17S

Very good additional detail points. Beyond 50 miles from TOD, it seems that an MCP altitude knob press will initiate a cruise descent as described. The CRZ page will remain active. However within 50 miles of TOD, that same knob press will cause the FMC to change its active Vnav page from "CRZ" to "DES", blank the CRZ page (short of the boxes that will allow a manually initiated return to CRZ mode) and display the path deviation scale on the ND. Additionally, the altitude will now recapture any lower MCP altitude selection in VNAV ALT/SPD while leaving the FMC at the DES page. My assumption is that these events indicate that the FMC has shifted to the descent mode and then uses a DES page, descent mode to facilitate the descent other than the normal VNAV PATH/IDLE>HOLD. The FMAs annunciate a VNAV SPD/THR style Descend Now mode set . . . and that's about all the DES page has left to work with, as far as I know. Are there others? It kinda sounded like he was within 50 miles. Most certainly though, maybe not. And BTW, check out the right FMA for the pitch mode annunciation. (No worries. After it gets late where you are it gets early where I am!)

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Guest shogun007

G'day Pablo, perhaps this extract from the Boeing 747-400 Flight Crew Operations Manual will give you a better understanding of what is happening when you push the Altitude Selector. Feel free to email me at shogun07@bigpond.net.auif you have any more questions, I'd be glad to answer them for you.Regards Steve744 Co-Pilot2 Altitude SelectorRotate

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